Mitsubishi NF50-HP, NF30-SP, NF50-HRP, NF60-HP, NF100-SP Technical Notes

MOULDED CASE CIRCUIT BREAKERS

TECHNICAL NOTES

ADVANCED AND EVER ADVANCING

00

A

We have the pleasure of providing all our customers with the technical information for Mitsubishi
moulded case circuit breakers. This indicates the fundamental data of our circuit breakers
regarding the applicable standards, constructional principles, and operational performances.
Please refer to the catalogue of our circuit breakers for details of specifications.
Also please stand in need of the handling and maintenance manual for maintaning the circuit
breakers in service continuously.
We do hope they are available for all our customers to built more efficient systems.

CONTENTS

1. INTRODUCTION ...............................................2

2. FEATURES .......................................................3

2.1 Arc-Extinguishing Device – ISTAC.................. 3

2.2 Digital ETR ......................................................... 4

3. CONSTRUCTION AND OPERATION............... 6

4. CHARACTERISTICS AND

PERFORMANCE............................................. 11

4.1 Overcurrent-Trip Characteristics ................... 11

4.2 Short-Circuit Trip Characteristics.................. 11

4.3 Effects of Mounting Attitudes ........................ 12

4.4 DC Tripping Characteristics of AC-Rated

MCCBs.............................................................. 12

4.5 Frequency Characteristics ............................. 12

4.6 Switching Characteristics............................... 13

4.7 Dielectric Strength........................................... 13

5. CIRCUIT BREAKERS SELECTION ............... 14

5.1 Circuit Breakers Selection Table ................... 14

6. PROTECTIVE CO-ORDINATION ................... 39

6.1 General ............................................................. 39

6.2 Interrupting Capacity Consideration ............. 40

6.3 Selective-Interruption...................................... 41

6.4 Cascade Back-up Protection.......................... 46

2

t Let-Through Characteristics and Current

6.5 I

Limiting Characteristics.................................. 48

6.6 Protective Coordination with Wiring ............. 49

6.7 Protective Coordination with

Motor Starters .................................................. 52

6.8 Coordination with Devices on the

High-Voltage Circuit ........................................ 54

7. SELECTION .................................................... 57

7.1 For Motor Branch Circuits .............................. 57

7.2 For Lighting and Heating Branch Circuits .... 57

7.3 For Main Circuit ............................................... 58

7.4 For Welding Circuits ....................................... 58

7.5 For Transformer-Primary Use ........................ 60

7.6 For Capacitor Circuits..................................... 61

7.7 For Thyristor Circuits...................................... 62

7.8 Selection of MCCBs in inverter circuit .......... 68

8. ENVIRONMENTAL CHARACTERISTICS ...... 70

8.1 Atmospheric Environment.............................. 70

8.2 Vibration-Withstand Characteristics ............. 71

8.3 Shock-Withstand Characteristics .................. 72

9. SHORT-CIRCUIT CURRENT

CALCULATIONS ............................................ 73

9.1 Purpose ............................................................ 73

9.2 Definitions ........................................................ 73

9.3 Impedances and Equivalent Circuits of

Circuit Components ........................................ 73

9.4 Classification of Short-Circuit Current.......... 76

9.5 Calculation Procedures .................................. 77

1

1. INTRODUCTION

Mitsubishi Advancing Technology

Mitsubishi, the leading manufacturer of circuit break­ers, has been providing customers with a wide range
of highly reliable and safe moulded case circuit break­ers (MCCB) and earth-leakage circuit breakers
(ELCB), corresponding to the needs of the age.

Since production began in 1933 many millions of
Mitsubishi ACBs, MCCBs and MCBs have been sold
throughout many countries.

In 1985 a new design concept for controlling arc en­ergies within MCCBs – vapour jet control (VJC) – was
introduced and significantly improved performance. It
is provided the technological advance for a new ‘su­per series’ range of MCCBs and is used in all present
ratings from 3 to 1600 amps.

In 1995 Mitsubishi offers the new PSS (Progressive
Super Series) breakers having ratings from 3 to 250
amps that concentrate the most advanced technolo­gies into a compact body. Their four major features
are:

• New circuit-breaking technology ISTAC for a higher
current-limiting performance, upgrading the circuit­breaking capability.

• Electronic circuit breakers with the Digital ETR pro­tecting the circuit accurately.

• One-frame, one-size design allowing efficient panel
design.

• Cassette-type internal accessories that allow instal­lation by the user.

Progressive Super Series, an integration of technol­ogy and know-how from this comprehensive electronic
product manufacturer, will create its own fields of ap­plication with its excellent performance.

A Brief Chronology

1933 Moulded case circuit breaker production

begins.

1952 Miniature circuit breaker production be-

gins.

1968 Manufacture commences of short-time-

delayed breakers.

1969 Production and sale of first residual cur-

rent circuit breakers.

1970 170kA breaking level ‘permanent power

fuse’ integrated MCCBs is introduced.

1973 Introduction of first short-time delay and

current-limiting selectable breakers go on
sale.

1974 First MELNIC solid-state electronic trip

unit MCCBs are introduced.

1975 ELCBs with solid-state integrated circuit

sensing devices are introduced.

1977-1979 Four new ranges of MCCBs are intro-

duced – economy, standard, current lim­iting, ultra current limiting and motor rated
designs – a comprehensive coverage of
most application requirements.

1982 Compact ACBs with solid-state trip de-

vices and internally mounted accessories
introduced.

1985-1989 Super series MCCBs with VJC and ETR

are developed and launched – awarded
the prestigious Japanese MInister of Con­struction Prize.

1990 New 200kA level U-series MCCBs super

current limiting breakers are introduced.

1991 Super-NV ELCBs and Super-AE ACBs

are introduced.

1995 Progressive Super Series 30~250 amps

are introduced.

1997 Progressive Super Series 400~800 amps

are introduced.

2

2. FEATURES – Advanced MCCB Design Technol-

ogy & Performance

2.1 Arc-Extinguishing Device – ISTAC

Mitsubishi has developed an epoch-making ISTAC
technology to realize an improved current-limiting and

breaking performance within a smaller breaking space.
Introduction of ISTAC technology upgrades the cur-

rent-limiting, selective-breaking, and cascade-break­ing performance. The maximum peak let-through cur-

rent Ip decreases to about 80% (compared with
Mitsubishi’s 100AF). The passing energy I2t de­creases to about 65% (compared with Mitsubishi’s

100AF). The smaller breaking space has led to an
improved function, a smaller size, and a standardiza­tion of the breakers.

Triple forces accelerating

The triple forces generated by a newly designed cur­rent pass and the Vapor Jet Control (VJC) insulat­ing materials which makes up a slot-type breaking
construction accelerate the movable conductor, and
separate the contacts faster than ever before in short­breaking.

Electromagnetic attractive force which works between
a current of the movable conductor and a current of
the fixed upper conductor.

Electromagnetic repulsive force which works between
a current of the movable conductor and a current of
the fixed lower conductor.

Pressure which works on the movable conductor by
gas generated in the slot.

The VJC suppresses the emergence of carbide prod­ucts in breaking a current and contribute to the recov­ery of insulation immediately thereafter.
The VJCs on the fixed and movable contacts work
together to forcefully reduce the arc spot and rapidly
contract the total arc being extinguished.

Movable contact

Upper,
fixed-contact
conductor

Lower,
fixed-contact
conductor

Pressure

Arc

3

Movable
contact VJC

Fixed contact
VJC

Vapor jet control (VJC)

Vapor Jet Controllers made of insulating material are
arranged around the contacts where they control the
arc as follows:

1. The arc spot is forcibly reduced by the arrange­ment of the insulating material.

2. The arc column is contracted.

3. Adiabatic expansion cools the arc.

4. The arc is transferred at the optimum moment to
the arc-extinguishing chamber by the arrangement
of the Vapor Jet Controllers.

Repulsive
force

Movable
contact

2

1

Attractive
force

Current A

Current B

Current C

Current

Upper,
fixed-contact
conductor

Lower,
fixed-contact
conductor

Arc control by slot-breaking

The VJC of the fixed contact incorporates newly de­veloped insulation made of ceramic fiber and metal
hydroxide. The substantially improves the VJC effect.
The arc-extinguishing gas energies to improve the
capability of extinguishing the arc.

3

2.2 Digital ETR (Electronic Trip Relay)

Sampling and A/D
conversion

Calculating
the digitally
effective value

Processing
the long time-delay
pre-alarm
characteristics

Mitsubishi’s electronic MCCBs are equipped with a
digital ETR to enable fine protection.
The digital ETR contains Mitsubishi’s original double
IC (8 bit microcomputer and custom-IC).

Digital detection of the effective value

Electronic devices such as an inverter distort the cur­rent waveform. Mitsubishi’s PSS electronic breakers
are designed to detect digitally the effective value of
the current to minimize over-current tripping errors.
This enables fine protection for the system.

I : Instantaneous

Power-source side terminal

Breaking mechanism

CT

CT

CT

CT

Load-side
terminal

Load-current indication LED (70%)

PSS

Rectifying circuit

WDT

Test input

Trip coil

Custom IC

Microcomputer

I

CV

A/D

convertor

CPU

Characteristics
setting part

SSW
LSW

PSW

Input and
output

circuit
CV : Constant

voltage
circuit

Phase­selection
sampling
circuit

Short
time-delay
soft ware

Trigger circuit

Over-current
indication LED

Pre-alarm
indication LED

Pre-alarm
output

LSW : Long
time-delay
soft ware

PSW : Pre-alarm
soft ware

WDT : Watch-dog
timer
circuit

Processing of the digital ETR

Standard equipped pre-alarm system

Mitsubishi’s PSS electronic breakers have a pre-alarm
system as a standard. When the load current exceeds
the set pre-alarm current, the breaker lights up an LED
and outputs a pre-alarm signal.

4

1×10

3

1×10

I

P

2

1×10

Pre-alarm

10

current

Time (s)

0.1

0.01

1

Load current

4

I

r

10

Current setting

T

L

I

s

T

s

Current (A)

High-voltage fuse­Allowable short-time
characteristics

Long time-delay
operating time

Short time-delay
tripping current

Short time-delay
operating time

Instantaneous
tripping current

I

i

2

10

Current-Converted value
on the high-voltage side

Switch
with fuse

High
voltage

Low

voltage

Transformer
MCCB

(electronic)

3

10

M

Load

2.3 Equipment of High Technology

Series

NF-S

NF-C

NF-U

Type

NF30-SP
NF50-HP
NF50-HRP
NF60-HP
NF100-SP
NF100-HP
NF100-SEP
NF100-HEP
NF160-SP
NF160-HP
NF250-SP
NF250-HP
NF250-SEP
NF250-HEP
NF400-SP
NF400-SEP
NF400-HEP
NF400-REP
NF630-SP
NF630-SEP
NF630-HEP
NF630-REP
NF800-SEP
NF800-HEP
NF800-REP
NF1000-SS
NF1250-SS
NF1600-SS
NF50-CP
NF60-CP
NF100-CP
NF250-CP
NF400-CP
NF630-CP
NF800-CEP
NF100-RP
NF100-UP
NF225-RP
NF225-UP
NF400-UEP
NF630-UEP
NF800-UEP
NF1250-UR

ISTAC

●

●

●

●

●

●

●

●

●

●

●

●

Advanced Technology

VJC

Digital-ETR

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

Analog-ETR

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

5

3. CONSTRUCTION AND OPERATION

3.1 General

The primary components are: a switching mechanism,
an automatic tripping device (and manual trip button),
contacts, an arc-extinguishing device, terminals and
a molded case.

Arc-Extinguishing Device

Mitsubishi MCCBs feature excel­lent arc-extinguishing perfor­mance by virtue of the optimum
combination of grid gap, shape,
and material.

Magnetic flux

Grid

Magnetic
force

Arc extinction

Switching Mechanism

The contacts open and close rap­idly, regardless of the moving
speed of the handle, minimizing
contact wear and ensuring safety.

Rapid

movement

Arc

Contact

Link-mechanism
operation

Molded case
(Base)

Terminal

Molded case
(Cover)

Automatic tripping device

Handle

1. Trip indication
The automatically tripped condi­tion is indicated by the handle in
the center position between ON
and OFF, the yellow (or white)
line cannot be seen in this posi­tion.

2. Resetting
Resetting after tripping is per­formed by first moving the han­dle to the OFF position to en­gage the mechanism, then re­turning the handle to ON to re­close the circuit.

3. Trip-Free
Even if the handle is held at
ON, the breaker will trip if an
overcurrent flows.

Trip Button (Push to Trip)

Enables tripping mechanically
from outside, for confirming the
operation of the accessory switch­es and the manual resetting func­tion.

6

Fig. 3.1 Type NF100-SP Construction

ON

ON OFF Trip

4. Contact On Mechanism
Even in the worst case in which
welding occurs owing to an
overcurrent, the breaker will trip
and the handle will maintain to
ON, indicating the energizing
state.

OFF

Handle indication

OFF

ON

3.2 Switching Mechanism

Spring tension line

Toggle link

Cradle

Bracket

Spring

a) On

b) Off

c) Tripped

ON to OFF dead-point line

OFF to ON dead-point line

Handle centered; indicates
tripped condition

The ON, OFF and TRIPPED conditions are shown in
Fig. 3.2. In passing from ON to OFF, the handle ten­sion spring passes through alignment with the toggle
link (“dead point” condition). In so doing, a positive,
rapid contact-opening action is produced; the OFF to
ON contact closing acts in a similar way (“quick make”
and “quick break” actions). In both cases the action of
the contacts is always rapid and positive, and inde­pendent of the human element – i.e., the force or
speed of the handle.

In auto tripping a rotation of the bracket releases
the cradle and operates the toggle link to produce the
contact-opening action described above. In the tripped
condition the handle assumes the center position be­tween on and off, providing a visual indication of the
tripped condition. Also, auto trip is “trip free,” so that
the handle cannot be used to hold the breaker in the
ON condition. The protective contact-opening func­tion cannot be defeated.

In multipole breakers the poles are separated by
integral barriers in the molded case. The moving con­tacts of the poles are attached to the central toggle
link by a common-trip bar, however, so that tripping,
opening and closing of all poles is always simulta­neous. This is the “common trip” feature, by which
single phasing and similar unbalance malfunctions are
effectively prevented.

Fig. 3.2 Switching Mechanism Action

3.3 Automatic Tripping Device

There are three types of device, the thermal-magnetic
type, the hydraulic-magnetic type and the electronic
trip relay type.

7

■Automatic Tripping Devices

●Thermal-Magnetic Type (100~630A Frame)

Bimetal

Heater

●Thermal-Magnetic Type (1000~4000A Frame)

Latch

Bimetal

Armature

Heater

Trip bar

Latch

Trip bar

Armature
Stationary core

1. Time-Delay Operation
An overcurrent heats and warps the bi­metal to actuate the trip bar.

2. Instantaneous Operation
If the overcurrent is excessive, the
amature is attracted and the trip bar ac­tuated.

Fig. 3.3

1. Time-Delay Operation
An overcurrent heats and warps the bi­metal to actuate the trip bar.

2. Instantaneous Operation
If the overcurrent is excessive, magneti­zation of the stationary core is strong
enough to attract the armature and ac­tuate the trip bar.

Fig. 3.4

●Hydraulic-Magnetic Type (30~60A Frame)

Armature

Trip bar

Pipe

Pole piece

Damping spring
Coil

Silicon oil

Moving core

Fig. 3.5

●Principle of Electronic Trip Relay (ETR) Operation

(100~800A Frame)

Power-source side terminal

Breaking mechanism

Custom IC

CT

CT

CT

Load-side
terminal

CT

CV

PSS

Rectifying circuit

WDT

Test input

Load-current indication LED (70%)

Trip coil

Microcomputer

I

A/D

convertor

CPU

Characteristics
setting part

SSW
LSW

PSW

Input and
output

Trigger circuit

Over-current
indication LED

Pre-alarm
indication LED

Pre-alarm
output

(1000~1600A Frame)

Power-supply side terminal

Switching mechanism

Trip coil

Special IC

Peak
conversion

Rectifier circuit

Test
terminals

and
largest-phase
selection

Effective value
conversion
and
largest-phase
selection

Test-signal
generator
circuit

Overcurrent display
LED

Fig. 3.6

CT

Load-side
terminal

CT

CT

Instan­taneous
circuit

Short­delay
circuit

Long­delay
circuit

1. Time-Delay Operation
At an overcurrent flow, the magnetic
force of the coil overcomes the spring,
the core closes to the pole piece, attracts
the armature, and actuates the trip bar.
The delay is obtained by the viscosity of
silicon oil.

2. Instantaneous Operation
If the overcurrent is excessive, the ar­mature is instantly attracted, without the
influence of the moving core.

1. The current flowing in each phase is
monitored by a current transformer (CT).

2. Each phase of the transformed current
undergoes full-phase rectification in the
rectifier circuit.

3. After rectification, each of the currents
are converted by a peak-conversion and
an effective-value conversion circuit.

4. The largest phase is selected from the

Trigger circuit

converted currents.

5. Each time-delay circuit generates a time
delay corresponding to the largest
phase.

6. The trigger circuit outputs a trigger sig­nal.

7. The trip coil is excited, operating the
switching mechanism.

8

Table 3.1 Comparison of Thermal-Magnetic, Hydraulic-Magnetic and Electronic Types

Item

Operating current is affected by ambient
temperature (bimetal responds to absolute
temperature not temperature rise).

Ambient
temperature

Thermal-magnetic type

Low temperature

Standard temperature

Hydraulic-magnetic type

Affected only to the extent that the damp­ing-oil viscosity is affected.

Low temperature

Negligible effect

Electronic type

Frequency

Distorted
wave

Operating time

High temperature

Current

Negligible effect up to several hundred Hz;
above that the instantaneous trip is affec­ted due to increased iron losses.

Low frequency

High frequency

Operating time

Current

Negligible effect up to 630A;
Above that operating current decreases
due to increase of a fever.

Above 700A

Operating time

Current

Negligible effect.

Operating time

High temperature

Current

Trip current increases with frequency, due
to increased iron losses.

High frequency

Low frequency

Operating time

Current

IF distortion is big, minimum operating cur­rent increases.

Small current width

Current width

Operating time

Current

Mounting attitude changes the effective
weight of the magnetic core.

Operating time

Current

Tripping current of some types decrease
due to CT or condition of operating circuit
with high frequency, and others increase.

Operating time

Current

For peak value detection, operating current
drops.

Peak value
detection

Operating time

Current

Negligible effect

Mounting
attitude

Flexibility
of operating
characteristics

Flexibility of
rated current

Operating time

Current

Bimetal must provide adequate deflection
force and desired temperature characteris­tic. Operating time range is limited.

Operating time

Current

Units for small rated currents are physically
impractical.

Horizontal

ON OFF

Current

OFF

ON

Ceiling

Operating time

Oil viscosity, cylinder, core and spring de­sign, etc., allow a wide choice of operating
times.

Operating time

Current

Coil winding can easily be designed to suit
any ampere rating.

Operating time

Current

Operating time can be easily shortened.
To lengthen operating time is not.

Operating time

Current

Within the range of 50(60)~100% of rated
current, any ampere rating are practical.
Also, to lower the value of short-time delay
or instantaneous trip can be easily done
comparatively.

9

3.4 Contacts

A pair of contacts comprises a moving contact and a
fixed contact. The instants of opening and closing
impose the most severe duty. Contact materials must
be selected with consideration to three major criteria:

1. Minimum contact resistance

2. Maximum resistance to wear

3. Maximum resistance to welding
Silver or silver-alloy contacts are low in resistance,

but wear rather easily. Tungsten, or majority-tungsten
alloys are strong against wear due to arcing, but rather
high in contact resistance. Where feasible, 60%+ sil­ver alloy (with tungsten carbide) is used for contacts
primarily intended for current carrying, and 60%+ tung­sten alloy (with silver) is used for contacts primarily
intended for arc interruption. Large-capacity MCCBs
employ this arrangement, having multicontact pairs,
with the current-carrying and arc-interruption duties
separated.

3.5 Arc-Extinguishing Device

Arcing, an inevitable aspect of current interruption,
must be extinguished rapidly and effectively, in nor­mal switching as well as protective tripping, to mini­mize deterioration of contacts and adjacent insulat­ing materials. In Mitsubishi MCCBs a simple, reliable,
and highly effective “de-ion arc extinguisher,” consist­ing of shaped magnetic plates (grids) spaced apart in
an insulating supporting frame, is used (Fig. 3.7). The
arc (ionized-path current) induces a flux in the grids
that attracts the arc, which tends to “lie down” on the
grids, breaking up into a series of smaller arcs, and
also being cooled by the grid heat conduction. The
arc (being effectively longer) thus requires far more
voltage to sustain it, and (being cooler) tends to lose
ionization and extinguish. If these two effects do not
extinguish the arc, as in a very large fault, the elevated
temperature of the insulating frame will cause gas­sing-out of the frame material, to de-ionize the arc.
Ac arcs are generally faster extinguishing due to the
zero-voltage point at each half cycle.

3.8 Trip Button

This is a pushbutton for external, mechanical tripping
of the MCCB locally, without operating the external­accessory shunt trip or undervoltage trip, etc. It en­ables easy checking of breaker resetting, control-cir­cuit devices associated with alarm contacts, etc., and
resetting by external handle.

Supporting
frame

Grids

Fig. 3.7 The De-Ion Arc Extinguisher

Induced flux

Grid

Attraction force

Fig. 3.8 The Induced-Flux Effect

Arc

3.6 Molded Case

The integral molded cases used in Mitsubishi MCCBs
are constructed of the polyester resin containing glass
fibers, the phenolic resin or glass reinforced nylon.
They are designed to be suitably arc-, heat- and gas­resistant, and to provide the necessary insulating
spacings and barriers, as well as the physical strength
required for the purpose.

3.7 Terminals

These are constructed to assure electrical efficiency
and reliability, with minimized possibility of localized
heating. A wide variety of types are available for ease
of mounting and connection. Compression-bonded
types and bar types are most commonly used.

10

Rated current

(A)

30 or less

31~63

64~100

101~250
251~400
401~630

631~800

801~1000

1001~1250
1251~1600

1601~2000
2001~4000

Tripping time

(minutes, max.)

200%

8.5
4

8.5
8

10
12

14
16
18

20
22
24

130%

60
60

120
120

120
120

120
120
120

120
120
120

120
120
120

120
120
120

120
120
120

60
60

120

105%

Non-Tripping time

(minutes, max.)

4. CHARACTERISTICS AND PERFORMANCE

4.1 Overcurrent-Trip Characteristics (Delay

Tripping)

Tripping times for overcurrents of 130 and 200% of
rated current are given in Table 4.1, assuming ambi­ent temperatures of 40°C, a typical condition inside
of panelboards. The figures reflect all poles tested to­gether for 130% tripping, and 105% non-tripping.
Within the range of the long-delay-element (thermal
or hydraulic) operation, tripping times are substan­tially linear, in inverse relationship to overcurrent mag­nitude.

The tripping times are established to prevent ex­cessive conductor-temperature rise; although times
may vary among MCCBs of different makers, the lower
limit is restricted by the demands of typical loads: tung­sten-lamp inrush, starting motor, mercury-arc lamps,
etc. The tripping characteristics of Mitsubishi MCCBs
are established to best ensure protection against ab­normal currents, while avoiding nuisance tripping.

4.1.1 Ambient Temperature and Thermal Tripping

Fig. 4.1 is a typical ambient compensation curve
(curves differ according to types and ratings), show­ing that an MCCB rated for 40°C ambient use must
be derated to 90% if used in a 50°C environment. In
an overcurrent condition, for the specified tripping time,
tripping would occur at 180% rated current, not 200%.
At 25°C, for the same tripping time, tripping would
occur at 216%, not 200%.

4.1.2 Hot-State Tripping

The tripping characteristics described above reflect
“cold-state tripping” – i.e., overloads increased from
zero – and the MCCB stabilized at rated ambient. This
is a practical parameter for most uses, but in intermit­tent operations, such as resistance welding, motor
pulsing, etc., the “hot state” tripping characteristic must
be specified, since over-loads are most likely to oc­cur with the MCCB in a heated state, while a certain
load current is already flowing.

Where the MCCB is assumed to be at 50% of rat­ing when the overload occurs, the parameter is called
the 50% hot-state characteristic; if no percentage is
specified, 100% is assumed. Hot-state ratings of 50%
and 75% are common.

4.2 Short-Circuit Trip Characteristics (In-

stantaneous Tripping)

For Mitsubishi MCCBs with thermal-magnetic trip units
the instantaneous-trip current can be specified inde­pendently of the delay characteristic, and in many
cases this parameter is adjustable offering consider­able advantage in coordination with other protection
and control devices. For example, in coordination with
motor starters, it is important to set the MCCB instan­taneous-trip element at a lower value than the fusing
(destruction) current of the thermal overload relay

(OLR) of the starter.

For selective tripping, it must be remembered that
even though the branch-MCCB tripping time may be
shorter than the total tripping time of the main MCCB,
in a fault condition the latter may also be tripped be­cause its latching curve overlaps the tripping curve of
the former. The necessary data for establishing the
required compatibility is provided in the Mitsubishi
MCCB sales catalogues.

The total clearing time for the “instantaneous” trip­ping feature is shown in Fig. 4.3; actual values differ
for each MCCB type.

Table 4.1 Overcurrent Tripping Times

120

110

108

100

20 25

% rating compensation

Fig. 4.1 Typical Temperature-Compensation Curve

Hot state

Operating time

Fig. 4.2 Hot-State-Tripping Curve

40

30

90

80

Ambient temperature (:)

Cold state



Current

50 60

11

Total
clearing
time

Fig. 4.3 Instantaneous Tripping Sequence

Latching
(relay)
time

Electromagnet
oparating
time

Floor-mounted

Mechanical
delay
time

Time for
contacts to
open

Arcing
time

Arc­extinguishing
time

4.3 Effects of Mounting Attitudes

Instantaneous tripping is negligibly affected by mount­ing attitude, for all types of MCCB. Delay tripping is
also negligibly affected in the thermal types, but in
the hydraulic-magnetic types the core-weight effect
becomes a factor. Fig. 4.4 shows the effect, for verti­cal-surface mounting and for two styles of horizontal­surface mounting.

(vertical plane)

100%

Wall-mounted
(horiz. or vert. attitude)

Ceiling-mounted

Tripping time

Overcurrent

Fig. 4.4 Effect of Mounting Attitude on the Hydraulic-

Magnetic MCCB Tripping Curves

Fig. 4.5 Effects of Nonvertical-Plane Mounting on Current

4.4 DC Tripping Characteristics of AC-Rated MCCBs

Table 4.2 DC Tripping Characteristics

Trip unit

Thermal
magnetic

Hydraulic
magnetic

Long delay

No effect below 630A
frame. Above this, AC
types cannot be used
for DC.

DC minimum-trip
values are 110~140%
of AC values.

Instantaneous

DC inst.-trip current is
approx. 130% of AC
value.

107%

107%

Rating

ON

ON

ON

ON

ON

ON

100%

93%

ON

ON

93%

Tripping curve

AC

Tripping time

Tripping time

DC

Overcurrent

AC

DC

Overcurrent

90%110%

4.5 Frequency Characteristics

At commercial frequencies the characteristics of
Mitsubishi MCCBs of below 630A frame size are vir­tually constant at both 50Hz and 60Hz (except for the
E Line models, the characteristics of MCCBs of 2000A
frame and above vary due to the CT used with the
delay element).

At high frequencies (e.g., 400Hz), both the current
capacity and delay tripping curves will be reduced by
skin effect and increased iron losses.

Performance reduction will differ for different types;
at 400Hz it will become 80% of the rating in breakers
of maximum rated current for the frame size, and 90%

12

of the rating in breakers of half of the maximum rating
for the frame size.

The instantaneous trip current will gradually in­crease with frequency, due to reverse excitation by
eddy currents. The rise rate is not consistent, but
around 400Hz it becomes about twice the value at
60Hz. Mitsubishi makes available MCCBs especially
designed for 400Hz use. Apart from operating char­acteristics they are identical to standard MCCBs (S
Line).

4.6 Switching Characteristics

Frame size
100 or less

225

400, 630

800
1000~2000
2500, 3000
3200, 4000

Operations per hour

120
120

60
20
20
10
10

Number of operations

Without current

8500
7000
4000
2500
2500
1500
1500

With current

1500
1000
1000

500
500
500
500

Total

10000

8000
5000
3000
3000
2000
2000

The MCCB, specifically designed for protective inter­ruption rather than switching, and requiring high-con­tact pressure and efficient arc-extinguishing capabil­ity, is expected to demonstrate inferior capability to
that of a magnetic switch in terms of the number of
operations per minute and operation life span. The
specifications given in Table 4.3 are applicable where
the MCCB is used as a switch for making and break-

Table 4.3 MCCB Switching Endurance

ing rated current.

Electrical tripping endurance in MCCBs with shunt
or undervoltage tripping devices is specified as 10%
of the mechanical-endurance number of operations
quoted in IEC standards.

Shunt tripping or undervoltage tripping devices are
intended as an emergency trip provision and should
not be used for normal circuit-interruption purposes.

4.7 Dielectric Strength

In addition to the requirements of the various interna­tional standards, Mitsubishi MCCBs also have the
impulse-voltage withstand capabilities given below
(Table 4.4). The impulse voltage is defined as sub-

Table 4.4 MCCB Impulse Withstand Voltage (Uimp)

Type

NF

Line

MB

MB30-CS
MB30-SP MB50-CP MB50-SP

MB100-SP MB225-SP

NF30-SP NF50-HP NF60-HP
NF50-HRP NF100-SP NF100-HP NF100-SEP NF100-HEP
NF160-SP NF160-HP NF250-SP NF250-HP NF250-SEP NF250-HEP

S

NF400-SP NF400-SEP NF400-HEP NF400-REP NF630-SP NF630-SEP
NF630-HEP NF630-REP NF800-SEP NF800-HEP NF800-REP

NF800-REP NF1000-SS NF1250-SS NF1600-SS
NF30-CS
NF50-CP NF60-CP NF100-CP NF250-CP

C

NF400-CP NF630-CP NF800-CEP
NF100-RP NF100-UP NF225-RP NF225-UP

U

NF400-UEP NF630-UEP NF800-UEP

stantially square-wave, with a crest length of

0.5~1.5µsec and a tail-length of 32~48µsec. The volt­age is applied between line and load terminals (MCCB
off), and between live parts and ground (MCCB on).

Impulse-voltage (kA)

4
6

6

8
4

6
8
6
8

13

5. CIRCUIT BREAKER SELECTION

5.1 Circuit Breaker Selection Table

Following Table shows various characteristics of each breaker to consider selection and coordination with
upstream devices or loads.

Characteristics

Standard : Standard characteristics MCCBs
Low-inst : Low-inst. MCCBs for Discrimination

When a power fuse (PF) is used as a high-voltage protector, it must be coordinated
with an MCCBs on the secondary side.

PF short-time tolerancs
capacity

Pf.

MCCB
operating
characteristic
curve

Tr.

MCCB1

MCCB2

Generator: Generator-Protection MCCBs

These MCCBs have long-time-delay operation shorter than standard type and low
instantaneous operation.

Mag-Only : Magnetic trip only MCCBs

These are standard MCCBs minus the thermal tripping device. They have no time­delay tripping characteristic, providing protection only against large-magnitude short­circuit faults.

Low-inst.MCCBs

Time

Current

14

CIRCUIT BREAKER SELECTION TABLE

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

690V
500V
440V
400V
230V

Hydraulic-magnetic
Fixed ampere rating and
instantaneous

NF30-CS

3, 5, 10, 15, 20, 30

500

–
–

1.5/1.5 (415V)

1.5/1.5 (380V)

2.5/2 (240V)
23

339± 17

566± 28
10 132 ± 57
15 198 ± 86
20 265 ± 115
30 397 ± 172

30

NF30-SP

3, 5, 10, 15, 20, 30

600

–

2.5/1

2.5/1
5/2
5/2

23

Hydraulic-magnetic
Fixed ampere rating and
instantaneous

333± 10

555± 17
10 110 ± 35
15 165 ± 52
20 220 ± 70
30 330 ± 105

50

NF50-CP

10, 15, 20, 30, 40, 50

600

–

2.5/1

2.5/1
5/2
5/2

23

Hydraulic-magnetic
Fixed ampere rating and
instantaneous

10 110 ± 35
15 165 ± 52
20 220 ± 70
30 330 ± 105
40 440 ± 140
50 550 ± 175

Low-inst

Generator

Mag-Only

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

–

–

–

–

–

–

–

Magnetic

–

–

Fixed ampere rating
instantaneous

330± 6

550± 10
10 100 ± 20
15 150 ± 30
20 200 ± 40
30 300 ± 60

–

–

–

–

–

–

23

Magnetic
Fixed ampere rating
instantaneous

10 100 ± 20
15 150 ± 30
20 200 ± 40
30 300 ± 60
40 400 ± 80
50 500 ± 100

–

–

–

–

–

–

23

15

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

690V
500V
440V
400V
230V

50

NF50-HP

10, 15, 20, 30, 40, 50

600

–

7.5/4
10/5
10/5

25/13

234

Hydraulic-magnetic
Fixed ampere rating and
instantaneous

10 110 ± 35
15 165 ± 52
20 220 ± 70
30 330 ± 105
40 440 ± 140
50 550 ± 175

60

NF60-CP

10, 15, 20, 30, 40, 50, 60

600

–

2.5/1

2.5/1
5/2
5/2

23

Hydraulic-magnetic
Fixed ampere rating and
instantaneous

10 110 ± 35
15 165 ± 52
20 220 ± 70
30 330 ± 105
40 440 ± 140
50 550 ± 175
60 660 ± 210

Low-inst

Generator

Mag-Only

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

–

–

–

–

–

–

234

Magnetic
Fixed ampere rating and
instantaneous

10 100 ± 20
15 150 ± 30
20 200 ± 40
30 300 ± 60
40 400 ± 80
50 500 ± 100

–

–

–

–

–

–

23

Magnetic
Fixed ampere rating and
instantaneous

10 100 ± 20
15 150 ± 30
20 200 ± 40
30 300 ± 60
40 400 ± 80
50 500 ± 100
60 600 ± 120

16

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Low-inst

Generator

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

690V
500V
440V
400V
230V

Hydraulic-magnetic
Fixed ampere rating and
instantaneous

60

NF60-HP

10, 15, 20, 30, 40, 50, 60

600

–

7.5/4
10/5
10/5

25/13

234

10 110 ± 35
15 165 ± 52
20 220 ± 70
30 330 ± 105
40 440 ± 140
50 550 ± 175
60 660 ± 210

–

–

–

–

–

100

NF100-CP NF100-SP

50, 60, 75, 100

600

–

7.5/4
10/5
10/5

25/13

23

Thermal, magnetic
Fixed ampere rating and
instantaneous

50 750 ± 150
60 900 ± 180
75 1125 ± 225

100 1500 ± 300

23

Thermal, magnetic
Fixed ampere rating and
instantaneous

50 300 ± 60
60 360 ± 72
75 450 ± 90

100 600 ± 120

–

–

15, 20, 30, 40, 50, 60, 75, 100

Thermal, magnetic
Fixed ampere rating and
instantaneous

100 1500 ± 300

Thermal, magnetic
Fixed ampere rating and
instantaneous

100 600 ± 120

690

–

15/8
25/13
30/15
50/25

234

15 225 ± 45
20 300 ± 60
30 450 ± 90
40 600 ± 120
50 750 ± 150
60 900 ± 180
75 1125 ± 225

234

15 90 ± 18
20 120 ± 24
30 180 ± 36
40 240 ± 48
50 300 ± 60
60 360 ± 72
75 450 ± 90

–

–

Mag-Only

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

–

234 2 3

Magnetic
Fixed ampere rating and
instantaneous

10 100 ± 20
15 150 ± 30
20 200 ± 40
30 300 ± 60
40 400 ± 80
50 500 ± 100
60 600 ± 120

Magnetic
Fixed ampere rating and
instantaneous

100 1000 ± 200

–

50 500 ± 100
60 600 ± 120
75 750 ± 150

–

234

Magnetic
Fixed ampere rating and
instantaneous

15 150 ± 30
20 200 ± 40
30 300 ± 60
40 400 ± 80
50 500 ± 100
60 600 ± 120
75 750 ± 150

100 1000 ± 200

17

Frame (A) 100

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

690V
500V
440V
400V
230V

Thermal, magnetic
Adjustable ampere rating
and fixed instantaneous

NF100-CP T/A

15 ~ 20, 20 ~ 25, 25 ~ 40
40 ~ 63, 63 ~ 80, 80 ~ 100

600

–

7.5/4
10/5
10/5

25/13

23

15 ~ 20 225 ± 45
20 ~ 25 300 ± 60
25 ~ 40 375 ± 75
40 ~ 63 600 ± 120
63 ~ 80 945 ± 189
80 ~ 100 1200 ± 240

50

NF50-HRP

15, 20, 30, 40, 50

690

2.5/1
20/10
30/15
30/15
85/43

23

Thermal, magnetic
Fixed ampere rating and
instantaneous

15 225 ± 45
20 300 ± 60
30 450 ± 90
40 600 ± 120
50 750 ± 150

100

NF100-SP T/A

15 ~ 20, 20 ~ 25, 25 ~ 40
40 ~ 63, 63 ~ 80, 80 ~ 100

690

–

15/8
25/13
30/15
50/25

234

Thermal, magnetic
Adjustable ampere rating
and fixed instantaneous

15 ~ 20 225 ± 45
20 ~ 25 300 ± 60
25 ~ 40 375 ± 75
40 ~ 63 600 ± 120
63 ~ 80 945 ± 189
80 ~ 100 1200 ± 240

Low-inst

Generator

Mag-Only

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

–

–

–

–

–

–

–

–

–

Magnetic
Fixed ampere rating and
instantaneous

15 150 ± 30
20 200 ± 40
30 300 ± 60
40 400 ± 80
50 500 ± 100

–

–

–

–

–

–

23

–

–

–

–

–

–

–

–

–

18

Frame (A)

Type

Rated current In (A)

15, 20, 30, 40, 50, 60, 75, 100

100

NF100-HP T/ANF100-HP

15 ~ 20, 20 ~ 25, 25 ~ 40
40 ~ 63, 63 ~ 80, 80 ~ 100

NF100-RP

15, 20, 30, 40, 50, 60, 75, 100

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Low-inst

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

690V
500V
440V
400V
230V

690

5/3
30/15
50/25
50/25

100/50

234

Thermal, magnetic
Fixed ampere rating and
instantaneous

15 225 ± 45
20 300 ± 60
30 450 ± 90
40 600 ± 120
50 750 ± 150
60 900 ± 180
75 1125 ± 225

100 1500 ± 300

–

–

690

5/3
30/15
50/25
50/25

100/50

234

Thermal, magnetic
Adjustable ampere rating
and fixed instantaneous

15 ~ 20 225 ± 45
20 ~ 25 300 ± 60
25 ~ 40 375 ± 75
40 ~ 63 600 ± 120
63 ~ 80 945 ± 189
80 ~ 100 1200 ± 240

–

–

690

–

42/42
125/125
125/125
125/125

23

Thermal, magnetic
Fixed ampere rating and
instantaneous

15 225 ± 45
20 300 ± 60
30 450 ± 90
40 600 ± 120
50 750 ± 150
60 900 ± 180
75 1125 ± 225

100 1500 ± 300

–

–

Generator

Mag-Only

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

–

–

–

–

234

Magnetic
Fixed ampere rating and
instantaneous

15 150 ± 30
20 200 ± 40
30 300 ± 60
40 400 ± 80
50 500 ± 100
60 600 ± 120
75 750 ± 150

100 1000 ± 200

–

–

–

–

–

–

–

–

–

–

–

–

–

–

19

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Low-inst

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device

690V
500V
440V
400V
230V

Thermal, magnetic
Fixed ampere rating and
instantaneous

NF100-UP NF100-SEP

15, 20, 30, 40, 50, 60, 75, 100

690

10/5
200/200
200/200
200/200
200/200

234

15 225 ± 45
20 300 ± 60
30 450 ± 90
40 600 ± 120
50 750 ± 150
60 900 ± 180
75 1125 ± 225

100 1500 ± 300

–

–

15 ~ 20, 30 ~ 50, 60 ~ 100

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up, and instantaneous

Short time delay pick up current
Variation is within ±15% of
setting current

15 30-37.5-45-52.5-60-75­20 40-50-60-70-80-100-120­30 60-75-90-105-120-150­40 80-100-120-140-160­50 100-125-150-175-200­60 120-150-180-210-240­75 150-187.5-225-262.5-300-

100 200-250-300-350-400-

Instantaneous pick up current
Variation is within ±15% of
setting current

30 ~ 50 200 ~ 800
60 ~ 100 400 ~ 1600

100

690

–

15/8
25/13
30/15
50/25

34

2 to 10 Ir
90-105-120-150
140-160-200
180-210-240-300
200-240-280-320-400
250-300-350-400-500
300-360-420-480-600
375-450-525-600-750
500-600-700-800-1000

4 In ~ 16 In

–

–

NF100-HEP

15 ~ 20, 30 ~ 50, 60 ~ 100

690

5/3
30/15
50/25
50/25

100/50

34

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up, and instantaneous

Short time delay pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

15 30-37.5-45-52.5-60-75-

90-105-120-150

20 40-50-60-70-80-100-120-

140-160-200

30 60-75-90-105-120-150-

180-210-240-300

40 80-100-120-140-160-

200-240-280-320-400

50 100-125-150-175-200-

250-300-350-400-500

60 120-150-180-210-240-

300-360-420-480-600

75 150-187.5-225-262.5-300-

375-450-525-600-750

100 200-250-300-350-400-

500-600-700-800-1000
Instantaneous pick up current
Variation is within ±15% of
setting current

4 In ~ 16 In
30 ~ 50 200 ~ 800
60 ~ 100 400 ~ 1600

–

–

Generator

Mag-Only

20

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

–

–

Electronic trip relay

–

–

–

–

–

Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up, and instantaneous

Rating: 15 ~ 20A, 30 ~ 50A,
Inst. : Operating characteristics

–

3

60 ~ 100A
must be adjusted as

follows.
STD q 3 (Is setting)
LTD : minimum setting

L = 12sec setting)

(T

–

–

–

–

3

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up, and instantaneous

Rating: 15 ~ 20A, 30 ~ 50A,

60 ~ 100A

Inst. : Operating characteristics

must be adjusted as
follows.
STD q 3 (Is setting)
LTD : minimum setting

L = 12sec setting)

(T

–

–

–

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

690V
500V
440V
400V
230V

Thermal, magnetic
Fixed ampere rating and
instantaneous

NF160-SP

125, 150, 160

690

–

15/8
25/13
30/15
50/25

234

125 1750 ± 350
150 2100 ± 420
160 2240 ± 448

160

NF160-SP T/A

100 ~ 125, 125 ~ 160

690

–

15/8
25/13
30/15
50/25

234

Thermal, magnetic
Adjustable ampere rating and
fixed instantaneous

100 ~125 1400 ± 280
125 ~ 160 1400 ± 280

NF160-HP

125, 150, 160

690

5/3

30/8
50/13
50/13

100/25

234

Thermal, magnetic
Fixed ampere rating and
instantaneous

125 1750 ± 350
150 2100 ± 420
160 2240 ± 448

Low-inst

Generator

Mag-Only

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

–

–

–

–

–

–

234 234

Magnetic
Fixed ampere rating and
instantaneous

125 1250 ± 250
160 1600 ± 320

–

–

–

–

–

–

–

–

Magnetic
Fixed ampere rating and
instantaneous

125 1250 ± 250
160 1600 ± 320

–

–

–

–

–

–

–

21

Frame (A)

Type

Rated current In (A)

160

NF160-HP T/A

125, 150, 175, 200, 225, 250100 ~ 125, 125 ~ 160

NF250-CP

250

NF250-CP T/A

100 ~ 125, 125 ~ 160
150 ~ 200, 200 ~ 250

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Low-inst

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

690V
500V
440V
400V
230V

Thermal, magnetic
Adjustable ampere rating
and fixed instantaneous

690

5/3

30/8
50/13
50/13

100/25

234

100 ~ 125 1400 ± 280
125 ~ 160 1400 ± 280

–

–

–

600

–
10/5
15/8
18/9

30/15

23

Thermal, magnetic
Fixed ampere rating and
instantaneous

125 1750 ± 350
150 2100 ± 420
175 2450 ± 490
200 2800 ± 560
225 3150 ± 630
250 2500 ± 500

23

Thermal, magnetic
Fixed ampere rating and
instantaneous

6 In 4 In

125 750 ± 150 500 ± 100
150 900 ± 180 600 ± 120
175 1050 ± 210 700 ± 140
200 1200 ± 240 800 ± 160
225 1350 ± 270 900 ± 180
250 1500 ± 300 1000 ± 200

600

–
10/5
15/8
18/9

30/15

23

Thermal, magnetic
Adjustable ampere rating
and fixed instantaneous

100 ~ 125 1400 ± 280
125 ~160 1400 ± 280
150 ~200 2100 ± 420
200 ~250 2500 ± 500

–

–

–

Generator

Mag-Only

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

–

–

–

–

Magnetic

–

–

Fixed ampere rating and
instantaneous

125 1250 ± 250
150 1500 ± 300
175 1750 ± 350
200 2000 ± 400
225 2250 ± 450
250 2250 ± 450

–

–

–

23

–

–

–

–

–

–

22

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

690V
500V
440V
400V
230V

Thermal, magnetic
Fixed ampere rating and
instantaneous

NF250-SP

125, 150, 175, 200, 225, 250

690

–

15/8
25/13
30/15
50/25

234

125 1750 ± 350
150 2100 ± 420
175 2450 ± 490
200 2800 ± 560
225 3150 ± 630
250 2500 ± 500

250

NF250-SP T/A

100 ~ 125, 125 ~ 160
150 ~ 200, 200 ~ 250

690

–

–
25/20
30/22
50/42

234

Thermal, magnetic
Adjustable ampere rating
and fixed instantaneous

100 ~125 1400 ± 280
125 ~ 160 1400 ± 280
150 ~200 2100 ± 420
200 ~250 2500 ± 500

NF250-HP

125, 150, 175, 200, 225, 250

690

5/3

30/8
50/13
50/13

100/25

234

Thermal, magnetic
Fixed ampere rating and
instantaneous

125 1750 ± 350
150 2100 ± 420
175 2450 ± 490
200 2800 ± 560
225 3150 ± 630
250 2500 ± 500

Low-inst

Generator

Mag-Only

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

234

Thermal, magnetic
Fixed ampere rating and
instantaneous

6 In4 In
125 750 ± 150 500 ± 100
150 900 ± 180 600 ± 120
175 1050 ± 210 700 ± 140
200 1200 ± 240 800 ± 160
225 1350 ± 270 900 ± 180
250 1500 ± 300 1000 ± 200

–

–

–

234

Magnetic
Fixed ampere rating and
instantaneous

125 1250 ± 250
150 1500 ± 300
175 1750 ± 350
200 2000 ± 400
225 2250 ± 450
250 2250 ± 450

–

–

–

–

–

–

–

Magnetic

–

–

Fixed ampere rating and
instantaneous

125 1250 ± 250
150 1500 ± 300
175 1750 ± 350
200 2000 ± 400
225 2250 ± 450
250 2250 ± 450

–

–

–

–

–

–

234

23

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

690V
500V
440V
400V
230V

Thermal, magnetic
Adjustable ampere rating and
fixed instantaneous

250

100 ~ 125, 125 ~ 160
150 ~ 200, 200 ~ 250

690

5/3

30/8
50/13
50/13

100/25

234

100 ~125 1400 ± 280
125 ~160 1400 ± 280
150 ~200 2100 ± 420
200 ~250 2500 ± 500

NF225-RPNF250-HP T/A

125, 150, 175, 200, 225

690

–

42/42
125/125
125/125
125/125

23

Thermal, magnetic
Fixed ampere rating and
instantaneous

125 1750 ± 350
150 2100 ± 420
175 2450 ± 490
200 2800 ± 560
225 3150 ± 630

225

NF225-UP

125, 150, 175, 200, 225

690

10/5
200/200
200/200
200/200
200/200

234

Thermal, magnetic
Fixed ampere rating and
instantaneous

125 1750 ± 350
150 2100 ± 420
175 2450 ± 490
200 2800 ± 560
225 3150 ± 630

Low-inst

Generator

Mag-Only

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

24

Rating (A) and
Inst. (A)

–

–

–

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

690V
500V
440V
400V
230V

NF250-SEP

125-250

690

–

15/8
25/13
30/15
50/25

34

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up, and instantaneous

Short time delay pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

125 250-312.5-375-437.5-500-

625-750-875-1000-1250

150 300-375-450-525-600-

750-900-1050-1200-1500

175 350-437.5-525-612.5-700-

875-1050-1225-1400-1750

200 400-500-600-700-800-1000-

1200-1400-1600-2000

225 450-562.5-675-787.5-900-

1125-1350-1575-1800-2250

250 500-625-750-875-1000-

1250-1500-1750-2000-2500
Instantaneous pick up current
Variation is within ±15% of
setting current

4 ~ 14 In

125 ~ 250 1000 ~ 3500

250

NF250-HEP

125-250

690

5/3

30/8
50/13
50/13

100/25

34

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up, and instantaneous

Short time delay pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

125 250-312.5-375-437.5-500-

625-750-875-1000-1250

150 300-375-450-525-600-

750-900-1050-1200-1500

175 350-437.5-525-612.5-700-

875-1050-1225-1400-1750

200 400-500-600-700-800-1000-

1200-1400-1600-2000

225 450-562.5-675-787.5-900-

1125-1350-1575-1800-2250

250 500-625-750-875-1000-

1250-1500-1750-2000-2500
Instantaneous pick up current
Variation is within ±15% of
setting current

4 ~ 14 In

125 ~ 250 1000 ~ 3500

Low-inst

Generator

Mag-Only

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

–

–

–

3

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up, and instantaneous

Rating: 125 ~ 250A
Inst. : Operating characteristics

must be adjusted as
follows.
STD q 3 (Is setting)
LTD : minimum setting

L = 12sec setting)

(T

–

–

–

–

–

–

3

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up, and instantaneous

Rating: 125 ~ 250A
Inst. : Operating characteristics

must be adjusted as
follows.
STD q 3 (Is setting)
LTD : minimum setting

L = 12sec setting)

(T

–

–

–

25

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Number of poles
Automatic tripping

device

690V
500V
440V
400V
230V

Thermal, magnetic
Fixed ampere rating and
instantaneous

NF400-CP

250, 300, 350, 400

600

–

15/8
25/13
36/18
50/25

23

400A

NF400-SP

250, 300, 350, 400

690
10/10
30/30
42/42
45/45
85/85

234

Thermal, magnetic
Fixed ampere rating and
instantaneous

NF400-SEP

200 ~ 400

adjustable

690
10/10
30/30
42/42
45/45
85/85

34

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up and instantaneous

Low-inst

Generator

Mag-Only
(Inst trip only)

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

250 2500 ± 500
300 3000 ± 600
350 3500 ± 700
400 4000 ± 800

23

Thermal, magnetic
Fixed ampere rating and
instantaneous

6 In4 In
250 1500±300 1000±200
300 1800±360 1200±240
350 2100±420 1400±280
400 2400±480 1600±320

–

–

–

23

Magnetic
Fixed ampere rating and
instantaneous

250 2500 ± 500
300 3000 ± 600
350 3500 ± 700
400 4000 ± 800

250 3500 ± 700
300 4200 ± 840
350 4900 ± 980
400 5600 ± 1120

–

–

–

–

–

–

234

Magnetic
Fixed ampere rating and
instantaneous

250 2500 ± 500
300 3000 ± 600
350 3500 ± 700
400 4000 ± 800

Short time delay pick up current
Variation is within ±15% of
setting current

200 400-500-600-700-800-

225 450-562.5-675-787.5-

250 500-625-750-875-1000-

300 600-750-900-1050-1200-

350 700-875-1050-1225-

400 800-1000-1200-1400-

Instantaneous pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

1000-1200-1400-1600­2000

900-1125-1200-1500­1800-1350-1575-1800­2250

1250-1500-1750-2000­2500

1500-1800-2100-2400­3000

1400-1750-2100-2450­2800-3500

1600-2000-2400-2800­3200-4000

4 In ~ 16 I n
1600 ~ 6400

–

–

–

–

–

–

–

–

–

26

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Number of poles
Automatic tripping

device

690V
500V
440V
400V
230V

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up and instantaneous

400A

NF400-HEP NF400-REP

200 ~ 400

adjustable

690
10/10
50/50
65/65
70/70

100/100

34

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up and instantaneous

200 ~ 400

adjustable

690
15/10
70/35

125/63
125/63
150/75

3

NF400-UEP

200 ~ 400

adjustable

690

35/35
170/170
200/200
200/200
200/200

34

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up and instantaneous

Low-inst

Generator

Mag-Only
(Inst trip only)

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device
Rating (A) and

Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Short time delay pick up current
Variation is within ±15% of
setting current

200 400-500-600-700-800-

225 450-562.5-675-787.5-

250 500-625-750-875-1000-

300 600-750-900-1050-1200-

350 700-875-1050-1225-

400 800-1000-1200-1400-

Instantaneous pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

1000-1200-1400-1600­2000

900-1125-1200-1500­1800-1350-1575-1800­2250

1250-1500-1750-2000­2500

1500-1800-2100-2400­3000

1400-1750-2100-2450­2800-3500

1600-2000-2400-2800­3200-4000

4 In ~ 16 In
1600 ~ 6400

–

–

–

–

–

–

–

–

–

Short time delay pick up current
Variation is within ±15% of
setting current

200 400-500-600-700-800-

225 450-562.5-675-787.5-

250 500-625-750-875-1000-

300 600-750-900-1050-1200-

350 700-875-1050-1225-

400 800-1000-1200-1400-

Instantaneous pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

1000-1200-1400-1600­2000

900-1125-1200-1500­1800-1350-1575-1800­2250

1250-1500-1750-2000­2500

1500-1800-2100-2400­3000

1400-1750-2100-2450­2800-3500

1600-2000-2400-2800­3200-4000

4 In ~ 16 I n
1600 ~ 6400

–

–

–

–

–

–

–

–

–

Short time delay pick up current
Variation is within ±15% of
setting current

200 400-500-600-700-800-

225 450-562.5-675-787.5-

250 500-625-750-875-1000-

300 600-750-900-1050-1200-

350 700-875-1050-1225-

400 800-1000-1200-1400-

Instantaneous pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

1000-1200-1400-1600­2000

900-1125-1200-1500­1800-1350-1575-1800­2250

1250-1500-1750-2000­2500

1500-1800-2100-2400­3000

1400-1750-2100-2450­2800-3500

1600-2000-2400-2800­3200-4000

4 In ~ 16 I n
1600 ~ 6400

–

–

–

–

–

–

–

–

–

27

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Number of poles
Automatic tripping

device

690V
500V
440V
400V
230V

Thermal, magnetic
Fixed ampere rating and
instantaneous

630A

NF630-CP NF630-SP

500, 600, 630

600

–

18/9
36/18
36/18
50/25

23

500, 600, 630

690
10/10
30/30
42/42
45/45
85/85

234

Thermal, magnetic
adjustable ampere rating and
fixed instantaneous

NF630-SEP

300 ~ 630

adjustable

690
10/10
30/30
42/42
45/45
85/85

34

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up and instantaneous

Low-inst

Generator

Mag-Only
(Inst trip only)

28

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device
Rating (A) and

Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

500 5000 ± 1000
600 6000 ± 1200
630 6300 ± 1260

–

–

–

–

–

–

23

Magnetic
Fixed ampere rating and
instantaneous

500 5000 ± 1000
600 6000 ± 1200
630 6300 ± 1260

500 Lo 2500 ± 500

2 4000
3 5500

Hi 7000 ± 1400

600 Lo 3000 ± 600

2 4800
3 6600

Hi 8400 ± 1680

630 Lo 3150 ± 630

2 5040
3 6930

Hi 8820 ± 1764

–

–

–

–

–

–

234

Thermal, magnetic
adjustable ampere rating and
fixed instantaneous

500 Lo 2000 ± 400

2 3000
3 4000

Hi 5000 ± 1000

600 Lo 2400 ± 480

2 3600
3 4800

Hi 6000 ± 1200

630 Lo 2520 ± 504

2 3780
3 5040

Hi 6300 ± 1260

Short time delay pick up current
Variation is within ±15% of
setting current

300 600-750-900-1050-1200-

350 700-875-1050-1225-

400 800-1000-1200-1400-

500 1000-1250-1500-1750-

600 1200-1500-1800-2100-

630 1260-1575-1890-2205-

Instantaneous pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

1500-1800-2100-2400­3000

1400-1750-2100-2450­2800-3500

1600-2000-2400-2800­3200-4000

2000-2500-3000-3500­4000-5000

2400-3000-3600-4200­4800-6000

2520-3150-3780-4410­5040-6300

4 In ~ 15 I n
2520 ~ 9450

–

–

–

–

–

–

–

–

–

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Number of poles
Automatic tripping

device

690V
500V
440V
400V
230V

NF630-HEP

300 ~ 630

adjustable

690
15/15
50/50
65/65
70/70

100/100

34

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up and instantaneous

630A

NF630-REP

300 ~ 630

adjustable

690
20/15
70/35

125/63
125/63
150/75

3

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up and instantaneous

NF630-UEP

300 ~ 630

adjustable

690

35/35
170/170
200/200
200/200
200/200

34

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up and instantaneous

Low-inst

Generator

Mag-Only
(Inst trip only)

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device
Rating (A) and

Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Short time delay pick up current
Variation is within ±15% of
setting current

300 600-750-900-1050-1200-

350 700-875-1050-1225-

400 800-1000-1200-1400-

500 1000-1250-1500-1750-

600 1200-1500-1800-2100-

630 1260-1575-1890-2205-

Instantaneous pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

1500-1800-2100-2400­3000

1400-1750-2100-2450­2800-3500

1600-2000-2400-2800­3200-4000

2000-2500-3000-3500­4000-5000

2400-3000-3600-4200­4800-6000

2520-3150-3780-4410­5040-6300

4 In ~ 15 In
2520 ~ 9450

–

–

–

–

–

–

–

–

–

Short time delay pick up current
Variation is within ±15% of
setting current

300 600-750-900-1050-1200-

350 700-875-1050-1225-

400 800-1000-1200-1400-

500 1000-1250-1500-1750-

600 1200-1500-1800-2100-

630 1260-1575-1890-2205-

Instantaneous pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

1500-1800-2100-2400­3000

1400-1750-2100-2450­2800-3500

1600-2000-2400-2800­3200-4000

2000-2500-3000-3500­4000-5000

2400-3000-3600-4200­4800-6000

2520-3150-3780-4410­5040-6300

4 In ~ 15 In
2520 ~ 9450

–

–

–

–

–

–

–

–

–

Short time delay pick up current
Variation is within ±15% of
setting current

300 600-750-900-1050-1200-

350 700-875-1050-1225-

400 800-1000-1200-1400-

500 1000-1250-1500-1750-

600 1200-1500-1800-2100-

630 1260-1575-1890-2205-

Instantaneous pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

1500-1800-2100-2400­3000

1400-1750-2100-2450­2800-3500

1600-2000-2400-2800­3200-4000

2000-2500-3000-3500­4000-5000

2400-3000-3600-4200­4800-6000

2520-3150-3780-4410­5040-6300

4 In ~ 15 I n
2520 ~ 9450

–

–

–

–

–

–

–

–

–

29

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Number of poles
Automatic tripping

device

690V
500V
440V
400V
230V

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up and instantaneous

NF800-CEP

400 ~ 800

adjustable

600

–

18/9
36/18
36/18
50/25

3

800A

NF800-SEP

400 ~ 800

adjustable

690
10/10
30/30
42/42
45/45
85/85

34

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up and instantaneous

NF800-HEP

400 ~ 800

adjustable

690
15/15
50/50
65/65
70/70

100/100

34

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up and instantaneous

Low-inst

Generator

Mag-Only
(Inst trip only)

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device
Rating (A) and

Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Short time delay pick up current
Variation is within ±15% of
setting current

400 800-1000-1200-1400-

450 900-1150-1350-1575-

500 1000-1250-1500-1750-

600 1200-1500-1800-2100-

700 1400-1750-2100-2450-

800 1260-1575-1890-2205-

Instantaneous pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

1600-2000-2400-2800­3200-4000

1800-2250-2700-3150­3600-4500

2000-2500-3000-3500­4000-5000

2400-3000-3600-4200­4800-6000

2800-3500-4200-4900­5600-6300

2520-3150-3780-4410­5040-6300

4 In ~ 12 I n
3200 ~ 9600

–

–

–

–

–

–

–

–

–

Short time delay pick up current
Variation is within ±15% of
setting current

400 800-1000-1200-1400-

450 900-1150-1350-1575-

500 1000-1250-1500-1750-

600 1200-1500-1800-2100-

700 1400-1750-2100-2450-

800 1260-1575-1890-2205-

Instantaneous pick up current
Variation is within ±15% of
setting current

Electronic trip relay
Adjustable ampere rating,
instantaneous pick up current

Instantaneous pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

1600-2000-2400-2800­3200-4000

1800-2250-2700-3150­3600-4500

2000-2500-3000-3500­4000-5000

2400-3000-3600-4200­4800-6000

2800-3500-4200-4900­5600-6300

2520-3150-3780-4410­5040-6300

4 In ~ 12 In
3200 ~ 9600

–

–

–

–

–

–

34

2 to 10 Ir

Short time delay pick up current
Variation is within ±15% of
setting current

400 800-1000-1200-1400-

450 900-1150-1350-1575-

500 1000-1250-1500-1750-

600 1200-1500-1800-2100-

700 1400-1750-2100-2450-

800 1260-1575-1890-2205-

Instantaneous pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

1600-2000-2400-2800­3200-4000

1800-2250-2700-3150­3600-4500

2000-2500-3000-3500­4000-5000

2400-3000-3600-4200­4800-6000

2800-3500-4200-4900­5600-6300

2520-3150-3780-4410­5040-6300

4 In ~ 12 In
3200 ~ 9600

–

–

–

–

–

–

–

–

–

30

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Number of poles
Automatic tripping

device

690V
500V
440V
400V
230V

NF800-REP

400 ~ 800

adjustable

690
20/15
70/35

125/63
125/63
150/75

3

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up and instantaneous

800A

NF800-UEP

400 ~ 800

adjustable

690

35/35
170/170
200/200
200/200
200/200

34

Electronic trip relay
Adjustable ampere rating
Adjustable long time delay
operating time, short time delay
pick up and instantaneous

Low-inst

Generator

Mag-Only
(Inst trip only)

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device
Rating (A) and

Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Short time delay pick up current
Variation is within ±15% of
setting current

400 800-1000-1200-1400-

450 900-1150-1350-1575-

500 1000-1250-1500-1750-

600 1200-1500-1800-2100-

700 1400-1750-2100-2450-

800 1260-1575-1890-2205-

Instantaneous pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

1600-2000-2400-2800­3200-4000

1800-2250-2700-3150­3600-4500

2000-2500-3000-3500­4000-5000

2400-3000-3600-4200­4800-6000

2800-3500-4200-4900­5600-6300

2520-3150-3780-4410­5040-6300

4 In ~ 12 I n
3200 ~ 9600

–

–

–

–

–

–

–

–

–

Short time delay pick up current
Variation is within ±15% of
setting current

400 800-1000-1200-1400-

450 900-1150-1350-1575-

500 1000-1250-1500-1750-

600 1200-1500-1800-2100-

700 1400-1750-2100-2450-

800 1260-1575-1890-2205-

Instantaneous pick up current
Variation is within ±15% of
setting current

2 to 10 Ir

1600-2000-2400-2800­3200-4000

1800-2250-2700-3150­3600-4500

2000-2500-3000-3500­4000-5000

2400-3000-3600-4200­4800-6000

2800-3500-4200-4900­5600-6300

2520-3150-3780-4410­5040-6300

4 In ~ 12 In
3200 ~ 9600

–

–

–

–

–

–

–

–

–

31

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Low-inst

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

690V
500V
440V
400V
230V

Solid-state
Adjustable ampere rating
Adjustable short time delay pick up
Fixed instantaneous pick up

Short time delay pick up current
Variation is within ±10% of the setting current

Instantaneous pick up current 20000

Variation is within ±10% of the setting current

1000

NF1000-SS

500-600-700-800-900-1000

690
25/13
65/33
85/43
85/43

125/63

34

500 2500-3750-5000
600 3000-4500-6000
700 3500-5250-7000
800 4000-6000-8000
900 4500-6750-9000

1000 5000-7500-10000

Solid-state
Adjustable ampere rating
Adjustable instantaneous pick up

5-7.5-10 In

+4000

–2000

34

5-7.5-10 In 3-4.5-6 In 2-3-4 In
500 2500-3750-5000 1500-2250-3000 1000-1500-2000
600 3000-4500-6000 1800-2700-3600 1200-1800-2400
700 3500-5250-7000 2100-3150-4200 1400-2100-2800
800 4000-6000-8000 2400-3600-4800 1600-2400-3200
900 4500-6750-9000 2700-4050-5400 1800-2700-3600

1000 5000-7500-10000 3000-4500-6000 2000-3000-4000

Generator

Mag-Only
(Inst trip only)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

34

Solid-state
Adjustable ampere rating
Adjustable instantaneous pick up

Variation is within ±10% of the setting current

3-4.5-6 In 2-3-4 In
500 1500-2250-3000 1000-1500-2000
600 1800-2700-3600 1200-1800-2400
700 2100-3150-4200 1400-2100-2800
800 2400-3600-4800 1600-2400-3200
900 2700-4050-5400 1800-2700-3600

1000 3000-4500-6000 2000-3000-4000

34

Solid-state
Adjustable ampere rating
Adjustable instantaneous pick up

Variation is within ±10% of the setting current

5-7.5-10 In
500 2500-3750-5000
600 3000-4500-6000
700 3500-5250-7000
800 4000-6000-8000
900 4500-6750-9000

1000 5000-7500-10000

32

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Low-inst

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

690V
500V
440V
400V
230V

Solid-state
Adjustable ampere rating
Adjustable short time delay pick up
Fixed instantaneous pick up

Short time delay pick up current
Variation is within ±10% of the setting current

Instantaneous pick up current 20000

Variation is within ±10% of the setting current

1250

NF1250-SS

600-700-800-1000-1200-1250

690
25/13
65/33
85/43
85/43

125/63

34

600 3000-4500-6000
700 3500-5250-7000

800 4000-6000-8000
1000 5000-7500-10000
1200 6000-9000-12000
1250 6250-9375-12500

Solid-state
Adjustable ampere rating
Adjustable instantaneous pick up

600 3000-4500-6000 1800-2700-3600 1200-1800-2400

700 3500-5250-7000 2100-3150-4200 1400-2100-2800

800 4000-6000-8000 2400-3600-4800 1600-2400-3200
1000 5000-7500-10000 3000-4500-6000 2000-3000-4000
1200 6000-9000-12000 3600-5400-7200 2400-3600-4800
1250 6250-9375-12500 3750-5625-7500 2500-3750-5000

5-7.5-10 In

+4000
–2000

34

5-7.5-10 In 3-4.5-6 In 2-3-4 In

Generator

Mag-Only
(Inst trip only)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

34

Solid-state
Adjustable ampere rating
Adjustable instantaneous pick up

Variation is within ±10% of the setting current

3-4.5-6 In 2-3-4 In
600 1800-2700-3600 1200-1800-2400
700 2100-3150-4200 1400-2100-2800
800 2400-3600-4800 1600-2400-3200

1000 3000-4500-6000 2000-3000-4000
1200 3600-5400-7200 2400-3600-4800
1250 3750-5625-7500 2500-3750-5000

34

Solid-state
Adjustable ampere rating
Adjustable instantaneous pick up

Variation is within ±10% of the setting current

5-7.5-10 In
600 3000-4500-6000
700 3500-5250-7000
800 4000-6000-8000

1000 5000-7500-10000
1200 6000-9000-12000
1250 6250-9375-12500

33

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Low-inst

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device

690V
500V
440V
400V
240V

Solid-state
Adjustable ampere rating
Adjustable short time delay pick up
Fixed instantaneous pick up

Short time delay pick up current
Variation is within ±10% of the setting current

Instantaneous pick up current 20000

600-700-800-1000-1200-1250

600 3000-4500-6000
700 3500-5250-7000

800 4000-6000-8000
1000 5000-7500-10000
1200 6000-9000-12000
1250 6250-9375-12500

5-7.5-10 In

1250

NF1250-UR

690

–

85/42
125/65
125/65
170/85

34

+4000

–2000

–

–

Generator

Mag-Only
(Inst trip only)

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

–

–

–

–

–

–

–

34

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Breaking
capacity (kA rms)

IEC60947-2

Icu/Ics

Standard

Low-inst

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

690V
500V
440V
400V
230V

Solid-state
Adjustable ampere rating
Adjustable short time delay pick up
Fixed instantaneous pick up

Short time delay pick up current
Variation is within ±10% of the setting current

Instantaneous pick up current 20000

Variation is within ±10% of the setting current

1600

NF1600-SS

800-1000-1200-1400-1500-1600

690
25/13
65/33
85/43
85/43

125/63

34

800 2400-3600-4800
1000 3000-4500-6000
1200 3600-5400-7200
1400 4200-6300-8400
1500 4500-6750-9000
1600 4800-7200-9600

Solid-state
Adjustable ampere rating
Adjustable instantaneous pick up

800 2400-3600-4800 1600-2400-3200
1000 3000-4500-6000 2000-3000-4000
1200 3600-5400-7200 2400-3600-4800
1400 4200-6300-8400 2800-4200-5600
1500 4500-6750-9000 3000-4500-6000
1600 4800-7200-9600 3200-4800-6400

3-4.5-6 In

+4000
–2000

34

3-4.5-6 In 2-3-4 In

Generator

Mag-Only
(Inst trip only)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

34

Solid-state
Adjustable ampere rating
Adjustable instantaneous pick up

Variation is within ±10% of the setting current

3-4.5-6 In 2-3-4 In

800 2400-3600-4800 1600-2400-3200
1000 3000-4500-6000 2000-3000-4000
1200 3600-5400-7200 2400-3600-4800
1400 4200-6300-8400 2800-4200-5600
1500 4500-6750-9000 3000-4500-6000
1600 4800-7200-9600 3200-4800-6400

34

Solid-state
Adjustable ampere rating
Adjustable instantaneous pick up

Variation is within ±10% of the setting current

3-4.5-6 In

800 2400-3600-4800
1000 3000-4500-6000
1200 3600-5400-7200
1400 4200-6300-8400
1500 4500-6750-9000
1600 4800-7200-9600

35

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Interrupting
capacity (kA rms)

IEC157-1

P1/P2

Standard

Low-inst

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device

600V
500V
415V
380V
240V

Thermal, adjustable-magnetic
Fixed ampere rating and
adjustable instantaneous

Variation is within ±10% of the
Hi setting current

1800 3200-4000-4800-5600-

2000 3200-4000-4800-5600-

2000

NF2000-S NFE2000-S

1800, 2000

600

–
65/50
85/50
85/50

125/85

34

Lo 1 2 3

45Hi

6400-7200-8000

Lo 1 2 3

45Hi

6400-7200-8000

–

–

1200-1400-1600-1800-2000

600

–
65/50
85/50
85/50

125/85

34

Solid-state
Adjustable ampere rating,
adjustable short time delay pick up
and fixed instantaneous pick up

Short time delay pick up current
Variation is within ±10% of the
setting current

1200 3600-5400-7200
1400 4200-6300-8400
1600 4800-7200-9600
1800 5400-8100-10800
2000 6000-9000-12000

Instantaneous pick up current

3-4.5-6 In

30000 ± 3000

–

–

2500

NF2500-S

2500

600

–
65/50
85/50
85/50

125/85

3

Thermal, adjustable-magnetic
Fixed ampere rating and
adjustable instantaneous

Variation is within ±10% of the
Hi setting current

2500 4000-5000-6000-7000-

Lo 1 2 3

45Hi

8000-9000-10000

–

–

Generator

Mag-Only

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

–

–

–

–

34

Adjustable-magnetic
Fixed ampere rating and
adjustable instantaneous

Variation is within ±10% of the
Hi setting current

2000 3200-4000-4800-5600-

Lo 1 2 3

45Hi

6400-7200-8000

–

–

–

–

–

–

–

Adjustable-magnetic
Fixed ampere rating and
adjustable instantaneous

Variation is within ±10% of the
Hi setting current

2500 4000-5000-6000-7000-

–

–

–

–

3

Lo 1 2 3

45Hi

8000-9000-10000

36

Specifty frequency

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Interrupting
capacity (kA rms)

IEC157-1

P1/P2

Standard

Low-inst

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device

600V
500V
415V
380V
240V

Thermal, adjustable-magnetic
Fixed ampere rating and adjustable instantaneous

Variation is within ±10% of the Hi setting current

3000 (3200)

NF3200-S

2800, 3000, 3200

600

–
65/50
85/50
85/50

125/85

3

Lo 1 2 3 4 5 Hi
2800 5000-6600-8300-10000-11600-13300-15000
3000 5000-6600-8300-10000-11600-13300-15000
3200 5000-6600-8300-10000-11600-13300-15000

–

–

NFE3000-S

1800-2000-2500-3000

600

–
65/50
85/50
85/50

125/85

3

Solid-state
Adjustable ampere rating,
adjustable short time delay pick up
and fixed instantaneous pick up

Short time delay pick up current
Variation is within ±10% of the
setting current

1800 3600-5400-7200
2000 4000-6000-8000
2500 5000-7500-10000
3000 6000-9000-12000

Instantaneous pick up current

2-3-4 In

30000 ± 3000

–

–

Generator

Mag-Only

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

–

–

–

3

Adjustable-magnetic
Fixed ampere rating and adjustable instantaneous

Variation is within ±10% of the Hi setting current

Lo 1 2 3 4 5 Hi
3000 5000-6600-8300-10000-11600-13300-15000
3200 5000-6600-8300-10000-11600-13300-15000

–

–

–

–

–

–

–

Specifty frequency

37

Frame (A)

Type

Rated current In (A)

Rated insulation voltage Ui (V) AC

AC Interrupting
capacity (kA rms)

IEC157-1

P1/P2

Standard

Low-inst

Number of poles
Automatic tripping

device

Rating (A) and
Inst. (A)

Number of poles
Automatic tripping

device

600V
500V
415V
380V
240V

Thermal, adjustable-magnetic
Fixed ampere rating and adjustable instantaneous

Variation is within ±10% of the Hi setting current

NF4000-S

3600, 4000

600

–
65/50
85/50
85/50

125/85

3

Lo 1 2 3 Hi
3600 8300-10000-11600-13300-15000
4000 8300-10000-11600-13300-15000

–

–

4000

NFE4000-S

2500-3000-3500-4000

600

–
65/50
85/50
85/50

125/85

3

Solid-state
Adjustable ampere rating,
adjustable short time delay pick up
and fixed instantaneous pick up

Short time delay pick up current
Variation is within ±10% of the
setting current

2500 5000-7500-10000
3000 6000-9000-12000
3500 7000-10500-14000
4000 8000-12000-16000

Instantaneous pick up current

2-3-4 In

35000 ± 3500

–

–

Generator

Mag-Only

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

Number of poles

Automatic tripping
device

Rating (A) and
Inst. (A)

–

–

–

3

Adjustable-magnetic
Fixed ampere rating and adjustable instantaneous

Variation is within ±10% of the Hi setting current

Lo 1 2 3 Hi

4000 8300-10000-11600-13300-15000

–

–

–

–

–

–

–

38

Specifty frequency

6. PROTECTIVE CO-ORDINATION

6.1 General

Type of System

The primary purpose of a circuit protection system is to prevent damage to series connected equipment and to
minimise the area and duration of power loss. The first consideration is whether an air circuit breaker or moul­ded case circuit breaker is most suitable.
The next is the type of system to be used. The three major types are:
Fully Rated, Selective and Cascade Back-Up.

Fully Rated

This system is highly reliable, as all of the breakers are rated for the maximum fault level at the point of their
installation. Discrimination (selective interruption) can be incorporated in some cases. The disadvantage is that
high cost branch breakers may be necessary.

Selective-Interruption(Discrimination)

Selective Interruption requires that in the event of a fault, only the device directly before the fault will trip, and
that other branch circuits of the same or higher level will not be affected. The range of selective Interruption of
the main breaker varies considerably depending on the breaker used.

Cascade Back-Up Protection

This is an economical approach to the use of circuit breakers, whereby only the main (upstream) breaker has
adequate interrupting capacity for the maximum available fault current. The MCCBs downstream cannot handle
this maximum fault current and rely on the opening of the upstream breaker for protection.

The advantage of the cascade back-up approach is that it facilitates the use of low cost, low fault level breakers
downstream, thereby offering savings in both the cost and size of equipment.

As Mitsubishi MCCBs have a very considerable current limiting effect, they can be used to provide this ‘cas­cade back-up’ protection for downstream circuit breakers.

39

6.2 Interrupting Capacity Consideration

Table 1 230VAC

3ph trans.

capacity (kVA)

1ph trans.

capacity (kVA)

Interrupting

capacity

(kA)(sym)

30

50·60

100

160

~

250

400

Frame (A)

630
800

1000

~

4000

Table 2 440VAC

Trans. capacity

(kVA)

Interrupting

capacity

(kA)(sym)

30

50·60

100

160

~

250

400

Frame (A)

630
800

1000

~

4000

30 or less
20 or less

2.5

NF30-

NF30-

CS

SP
NF50-CP
NF60-CP

30 or less 50~100 150~300 500~1000 1500~2000

1.5

NF30-CS

NF30-SP

NF50-CP
NF60-CP

50~75 100 150~300

30~50

5 10 15 25 30 35 50 85 100 125 170 200

NF50-HP, NF60-HP NF50-HRP

NF100-CP

2.5 12585655036307.5 10 15 18 25

NF50-HP
NF60-HP

NF100-CP

NF250-CP

75

NF250-CP

NF400-CP

NF630-CP

NF800-CEP

NF1000-SS~NF1600-SS, NF2000-S~NF4000-S
NFE2000-S, NFE3000-S, NFE4000-S

NF400-CP

NF630-CP

NF800-CEP

NF1000-SS~NF1600-SS, NF2000-S~NF4000-S
NFE2000-S, NFE3000-S, NFE4000-S

100~150

NF50-HRP

NF100-SP
NF100-SEP

NF160-SP
NF250-SP
NF250-SEP

200~500

NF100-SP
NF100-SEP

NF160-SP
NF250-SP
NF250-SEP

NF400-SP, NF400-SEP

NF630-SP,
NF630-SEP

NF800-SEP

42

NF100-HP
NF100-HEP
NF160-HP

NF250-HP
NF250-HEP

NF400-SP, NF400-SEP

NF400-HEP

NF630-SP,

NF630-HEP

NF630-SEP

NF800

NF800-HEP

-SEP

NF100-HP
NF100-HEP

NF160-HP
NF250-HP
NF250-HEP

NF400

-HEP
NF630

-HEP
NF800

-HEP

C Series

NF100-RP

NF225-RP

NF400-REP

NF630-REP
NF800-REP

2000~3000500~1500

–

NF100­RP
NF225-

RP

NF400

-REP
NF630

-REP
NF800

-REP

2500~5000

NF1250­UR

NF100-UP

NF225-UP

NF400-UEP
NF630-UEP

NF800-UEP
NF1250-

UR
S Series

200

NF100-UP

NF225-UP

NF400-UEP

NF630-UEP
NF800-UEP

40

6.3 Selective-Interruption (Discrimination)

6.3.1 Selective-Interruption Combination

Following tables show combinations of main-circuit
selective coordination breakers and branch breakers
and the available selective tripping current at the set­ting points at the branch-circuits.

Continuous supply

Main breaker

Branch breaker

Selection Conditions

1. The main breaker rated current, STD operating time
and INST pickup current are to be set to the maxi­mum values.

2. When selecting the over-current range, also check
the conformity using the other characteristic curves.

Selective interruption combinations (MCB-MCCB)

230VAC (Sym. kA)

Main Breaker

Note 1

Branch
Breaker

BH-D6
TYPE B

BH-D6
TYPE C

Icu : Rated breaking capacity
Note1 : Reted currents of main breakers are maximum values.
Note2 : Reted currents of branch breakers are 50A or less.

Icu(kA) 50 50

6

6

NF100-SEP NF250-SEP

1.6 Note21.6 Note2

1.6 Note2

Healthy circuit

Branch
breaker

3.5

3.5

Main breaker

Short-circuit point

STD pick up current.
Set up STD operating time in the

maximum value.

Set up inst pick up current
in the maximum value.

41

Selective interruption combinations (MCCB-MCCB)

230VAC (Sym. kA)

Main Breaker

Branch
Breaker

NF30-SP
NF50-HP

NF60-HP
NF50-HRP
NF100-SP

NF100-SP T/A
NF100-SEP

NF100-HP
NF100-HP T/A
NF100-HEP

NF160-SP
NF160-SP T/A

NF250-SP
NF250-SEP

S

NF250-SP T/A
NF160-HP

NF160-HP T/A
NF250-HP
NF250-HEP
NF250-HP T/A
NF400-SP
NF400-SEP
NF400-HEP
NF630-SP

NF630-SEP
NF630-HEP
NF50-CP

NF60-CP
NF100-CP

NE100-CP T/A

C

NF250-CP
NF250-CP T/A
NF400-CP
NF630-CP
NF100-RP
NF100-UP
NF225-RP
NF225-UP

U

NF400-UEP
NF630-UEP
NF800-UEP

Icu : Rated breaking capacity
Note : Reted currents main breakers are maximum values.

Icu(kA) 50 100 50 100 85 100 85 100 50 85 100 125

5 1.61.63.53.55555555 5
25 1.6 1.6 3.5 3.5 10 10 20 20 20 20 20 25
85 – – 3.5 3.5 10 10 20 20 20 65 65 85

50 – – 3.5 3.5 7.5 7.5 15 15 15 15 15 50

100 – – 3.5 3.5 10 10 25 25 25 25 25 100

50 ––––6.46.4101010101050
50 ––––––101010101050

50 ––––6.46.4101010101050
50 ––––6.46.4101010101050

100 ––––6.46.4101010101050
100 ––––––101010101050

100 ––––6.46.4101010101050
100 ––––6.46.4101010101050

85 ––––––––10101020
85 ––––––9.59.510101020

100 ––––––9.59.510101020

85 ––––––––––– 20

100 ––––––––––– 20

5 1.61.63.53.55555555 5

25 – – 3.5 3.5 7.5 7.5 10 10 10 10 15 25
30 ––––––7.57.57.57.57.525

30 ––––6.46.47.57.57.57.57.525
50 ––––––––10101020
50 ––––––––––– 20

125 – – 3.5 3.5 22 22 65 65 50 85 85 125
200 – – 3.5 3.5 22 22 65 65 50 85 85 125
125 ––––––––185050125
200 ––––––––185050125
200 ––––––––15151525
200 ––––––––––– 20
200 ––––––––––– –

NF100-SEP

NF100-HEP

NF250-SEP

NF250-HEP

NF400-SEP

NF400-HEP

NF630-SEP

NF630-HEP

NF800-CEP

NF800-SEP

NF800-HEP

NF1000-SS

NF1200-SS

NF1600-SS

42

Selective interruption combinations

440VAC (Sym. kA)

Main Breaker

Branch
Breaker

NF30-SP
NF50-HP

NF60-HP
NF50-HRP
NF100-SP

NF100-SP T/A
NF100-SEP

NF100-HP
NF100-HP T/A
NF100-HEP

NF160-SP
NF160-SP T/A

NF250-SP
NF250-SEP

S

NF250-SP T/A
NF160-HP

NF160-HP T/A
NF250-HP
NF250-HEP
NF250-HP T/A
NF400-SP
NF400-SEP
NF400-HEP
NF630-SP

NF630-SEP
NF630-HEP
NF50-CP

NF60-CP
NF100-CP

NE100-CP T/A

C

NF250-CP
NF250-CP T/A
NF400-CP
NF630-CP
NF100-RP
NF100-UP
NF225-RP
NF225-UP

U

NF400-UEP
NF630-UEP
NF800-UEP

Icu : Rated breaking capacity
Note : Reted currents main breakers are maximum values.

Icu(kA) 25 50 25 50 42 65 42 65 36 42 65 85

2.5 1.6 1.6 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
10 1.6 1.6 3.5 3.5 7.5 7.5 10 10 10 10 10 10
30 – – 3.5 3.5 7.5 7.5 15 15 15 18 18 30

25 – – 3.5 3.5 5 5 10 10 10 10 10 22

50 – – 3.5 3.5 7.5 7.5 18 18 18 18 18 50

25 ––––6.46.4101010101022
25 ––––––101010101022

25 ––––6.46.4101010101022
25 ––––6.46.4101010101022

50 ––––6.46.4101010101022
50 ––––––101010101022

50 ––––6.46.4101010101022
50 ––––6.46.4101010101022
42 ––––––––10101020
42 ––––––9.59.510101020
65 ––––––9.59.510101020

42 ––––––––––– 20
65 ––––––––––– 20

2.5 1.6 1.6 3.5 3.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5

10 – – 3.5 3.5 5 5 10 10 10 10 10 10
15 ––––––7.57.57.57.57.515

15 ––––6.46.47.57.57.57.57.515
25 ––––––––10101020
36 ––––––––––– 20

125 – – 3.5 3.5 15 15 30 30 30 42 50 85
200 – – 3.5 3.5 15 15 30 30 30 42 50 85
125 ––––––151515252585
200 ––––––151515252585
200 ––––––9.59.515151525
200 ––––––––––– 20
200 ––––––––––– –

NF100-SEP

NF100-HEP

NF250-SEP

NF250-HEP

NF400-SEP

NF400-HEP

NF630-SEP

NF630-HEP

NF800-CEP

NF800-SEP

NF800-HEP

NF1000-SS

NF1200-SS

NF1600-SS

43

Selective interruption combinations (ACB-MCCB)

230VAC (Sym. kA)

Main Breaker

Icu

Branch Breaker

(kA)

NF100-CP
NF100-SP
NF100-SP T/A
NF100-HP
NF100-HP T/A

100

100
NF100-SEP
NF100-HEP

100
NF160-SP
NF160-SP T/A
NF160-HP
NF160-HP T/A

100

100
NF250-CP
NF250-CP T/A
NF250-SP
NF250-SP T/A
NF250-HP
NF250-HP T/A

100

100
NF250-SEP
NF250-HEP

100
NF400-CP
NF400-SP
NF400-SEP
NF400-HEP

100
NF630-CP
NF630-SP
NF630-SEP
NF630-HEP

100
NF800-CEP
NF800-SEP
NF800-HEP

• The values in the table represent the max. rated current for both Series AE-SS air circuit breakers and branch breakers, and the
selective co-ordination applies when the AE-SS series air circuit breakers instantaneous pick up is set to maximum.

• The numerals shown in parentheses are for AE-SS with MCR. (When set MCR.)

100

Ics(kA)

25
50
50

50

50
50

30
30
50
50

50

50
85
85

50
50
85

50
85

AE630-SS

65
15(25)
16(50)
16(50)
24(65)
24(65)
16(50)
24(65)
15(50)
15(50)

9.4(65)

9.4(65)

9.4(30)

9.4(30)
15(50)
15(50)

9.4(65)

9.4(65)
15(50)

9.4(65)

–
–
–
–
–
–
–
–
–
–
–

AE1000-SS

65

25
45(50)
45(50)

65

65

50

65
24(50)
24(50)
25(65)
25(65)

22.5(30)

22.5(30)
24(50)
24(50)
25(65)
25(65)
24(50)
25(65)
15(50)
15(65)
15(65)
15(65)

–

–
15(65)
15(65)

–

–

–

AE1250-SS

65
25
50
50
65
65
50

65
30(50)
30(50)
40(65)
40(65)

30

30
30(50)
30(50)
40(65)
40(65)
30(50)
40(65)
20(50)
20(65)
20(65)
20(65)

18.7(50)

18.7(50)

18.7(65)

18.7(65)

18.7(50)

18.7(65)

18.7(65)

AE1600-SS

65
25
50
50
65
65
50

65
42(50)
42(50)

65

65

30

30
42(50)
42(50)

65

65
42(50)

65
30(50)
30(65)
30(65)
30(65)
24(50)
24(50)
24(65)
24(65)
24(50)
24(65)
24(65)

AE2000-SS

85
25
50
50
85
85
50
85
50
50
85
85
30
30
50
50
85
85
50

85
48(50)
48(65)
48(65)
48(65)
30(50)
30(50)
30(65)
30(65)
30(50)
30(65)
30(65)

AE2500-SS

85
25
50
50
85
85
50
85
50
50
85
85
30
30
50
50
85
85
50
85
50
70
70

70
40(50)
40(50)
40(65)
40(65)
40(50)
40(65)
40(65)

AE3200-SS

85
25
50
50
85
85
50
85
50
50
85
85
30
30
50
50
85
85
50
85
50
85
85
85
50

50
60(65)
60(65)

50
60(65)
60(65)

AE4000-SSC

85
25
50
50
85
85
50
85
50
50
85
85
30
30
50
50
85
85
50
85
50
85
85
85
50
50
85
85
50
85
85

44

440VAC (Sym. kA)

Main Breaker

Icu

Branch Breaker

(kA)

NF100-CP
NF100-SP
NF100-SP T/A
NF100-HP
NF100-HP T/A
NF100-SEP
NF100-HEP
NF160-SP
NF160-SP T/A
NF160-HP
NF160-HP T/A
NF250-CP
NF250-CP T/A
NF250-SP
NF250-SP T/A
NF250-HP
NF250-HP T/A
NF250-SEP
NF250-HEP
NF400-CP
NF400-SP
NF400-SEP
NF400-HEP
NF630-CP
NF630-SP
NF630-SEP
NF630-HEP

100
NF800-CEP
NF800-SEP
NF800-HEP

• The values in the table represent the max. rated current for both Series AE-SS air circuit breakers and branch breakers, and the
selective co-ordination applies when the AE-SS series air circuit breakers instantaneous pick up is set to maximum.

• The numerals shown in parentheses are for AE-SS with MCR. (When set MCR.)

100

Ics(kA)

10
30
30
50
50
25
50
25
25
50
50
15
15
25
25
50
50
25
50
25
42
42
65
50
50
85

50
85

AE630-SS

65

9.8(10)
9(30)
9(30)

16(50)
16(50)

9(25)

16(50)

7(25)
7(25)
9(50)
9(50)

9.4(15)

9.4(15)
7(25)
7(25)
7(50)
7(50)
7(25)
7(50)

–
–
–
–
–
–
–
–
–
–
–

AE1000-SS

65

10
20(30)
20(30)

50

50
20(25)

50
14(25)
14(25)
15(50)
15(50)

15

15
14(25)
14(25)
15(50)
15(50)
14(25)
14(50)
15(25)
15(42)
15(42)
15(42)

–

–
15(65)
15(65)

–

–

–

AE1250-SS

65

10
25(30)
25(30)

50

50

25

50
19(25)
19(25)
25(50)
25(50)

15

15
19(25)
19(25)
25(50)
25(50)
19(25)
19(50)
18(25)
18(42)
18(42)
18(65)

18.7(50)
–

18.7(65)

18.7(65)

18.7(50)

18.7(65)

18.7(65)

AE1600-SS

65
10
30
30
50
50
25
50
25

25
42(50)
42(50)

15

15

25

25
42(50)
42(50)

25
25(50)
24(25)
24(42)
24(42)
24(65)
24(50)
24(50)
24(65)
24(65)
24(50)
24(65)
24(65)

AE2000-SS

85
10
30
30
50
50
25
50
25
25
50
50
15
15
25
25
50
50
25
50

25
30(42)
30(42)
30(65)
30(50)
30(50)
30(65)
30(65)
30(50)
30(65)
30(65)

AE2500-SS

85
10
30
30
50
50
25
50
25
25
50
50
15
15
25
25
50
50
25
50

25
30(42)
30(42)
39(65)
37(50)
37(50)
37(65)
37(65)
37(50)
37(65)
37(65)

AE3200-SS

85
10
30
30
50
50
25
50
25
25
50
50
15
15
25
25
50
50
25
50
25
42
42

48(65)

50

50
63(65)
63(65)

50
63(65)
63(65)

AE4000-SSC

85
10
30
30
50
50
25
50
25
25
50
50
15
15
25
25
50
50
25
50
25
42
42

62(65)

50
50
85
85
50
85
85

45

6.4 Cascade Back-up Protection

6.4.1 Cascade Back-up Combinations

Following tables show the available MCCB combina­tions for cascade interruption and their interrupting
capacity.

440VAC

Interrupting capacity (kA)

Main

SCU

MCCB

Branch
MCCB

NF100-SP

NF100-HP

NF160-SP

NF160-HP

NF250-SP

NF250-HP

NF400-SP

25 50 25 50 25 50 42 65

NF400-HEP

NF400-REP

NF630-SP

NF630-HEP

125

42 65
NF30-SP
MB30-SP

2.5 10 14 5 5 5 5

––––––––––––
MB50-CP
MB50-SP

NF50-HP
NF60-HP
NF50-HRP
NF100-SP
MB100-SP
NF100-HP

S

NF160-SP
NF160-HP
NF250-SP

14 20 15 10 15 10 15 10 10 10 10 10 10 10 10 10

7.5
20 30 18–18–15 15 15 14 14 14

10

–

30

50–42–42

–

25

50–42–42 35 35 35 35 35 35 35 35 35 30

–––––––

50

––––––

25

–––––––

50

––––––

25

––––––––––––––––

65 65–65 65–65 65

35 50 50 35 50 50 35 50 50 50

65 65–65 65–65 65 65

35 50 50 35 50 50 35 50 50

MB225-SP
NF250-HP
NF400-SP
NF400-SEP
NF630-SP
NF630-SEP
NF50-CP
NF60-CP
NF100-CP

C

NF250-CP
NF400-CP
NF630-CP

–––––––

50

–––––––

42

––––––––––

42

10 14 5 5 5 5

2.5

10

20 30 14 14 14 14 14 14 14 14 14 14 14 14 14 14

––

15
25
36

25 25 25 25 30 30 30 25 25 25 20 20 20 20 20 20–18 18 18

––––––
–––––––––

65 65–65 65–65 65
65 65–65 65–65 65

65 65–65 65

––––––––––––

35 35 35 35 35 35 35 35 35 30 30 30

42 50 50 42 50 50 42 42 42

Main breaker

Branch
breaker

NF630-REP

NF800-SEP

NF800-HEP

NF800-REP

NF1000-SS, NF1250-SS, NF1600-SS

NF2000-S, NF2500-S

125

42 65

125

85 85 85 15 25 36 36

–––

––––––––––

NF400-CP

NF630-CP

NF800-CEP

NF3200-S, NF4000-S

NF250-CP

NF100-RP

125 200 125 200 200 200 200 125

–––

5

10 10–50

35

125

125
125 125
125 200 125 200 200

––––––

–––––––

125 200 125 200
125 200 125 200 200

––––––––

––––––––
–––––––––
–––––––––
–––––––––––

––––––––––––

–––

5

–––

14 14 14

35

125

125 20050125

––
––––––––
–––––––––

Branch
breaker

Fault point

NF225-RP

NF225-UP

NF400-UEP

NF100-UP

35 50

NF630-UEP

––––

50 50 10 10 10 10

––––

50 50

85 85

50 50 35 30

85 85 65

125 200

85 85 85

125 200 200 200 200
125 200

85 85 85

125 200 200 200 200

200 200 200

200 200

35 50 5

–––

14 14 14 14

125 200

50 30 25 20
50 50 50 30

200 200

NF800-UEP

NF1250-UR

–

–

65

–

65

–

–

42

46

230VAC

Interrupting capacity (kA)

Main
MCCB

SCU

Branch
MCCB

NF30-SP
MB30-SP
MB50-CP
MB50-SP
NF50-HP
NF60-HP
NF50-HRP
NF100-SP
MB100-SP

S

NF100-HP
NF160-SP
NF160-HP
NF250-SP
MB225-SP
NF250-HP
NF400-SP
NF400-SEP
NF630-SP
NF630-SEP
NF50-CP
NF60-CP
NF100-CP

C

NF250-CP
NF400-CP
NF630-CP

NF100-SP

NF100-HP

NF160-SP

NF160-HP

NF250-SP

NF250-HP

NF400-SP

NF400-HEP

NF400-REP

NF630-SP

NF630-HEP

NF630-REP

NF800-SEP

NF800-HEP

NF800-REP

NF1000-SS, NF1250-SS, NF1600-SS

NF2000-S, NF2500-S

50

100501005010085100 12585100 12585100 125 125 125 125

5 425010101010

42 85 35 35 35 35 30 30 30 30 30 30

10

50

100

25
85
50

100

50

100

50

100

85

85

5

25
30
50
50

50 50 50 50 50 50 50 50 50 50
––––––––––––––––––––––
–

–

100

85–85 85 85 85 85 85 85

––––––––––––––––––––––
–––85–
––––––––––––––––––––––––

–––85–
––––––––––––––––––––––––
–––––––––––––––

–––––––––––––––

35 50 10 10 10 10
35 85 50 50 50 50 50 50 50 50 50 50

––

50 50 50 50 50 50 50 50 50 50
––––––

–––––––––

––––––––––––

––––––
–––––––

––––––––––

85 85 85 85 85 85 85 70 70 70 70

85 85 85 85 85 85 85 70 70 70 70

––––––––

––––––––

100 100 100

100 100 100

––––––––––––

–––––––
–––––––

85 85 85 85 85 85 85 85 85 85 85 85

85 85 85 85 85 85 85 85 85

NF400-CP

NF630-CP

NF3200-S, NF4000-S

NF250-CP

30 50 50 50

–––

7.5

NF800-CEP

125 200 125 200 200 200 200 170

125 200

25 14 14–125 20085125

30 30–125 20085125

125 200 125 200 200 125 125
125 200 125 200 200 125 125
125 200 125 200 200 125 125

––––––––

–––––––––

–––

7.5

125 200

30 30–125 20085125

–––

35 35
––––––––
–––––––––

NF225-RP

NF100-RP

NF225-UP

NF100-UP

35 50

125 200 200 125 125
125 200 200 200 200

125 200 200 125 125
125 200 200 200 200

35 50

125 200 200

NF400-UEP

NF630-UEP

NF800-UEP

––––

––––
––––

–
–
–

70

–

70

–

200 200 200 100

200 200 100

––––

––

50 50

–

50 50

200 200 200

200 200

85
85

NF1250-UR

47

6.5 I2t let-Through and Current Limiting Characteristics (440 VAC)

2

I

t let-through characteristics Current limiting characteristics

(✕106)

10

8
6

4

2

-s)

2

t (A

1

2

0.8

0.6

Max. I

0.4

0.2

0.1
1 2 4 10 20 40 60 80 10086

(✕106)

20

10

5.0

2.0

-s)

2

t (A

1.0

2

Max. I

0.5

0.2

NF100-HP,NF250-HP

NF250-HP

NF100-HP
(40~100A)

NF100-HP
NF100-HP

NF100-HP

short-circuit current,sym.r.m.s.(kA)

NF100-RP,NF100-UP,NF225-RP,NF225-UP

NF225-RP

NF225-UP

NF100-RP

NF100-UP

(30A)
(20A)

(15A)

100

80
60

40

20

Prospective short-circuit

10

8
6

4

current,asym.peak

Max. Iet-through current(kA)

2

1

1 2 4 10 20 40 60 80 10086

short-circuit current,sym.r.m.s.(kA)

200

100

Max. Iet-through current(kA)

NF100-RP,NF100-UP,NF225-RP,NF225-UP

50

30
20

10

5

Prospective short-circuit

current,asym.peak

NF100-HP,NF250-HP

NF225-RP

NF100-RP

NF250-HP
NF100-HP

(40~100A)
NF100-HP(30A)
NF100-HP(20A)

NF100-HP(15A)

NF225-UP
NF100-UP

2

2

-s)
t (A

Max. I

(✕106)

0.8

0.6

0.4

0.2

0.1

40

20

10

8
6

4

2

1

0.1

0.05
3 10 20 50 100 200

short-circuit current,sym.r.m.s.(kA)

NF400-UEP,NF630-UEP,NF800-UEP

1 2 4 10 20 40 60 80100 20086

short-circuit current,sym.r.m.s.(kA)

NF630-UEP
NF800-UEP

NF400-UEP

1

3 10 20 50 100 200

short-circuit current,sym.r.m.s.(kA)

400

200

100

80
60

40

20

10

8
6

Max. Iet-through current(kA)

4

2

1

1 2 4 10 20 40 60 80100 20086

NF400-UEP,NF630-UEP,NF800-UEP

Prospective short-circuit

current,asym.peak

short-circuit current,sym.r.m.s.(kA)

NF630-UEP
NF800-UEP

NF400-UEP

48

6.6 Protective Coordination with Wiring

6.6.1 General Considerations

If it is assumed that the heat generated by a large
current passing through a wire is entirely dissipated
within the wire, the following expression is applicable
(for copper wires):

2

I

t=5.05✕10 log

(

)

S

I : Current(A, rms)
S : Wire cross-sectional area(mm2)
t : Current let-through time(s)
T : Wire temperature due to short circuit(°C)

To : Wire temperature before short circuit(°C)
Assume that short-circuit current occurs in a wire car­rying its rated current (hot state To=60°C). If 150°C is
the allowable temperature T, the following expression
is applicable (see also Fig. 6.13):

Table 6.4 Allowable Fault Conditions in Conductors

S

Wire size

mm

1

1.5

2.5
4
6

10
16
25
35
50
70

95
120
150
185
240

2

Allowable I2t

A2✕s

0.014✕10

0.032✕10

0.088✕10

0.224✕10

0.504✕10

1.40✕10

3.58✕10

8.75✕10

17.2✕10

35.0✕10

68.6✕10
126✕10
202✕10
315✕10
479✕10
806✕10

Notes: 1. Allowable I2t is calculated assuming that

all heat energy is dissipated in the
conductor, conductor allowable maximum
temperature exceeds 150°C, and hot
start is applied, at 60°C.

2. Is is an asym. value of allowable short­circuit current reduced to below the
allowable I2t, assuming half cycle
interruption for 16mm2 or less and one
cycle interruption for 25mm2 or more.

4

234+T

e

234+To

Is

Allowable short-circuit

current accoeding to I2t

kA, sym. (PF)

6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6

1.17 (0.9)

1.76 (0.9)

2.93 (0.9)

4.68 (0.9)

6.79 (0.8)

10.5 (0.6)

16.0 (0.5)

17.3 (0.3)

24.2 (0.3)

34.5 (0.3)

48.3 (0.3)

65.6 (0.3)

82.8 (0.3)
103 (0.3)
128 (0.3)
166 (0.3)

Allowable I2t=14000S

2

Considering let-through energy (∫i2dt) in a fault where
the protector has no current-limiting capability, if short­circuit occurs when let-through current is max., ∫i2dt
is:

Approx.

Approx.

2

Ie

(A ·s) in

71

2

Ie

(A ·s) in 1

34

2

2

1

cycle interruption

2

(Power factor is 0.5.)

cycle interruption

(Power factor is 0.3.)

where current le is the effective value of the AC com­ponent. Half-cycle interruption is applied to wire of up
to 14mm2, and one-cycle interruption to larger wires.
Table 6.4 is restrictive in that, e.g., in a circuit of fault
capacity of 5000A or more, 2.5mm2 wires would not
be permitted. In practice, the impedance of the con­ductor itself presents a limiting factor, as does the in­herent impedance of the MCCB, giving finite let­through I2t and Ip values that determine the actual
fault-current flow.

1000

700
500

300
200

100

70
50

Temperature rise(°C)

30
20

12 23344567856 1

Fig. 6.13 Temperature Rises Due to Current Flow in Copper Wires

×10

3

(A/mm

2) 2

·s

×10

4

6.6.2 600V Vinyl-Insulated Wire (Overcurrent)

Japanese Electrical Installations Technical Standards
(domestic) specify vinyl-insulated wire operating tem­perature as 60°C max., being a 30°C rise over a 30°C
ambient temperature. This is to offset aging deterio­ration attendant on elevated temperatures over long
periods. Criteria for elevated temperatures over short
periods have been presented in a study by B. W. Jones
and J. A. Scott (“Short-Time Current Ratings for Air­craft Wire and Cable,” AIEE Transactions), which pro­poses 150°C for periods of up to 2 seconds, and 100°C
for periods in the order of 20 seconds. These criteria
can be transposed to currents for different wire sizes
by the curves given in Fig. 6.14. Such figures, how­ever, must be further compensated for the difference
between vinyl materials used for aircraft and for

49

1000

800
600

500
400

300

200

100

60
50

40
30

Time (sec)

20

10

6
5

4
3

2

1.0

1.5

2.5

4.0

6.0

Wire sizes (mm2)

630

500

400

300

240

185

150

120

95

70

50

35

25

16

10

1

Fig. 6.14 Relation of Let-through Current to Time until 600V Vinyl-Insulated Wire Reaches a 70°C Temperature Rise.

ground use; ultimately, the temperature figure of 75°C
is derived (100°C per Jones and Scott, compensated)
as a suitable short-time limitation for wiring with heat­proof vinyl or styrene-butadene-rubber insulation.
Current transpositions for the range of wire sizes are
not presented, being non-standard ; however, Fig. 6.15
gives MCCB ratings for temperature limitations of 30°C
in normal operation, and 75°C for periods of up to 20

Wire size

(mm
MCCB
rating(A)

15
20
30
40
50

0.2

(In a Start from No Load State at Ambient Temperature of 30°C)

0.4 0.50.6

0.3 10.8 2 3

10

9203040

876540.1

Current (×102A)

seconds.
The apparent disparity of the ambient ratings of 30°C
for wiring against 40°C for MCCBs, is reconcilable in
that wiring, for the most part, is externally routed, while
MCCBs are housed in panelboards or the like. The
two figures can be used compatibly, without modifi­cation. It is further noted that, where MCCBs with long­delay elements of the thermal type are employed, the
effect of increased ambient, which would normally

2

)

1

1.5

2.5

4

6

10162535507095

120

185

derate the wiring, is adequately compensated by the
attendant decrease in thermal-region tripping time of

240

the MCCB.
The curves in Fig. 6.17 show the comparison of the
delay regions of MCCB tripping with allowable cur­rents in open-routed wiring. Fig. 6.16 shows the

Protected region

method required by the Japanese standards referred
to above, for derating wiring to be routed in conduit.

50 100 1000200 300 5008060

60

75
100
125

Open wiring

150
175

Time.

Routed in conduit

200
225
250
300
350

Unprotected region

!2!

Current

!

2

1

=Correction factor

!

1

400

Fig. 6.15 MCCBs and Wiring Sizes

Fig. 6.16 Wire Derating Method, for Conduit Routing

50

2000

2000

1000

800
600

400
300

200

100

80
60

40
30

20

Tripping time (s)

10

8
6

4
3

MCCB rating

2

1

0.8

0.6

0.5
10 100 1000 10000

2000

1000

800
600

400
300

200

100

80
60

40
30

20

Tripping time (s)

10

8
6

4
3

2

15A
20A
30A
40A
50A

MCCB rating

Wire size

1.0mm

1.5mm

2.5mm

4.0mm

6.0mm

Current (A)

a) 50A-fream MCCB

125A
150A
175A
200A
225A

1000

800

2
2
2
2
2

600
400

300
200

100

80
60

40
30

20

Tripping time (s)

10

8
6

4
3

MCCB rating

60A
75A

100A

Wire size

2

10mm

2

16mm

2

25mm

2

1

0.8

0.6

0.5
10 100 1000 10000

Current (A)

b) 100A-fream MCCB

2000

1000

800

Wire size
16mm

25mm
35mm
50mm
70mm

2
2
2
2
2

600
400

300
200

100

80
60

40
30

20

Tripping time (s)

10

8
6

4
3

MCCB rating

250A
300A
350A
400A

Wire size

2

50mm

2

70mm

2

95mm
120mm
150mm

2
2

2

1

0.8

0.6

0.5
10 100 1000 10000

Current (A)

c) 225A-fream MCCB

Fig. 6.17 600V Wire and MCCB Protection Compatibility

1

0.8

0.6

0.5
100 1000 10000 100000

Current (A)

d) 400A-fream MCCB

51

6.7

Protective Coordination with Motor Starters

Motor starters comprise a magnetic contactor and a
thermal overload relay, providing the nesessary
switching function for control of the motor, plus an
automatic cutout function for overload protection.
Mitsubishi Electric’s excellent line of motor starters
are available for a wide range of motor applications
and are compatible with Mitsubishi MCCBs.
Magnetic contactors are rugged switching devices
required to perform under severe load conditions with­out adverse affect. They are divided into Classes A
through D (by capacity); Class A, e.g., must be able
to perform 5 cycles of closing and opening of 10 times
rated current, followed by 100 closing operations of
the same current after grinding off 3/4 of the contact
thickness.
Current ratings of contactors usually differ according
to the circuit rated voltage, since voltage determines
arc energy, which limits current-handling capability.
Thermal overload relays (OLRs) employ bimetal ele­ments (adjustable) similar to those of MCCBs.
For compatibility with the magnetic contactor, the OLR
must be capable of interrupting 10 times the motor

full-load current without destruction of its heater ele­ment. Mitsubishi Type TH OLRs are normally capable
of handing 12 to 20 times rated current; in addition
there is available a unique saturable reactor for par­allel connection to the heaters of some types, giving
a fusion-proofing effect of 40~50 times.

6.7.1 Basic Criteria for Coordination

It is necessary to ensure that the MCCB does not trip
due to the normal starting current, but that the OLR
cutout curve intersects the MCCB thermal delay-trip­ping curve between normal starting current and 10
times full-load current. The MCCB instantaneous-trip­ping setting should be low enough to protect the OLR
heater element from fusion, in a short-circuit condi­tion.
The above criteria should ensure that either the MCCB
or the OLR will interrupt an overload, to protect the
motor and circuit wiring, etc. In practice it is desirable
for the MCCB instantaneous tripping to be set for about
15 times full-load current as a margin against tran­sients, such as in reclosing after power failure, Y-delta
switching, inching, etc.

Protection function

MCCB

Starter

combination

Fig. 6.18 Protective Coordination; MCCBs and Motor Starters

Motor overheat/burnout curve

MCCB trip. curve

OLR trip. curve

Current-time limitations of motor wiring

Intersection of MCCB and OLR trip curves

OLR heater fusion

Time

Motor starting
current

2134 56

Key

1 . Motor normal starting

current

2 . OLR-MCCB curve

intersection

3 . Transient peak of

motor current

Fig. 6.19 Protection Coordination Criteria for MCCBs and

Motor Starters

Current-time limitations of
MCCB-to-starter wiring

Current

4 . MCCB inst. trip current
5 . Protection limit; the

possible short-circuit at
the motor terminals must
be less than this value.

6 . MCCB rated interruption

capacity

Protects circuit wiring' control devices' and
OLR against fusion.

Protects the motor against overcurrents up
to 10 times rating.

MCCB

Magnetic contactor
and thermal overload
relay

6.7.2 Levels of Protection (Short Circuit)

In some cases it may be advantageous to allow the
starter to be damaged in the event of a short circuit,
provided that the fault is interrupted and the load side
is properly protected.
IEC standards defines 2 types of coordination, sum­marized as:

1. Type “1” coordination requires that, under short­circuit conditions, the contactor or starter shall
cause no danger to persons or installation and
may not be suitable for further service without re­pair and replacement of parts.

2. Type “2” coordination requires that, under short­circuit conditions, the contactor or starter shall
cause no danger to persons or installation and
shall be suitable for further use. The risk of con­tact welding is recognized, in which case the
manufacturer shall indicate the measures to be
taken as regards the maintenance of the equip­ment.

52

6.7.3 Motors with Long Starting Times

Residual+source V

Source V

✕Normal starting inrush current

The usual approach is to select a starter with a larger
current rating, but this method, of course, involves a
degree of sacrifice of protection. Mitsubishi provides
a unique solution to this problem in the form of a satu­rable reactor added to the OLR heater element. The
effect is to change the high-current characteristics,
so that nuisance tripping in starting is eliminated, with­out loss of overload protection. Mitsubishi saturable
reactors are adjusted to allow around 25~30 seconds
of continuous starting current.

6.7.4 Motor Breakers (M Line MCCBs) and
Magnetic Contactors

M Line MCCBs are provided with trip curves espe­cially suitable for motor protection, with ratings based
on motor full-load currents. They provide overcurrent
and short-circuit protection, and are normally used with
magnetic contactors. The need for protective coordi­nation (as with a regular MCCB plus a starter) is elimi­nated, and the reliability of protection in a short-cir­cuit condition is far higher than that of the heater of a
starter OLR. Where the motor starting time is long,
the MCCB tripping curve must be checked carefully,
since tripping times are rather short in the delay-trip
range. Care must also be taken with respect to surge
conditions such as inching, reversing, restart, Y-delta
starting, etc.

1. Superimposed DC Transient (Low Power-Factor
Effect) Fig. 6.20 shows that the power factor is about

0.3 at starting, causing a significant DC component,
so that the total transient inrush current may reach
about twice the value of the AC component, even
though the latter is of constant amplitude. Peak in­rush current (lt) of 1.4 x normal starting current (lo)
must be allowed for, in selecting the MCCB instanta­neous-trip setting.

2. Residual Voltage (Running Restart)
If residual (regenerative) voltages appearing at the
motor terminals are out of phase with the supply volt­age (at the time of reclosing after being interrupted,
before the motor speed is substantially reduced), the
cumulative effect of the line voltage and the residual
voltage is equivalent to the motor being directly sub­jected to a large line overvoltage, with a resulting ab­normal inrush current of:

This is a current magnification effect, which may be

1

1+

as much as 2 x in direct restarting, and

( )

3

x in Y-

delta-switching restarting. When the DC-transient fac­tor (§1 above) is added, the magnification becomes

2.4 in the case of direct restarting, and 1.9 for Y-delta
restarting.

6.7.5 Motor Thermal Characteristics

Overload currents in motors can lead to burnout, or
insulation damage resulting in shock or fire hazard;
the basic approaches to protection are (summarized
from Japanese standards):

1. MCCB + magnetic contactor + OLR

2. Motor breaker + magnetic contactor

3. Motor breaker alone
In 1, the OLR is the primary interrupter of overload,
and being adjustable, can be set for the true load re­quirement. Large overcurrent or short-circuit fault con­ditions are interrupted by the MCCB instantaneous
trip. In 2, the motor breaker is the protector for both
overload and short-circuit, and not being adjustable
must be selected carefully, for best coordination with
the load concerned. In 3, since the MCCB is relied on
not only for all protective functions but also for switch­ing, this arrangement should be reserved for applica­tions requiring infrequent motor starting and stopping.

6.7.6 Motor Starting Current

Motor starting times of up to 15 seconds are generally
considered safe; more than this is considered
undesirable; more than 30 seconds is considered
dangerous and should be avoided wherever possible.
For instantaneous tripping considerations, the MCCB
is normally set to 600% of the motor full-load current,
for trouble-free line-starting of an induction motor.
More detailed consideration is required where short­time inrush effects (current magnification) are involved,
such as in Y-delta switching, running restart, etc. Two
basic causations are as follows:

1.8

1.7

1.6

1.5

t

o

I

I

1.4

1.3

1.2

1.1

Fig. 6.20 Transient DC Component

34
32
30
28
26
24
22
20
18
16
14
12
10

Current magnification

8
6
4
2
0

Direct (line) starting
Inching duty
Reversing duty

-delta switching

0.75

0.2 0.4

Motor output (kW)

Fig. 6.21 Peak Inrush-Current Measurements

It

0

0.1 0.2 0.3 0.4 0.5 0.6 0.7
Power factor (lag)

1.5 2.2 3.7 5.5 7.5 22

b) Test results

30

2 Io

Backup
MCCB

Oscillo-

CT

graph

Motor
breaker

Contactor

M

a) Test connections

53

Thus, if normal starting current is assumed as 600%
of full-load current, the peak inrush becomes 1200%
in Y-delta restarting and 1600% in direct restarting.
The MCCB instantaneous-trip setting must be selected
at larger than these values.
Fig. 6.21 shows test date with respect to four condi­tions of transient inrush current, expressed as magni­fications of full-load current, measured on motors rated
from 0.2~30kW. The MCCB was used for line-start­ing switching, and the contactor for the other switch­ing duties. Phase matching between the line and re­sidual voltages was uncontrolled.
The oscillographs taken showed that the peak inrush
currents persist for about one-half cycle, followed by
a rapid decrease to normal starting-current level. From
the curves it can be concluded that peak inrush mag­nifications vary greatly depending on the duty involved;
for reversing duty, the MCCB instantaneous trip set­tings must be selected from 1600 ~ 3400% of full­load current. For line starting and Y-delta starting, the
range spans from 1000~2000%.

6.8 Coordination with Devices on the High­Voltage Circuit.

6.8.1 High-Voltage Power Fuse

The MCCB on the secondary (low-voltage) side of a
power transformer must have tripping characteristics
that provide protective coordination with the power
fuse (PF) on the high-voltage side (Fig. 6.22). The
MCCB must always trip in response to overcurrent,
to ensure that the PF does not fuse or deteriorate by
elevated temperature aging.
Fig. 6.23 shows the MCCB curve in relationship to
the deteriorated PF curve (if this is unavailable, the
average fusing curve reduced by 20% can usually be
assumed). The PF characteristic can be converted to
the secondary side, or the MCCB characteristic to the
primary side; the curves must not overlap in the
overcurrent region.
Where the MCCB instantaneous-tripping current of
the MCCB is adjustable, difficulties in matching the
curves can be overcome as shown, but a 10% mar­gin must be included to allow for the tolerance of the
MCCB tripping setting.
The shaded area in Fig. 6.23 belong to overcurrent
region, the overcurrent generally occur at the lower
circuit of MCCB2.
Thus, it may in some cases be better to accept a co­ordination between the PF and MCCB2, permitting a
mismatch between the PF and MCCB1.

PF

Tr

MCCB1

MCCB2

Fig. 6.22 Protective Coordination of MCCBs and HV-Side PF

Short-delay fusing
of PF (deteriorated)

MCCB tripping
curve

Time

Minimum
setting of
inst,-trip
current

Overcurrent

Fig. 6.23 Coordinated PF and MCCB Characteristics

6.8.2 Electronic MCCBs and HV PF

A basic requirement is that the deteriorated short-de­lay curve of the PF, and the short-delay trip curve of
Electronic MCCB, which is shifted +10% along the
current axis, do not overlap.
To facilitate matching, the rated current of the PF
should be as large as possible; however, there is an
upper limit, as seen from the following criteria:

1. The rated current should be 1.5~2 times the load
current.

2. To ensure protection in the event of a short circuit,
the PF must interrupt a current of 25 times the trans­former rating within 2 seconds.

3. To ensure that the PF neither deteriorates nor fuses
as a result of the transformer excitation surge
current, the short-delay deterioration curve of the
PF must be more than 0.1 seconds, at a current of
10 times the transformer rating. The “10 times”
factor becomes “15 times” in the case of a single­phase transformer.

54

PF

Tr

Electronic MCCB

Figs. 6.26 and 6.27 show the setup, and the coordinat­ed characteristics converted to the low-voltage side.
The turns ratio of the CT is 150:5, to match the rated
primary current of 87.5A. Considering cooperation of
the OCR with the upper-ranking substation OCR, the
OCR dial is normally set to 0.2 or less, or 1 second
max. if it has an instantaneous trip element. On the
Mitsubishi Type MOC-E general-purpose relay this is
equivalent to dial setting No. 2. Latching-curve over­lap, shown by the broken lines in Fig. 6.27, must be
allowed for. The instantaneous trip is set to 30A, in
accordance with §1, above.

Fig. 6.24 Protective Coordination of Electorinic MCCBs

Time

and PF

3 h

2

1 h

40
30

20

10 min

6
4

1 min

40
30
20

10 s

6
4
2

1 s

0.6

0.4

0.2

0.1

0.06

0.02

0.01

0.006

0.004

200

0.002
150

Fig. 6.25 Coordinated PF and Electronic MCCB

Electronic MCCB
characteristic curve

Short-delay tripping
current setting range

400

800

2,000 4,000 6,000

300

1,000

600

Current (A rms)

Characteristics

Deteriorated
curve of PF

10,000 40,000

20,000

150,000

6.8.3 MCCBs and HV-Side OCR

An overcurrent-relay remote tripping device (OCR) on
the HV side of the circuit must be coordinated with
the MCCBs on the LV side. The OCR setting must
take into consideration the coordination with the OCR
at the power-utility substation and, at the same time,
the following:

1. The setting of an OCR with an instantaneous-trip
element must be at least 10 times the transformer
current rating, to ensure that the excitation surge
of the latter does not trip the OCR.

2. To ensure short-circuit protection, the OCR must
operate within 2 seconds, at 25 times the
transformer rated current.

CB or S

CT ratio
150/5

Type MOC-E

CT

6.6kV/210V
3f1000 kVA

NF800-SEP
600A setting

Fig. 6.26 Electronic MCCBs in Coordination with an HV-

Side OCR

Tr

5A tap
dial < 2

OCR

NF800-SEP
800A setting

NF250-CP

For setting the Electronic MCCBs (800 and 600A ver­sions of Type NF800-SEP), the short-delay tripping
currents of both are set to MIN. NF800-SEP have neg­ligible latching inertia, so that the reset characteris­tics (except in the instantaneous-trip region) can be
regarded as the same as the tripping characteristics.
Further, there is very little tolerance variation between
units; thus, the tripping characteristics can be shown
as a single line.

If the NF800-SEP short-delay trip current is set at
MAX (where MIN and MAX respectively correspond
to 2 and 10 times rated current), a 600A rating setting
will correspond to 6000A tripping, and an 800A set­ting will correspond to 8000A tripping. In this case (at
MAX setting), short-delay latching of the NF800-SEP
will overlap the OCR latching (4710A, secondary con­version). But if the NF800-SEP and the OCR are all
set to MIN, so that the latching values do not exceed
4710A, good coordination will be achieved.

As the OCR has an instantaneous-trip element, set
at 30A (secondary conversion 28.3kA), the region of
selective interruption between the OCR and the
NF800-SEP will extend to this value.

Considering the coordination of the Electronic
MCCBs with the lower-level MCCBs (NF250-CP), it

55

can be seen from Fig. 6.27 that the maximum trip curve
(tolerance) of the C Line units matches well with the
NF800-SEP curves, with no danger of overlap.

3 h

Time

1 h

10 min

1 min

10 s

1 s

0.6

0.4

0.2

0.1

0.06

2

NF250-CP

40
30

20

6
4

2

30
20

6
4

2

175A

Max.

Min.

Min.

NF800-SEP
800A setting

Max.

MOC-E tripping curve
CT ratio 150/5
Tap 5
Dial #2

Latching curve

0.02

0.01

0.006

0.004
200 400 800 6,000 20,000 60,0002,000

0.002

150 300 600 1,000 4,000 10,000 40,000 80,000

Current (A, rms)

Inst. trip
setting 30A

Fig. 6.27 Coordinated OCR and Electronic MCCB Characteristics

56

7. SELECTION

Rated currents

MCCB trip curve

Starting current
and long-delay trip

Inrush and
inst. trip

Motor
starting
current

T

MS

I

MS

Current

Time

In selecting MCCBs for a particular application, in
addition to purely electrical aspects of load and distri­bution conductor systems, physical factors such as
panelboard configuration, installation environment,
ambient-temperature variations, vibration, etc. must
also be considered.

MCCBs are rated for an ambient of 40°C, and
where panelboard internal temperatures may exceed
this, the MCCBs installed should be derated in accor­dance with Table 7.1.

1. Actual load currents may exceed the nominal-val-

ues.

2. Load currents may increase with time, due to dete-

rioration of load devices (i.e., friction in motors).

3. Source voltage and frequency may vary.

Table 7.1 MCCB Deratings Due to Installation

Factors

Panelboard max.

internal temp. (°C)

50
55
60

Load allowable, due to

panelboard temp. (%)

90
80
70

7.1 Motor Branch Circuits

The following discussion assumes single motors and
cold-start operation.

7.1.1 General Considerations

The starting current (IMS) and time (TMS) for the mo­tor, and its full-load current, dictate the rated current,
long-delay trip and instantaneous-trip curves for the
MCCB as shown in Fig. 7.2. A safety-margin of up to
50% should be considered for the starting time, to
allow for voltage variations and increase in load fric­tion.

The instantaneous-trip curve should be at least 1.4
x normal starting current to allow for the effect of the
DC component attendant to the low power factor
(about 0.3) of the starting current. For -delta start­ing the unphased-switching allowance increases the

1.4 margin to 1.9. For running restarting the unphased­switching allowance increases the factor to 2.4.

Supply

system

Main,submain
or branch use

Regulations

Ambient

temperature

Service

purpose

Fig. 7.2 MCCB and Motor Starting

7.1.2 Motor Breaker

Ambient

conditions

Installation and

connection style

Wire

connection

?

Where starting times are relatively short and currents
are small, the Mitsubishi M Line motor breakers can
be used without the need for a motor starter.

7.2 For Lighting and Heating Branch Cir-

cuits

In such circuits, switching-surge magnitudes and times

Short circuit

Load

current

Load

Operation
conditions

are normally not sufficient to cause spurious tripping
problems; however, in some cases, such as mercury­arc lamps or other large starting-current equipment,
the methods presented in §7.1 above should be con­sidered.

In general, branch MCCBs should be selected so
that the total of ratings of the connected loads is not
more than 80% of the MCCB rating.

Fig. 7.1 MCCB Selection Consideration

57

7.3 For Main Circuits

2

7.3.1 For Motor Loads

The method of “synthesized motors” is recommended
– that is, the branch-circuit loads to be connected are
divided into groups of motors to be started simulta­neously (assumed), and then each group is regarded
as a single motor having a full-load current of the total
of the individual motors in the group. The groups are
regarded as being sequentially started.

The rating of the branch MCCB for the largest syn­thesized motor is designated IB max., those of the
subsequent synthesized motors as I1, I2, ...I

n-1

. The

rating of the main MCCB becomes:

I

= IB max + (I1 + I2 +...I

MAIN

n-1

) x D

where D is the demand factor (assumed as 1 if inde­terminate).

7.3.2 For Lighting and Heating, and Mixed Loads

For lighting and heating loads the rating of the main
MCCB is given as the total of the branch MCCB rat­ings times the demand factor. For cases where both
motor-load branches and lighting and heating
branches are served by a common main MCCB, the
summation procedures are handled separately, as
described in the foregoing, then grand-totalized to give
the main MCCB rating.

7.4 For Welding Circuits

7.4.1 Spot Welders

A spot welder is characterized by a short, heavy in­termittent load, switched on the transformer primary
side. The following points must be considered in
MCCB selection:

1. The intermittent load must be calculated in terms

of an equivalent continuous current.

2. The excitation transient surge due to the breaker

being on the transformer primary side must be al-

lowed for.

MCCB

Welder

2

W = I

Rt

1

1

and average heat produced:

W

t1 + t

I Rt

1

1

=

t1 + t

2

2

= I

Rβ = R(I1 β )

1

2

2

where β is the duty factor, defined as

total conduction time

total time

This is equivalent to heating by a continuous current of

I

β

1

.

In the example of Fig. 7.4:

I

= I

β = 1200 x 0.0625 = 300 (A)

e

1

i.e., a continuous current of 300A will produce the
average temperature. In practice, however, the instan­taneous temperature will fluctuate as shown in Fig.

7.5 and the maximum value (Tm) will be greater than
the average (Te) that would be produced by a con­tinuous current of 300A. The operation of an MCCB
thermal element depends on the maximum rather than
the average temperature, so it must be selected not
to trip at Tm; in other words, it is necessary to ensure
that its hot-start trip delay is at least as great as the
interval of current flow in the circuit. The rated current
of a “mag-only” MCCB (which does not incorporate a
thermal trip function) can be selected based on the
thermal equivalent current of the load, allowing a
margin of approximately 15% to the calculated value
to accommodate supply-voltage fluctuations, equip­ment tolerance, etc. Thus:

I

= Ie x 1.15 = 300 x 1.15 = 345 (A)

MCCB

The MCCB selected becomes the nearest standard
value above 345A.

Current

t

Time

(Duty factor b = =0.0625)

Fig. 7.4 Welder Intermittent Current

t

(3sec.) (45sec.)

3

3+45

2

1

I

1 = 1200A

Supply

Weld

workpiece

Control
timer

Fig. 7.3 Spot-Welder Circuit

The temperature rise of the MCCB and wiring de­pends on the thermal-equivalent continuous current.
To convert the welder intermittent current into a ther­mal-equivalent continuous value (Ie), consider the
current waveform (Fig. 7.4); load resistance (R) gives
power dissipation:

58

T

m

T

e

Temperature

Time

Fig. 7.5 Temperature Due to Intermittent Current

For practical considerations, rather than basing
selection on welding conditions, the MCCB should be
selected to accommodate the maximum possible duty,
based on the capacity and specifications of the welder.

If the welder rated capacity, voltage and duty fac­tor in Fig. 7.3 are 85kVA, 200V and 50% respectively,
the thermal-equivalent continuous current (Ie) be-

comes:

β

rated capacity

I

= x duty factor

e

rated voltage

85 + 10

=

200

3

x 0.5 = 300A

Hence, the MCCB rated current becomes:

I

= Ie x 1.15 = 300 x 1.15 = 345A

MCCB

(i.e., the next higher standard value).

The relationship between the duty factor, which
does not exceed the working limitations, and the maxi­mum permissible input Iβ at the above duty factor is:

I

300

I =

e

β

=

β

β

If the total period is taken as 60 seconds and the
duty factor is converted into the actual period during
which current flows, the above relationship can be
expressed graphically as in Fig. 7.6. Thus, although
the thermal equivalent current is 300A, the maximum
permissible input current for a duty factor of 50% (30
seconds current flow) is 425A. For a duty factor of

6.25% (3.75 sec current flow) it is 1200A. Even if the
secondary circuit of the welder were short circuited,
however, the resultant primary current would only in­crease by about 30% over the standard maximum
welding current. If this is 400kVA, the maximum pri­mary current I

I

max

β

is:

βmax

standard maximum input

= x 1.3

primary voltage

400 x 10

3

200

x 1.3 = 2600A=

Hence the maximum input current Iβ should be re­stricted to 2600A.

The 75% hot-start characteristic of the 350A Type
NF400-SP breaker is shown by the broken line in Fig.

7.6, and the temperature-rise characteristics up to the
upper limit of the welder, by the solid line. To ensure
protection of the welder from burnout, the delay-trip
characteristic is selected at higher than the solid line;
however, to establish MCCB protection criteria, it is
necessary to look at each welder individually.

Type NF400-SP·350A 75%

30

hot start

10"

7.4.2 MCCB Instantaneous Trip and Trans­former Excitation Surge

When a welding-transformer primary circuit is closed,
depending upon the phase angle at the instant of clo­sure, a transient surge current will flow, due to the
super-imposed DC component and the saturation of
the transformer core.

In order to prevent spurious tripping of protective
devices resulting from such surges, and also to main­tain constant welding conditions, almost all welders
currently available are provided with a synchronized
switch-on function, with or without wave-peak con­trol.

With synchronized switch-on, the measured ratio
between the RMS value of the primary current under
normal conditions and the maximum peak transient
current ranges from √2 ~ 2.

For nonsynchronized soft-starting-type welders the
measured ratio is a maximum of 4.

Maximum instantaneous transient surge excitation
currents for various starting methods are as follows:
Synchronized switch-on welders with wave peak con­trol:

I

= 2 x I

max

max

Synchronized switch-on welders without wave peak
control:

I

= 2 x I

max

βmax

Nonsynchronized switch-on welders with soft start:

I

= 4 x I

max

βmax

Nonsynchronized switch-on welders without soft start:

I

= 20 x I

max

βmax

If synchronized switch-on is employed, the tran­sient surge excitation currents are relatively consis­tent, so that the relationship I

max

= 2 I

βmax

is suffi-

cient.

For a synchronized switch-on type welder of maxi­mum primary input (I

I

= 2 x I

max

βmax

) = 2600A

βmax

= 2 x 2600 = 5200A

Since MCCB instantaneous trip currents are speci­fied in terms of RMS value, I

I

I

= = 3680A

inst

max

2

=

5200

2

The MCCB should be selected so that I

is as follows:

inst

is smaller

inst

than the lower tolerance limit, of the instantaneous
trip current.

3.75

Operating time (s)

0.6

425

Primary input current (A)

Fig. 7.6 Welder Temperature Rise and MCCB Trip Curve

7.4.3 Arc Welders

2"

An arc welder is an intermittent load specified. The
MCCB rating can by selected by converting the load
current into thermal-equivalent continuous current. If
this is taken as the rated current, however, the cur-

300026001200

rent duration per cycle will become relatively long, with
the attendant danger of thermal tripping of the MCCB.
In the total period of 10 minutes, if the duty factor is
50%, a 141% overload exists for 5 minutes; if the duty
factor is 40%, a 158% overload exists for 4 minutes;
and if the duty factor is 20%, a 224% overload exists

59

for 2 minutes. Thus:

I

MCCB

1.2 x P x 10

≥

3

E

where 1.2: Allowance for random variations in

arc-welder current, and supply-volt-

age fluctuations
P: Welder rated capacity (kVA)
E: Supply voltage (V)

The switching transient in the arc welder is mea­sured as 8~9 times the primary current. Consequently,
using 1.2 allowance, it is necessary to select instan­taneous-trip characteristics such that the MCCB does
not trip with a current of 11 times the primary current.

7.5 MCCBs for Transformer-Primary Use

Transformer excitation surge current may possibly
exceed 10 times rated current, with a danger of nui­sance tripping of the MCCB. The excitation surge
current will vary depending upon the supply phase
angle at the time of switching, and also on the level of
core residual magnetism. The maximum is as shown
for switching-point P in Fig. 7.7. During the half cycle
following switch-on the core flux will reach the sum of
the residual flux fr, plus the switching-surge flux 2fm.

The total, 2fm +fr, represents an excitation current in
excess of the saturation value. The decay-time con­stant of this tends to be larger for larger transformer
capacities. Table 7.2 shows typical values of excita­tion surge current, but as these do not take circuit
impedance into account, the actual values will be
larger. If both the primary leakage impedance and cir­cuit impedance are known, the surge current may be
derived by considering the transformer as an air core
reactor; otherwise the values in Table 7.2 should be
used. This table gives maximum values, however, that
are based on the application of rated voltages to rated
taps; it should be noted that supply overvoltage will
result in even larger surges.

Since it is the instantaneous-trip function of the
MCCB that responds to the transient current, ther­mal-magnetic MCCBs, which can more easily be
manufactured to handle high instantaneous-trip cur­rents, are advantageous over completely electromag­netic types, where the instantaneous-trip current is a
relatively small multiple of the rated current.

Table 7.2 Transformer Excitation Surge Currents

Capacity

(kVA)

5
10
15
20
30
50
75

100
150
200
300
500

First 1/2-cycle peak

(multiple)

1ph transformer 3ph transformer

37
37
35
35
34
34
29
28
24
22
18
17

Decay time constant

1

(Hz)

4
4
5
5
6
6
6
6
8
8
9

12

First 1/2-cycle peak

(multiple)

1

26
26
26
26
26
23
18
17
14
13
13
11

Note: 1 “Multiple” means the first 1/2-cycle peak as a multiple of the rated-current peak.
Table 7.3 Transformer Capacities and Primary-Side MCCBs

Tran.

kVA

5

7.5
10
15
20
30
50
75

100
150
200
300
500

1 phase 230V
NF100-SP ( 75)
NF100-SP ( 100)
NF250-SP ( 150)
NF250-SP ( 200)
NF400-SP ( 300)
NF400-SP ( 400)
NF630-SEP ( 600)
NF1000-SS ( 500)
NF1000-SS ( 500)
NF1000-SS ( 800)
NFE2000-S (1200)
NFE2000-S (1500)

—

MCCB Type (rated current (A))

1 phase 400V
NF100-SP ( 40)
NF100-SP ( 60)
NF100-SP ( 75)
NF250-SP ( 125)
NF250-SP ( 150)
NF250-SP ( 225)
NF400-SP ( 400)
NF630-SP ( 500)
NF630-SP ( 630)
NF1000-SS ( 500)
NF1000-SS ( 600)
NF1000-SS ( 900)
NFE2000-S (1400)

3 phase 230V
NF50-HP ( 50)
NF100-SP ( 40)
NF100-SP ( 60)
NF100-SP ( 100)
NF250-SP ( 125)
NF250-SP ( 175)
NF400-SP ( 250)
NF400-SP ( 300)
NF400-SP ( 400)
NF630-SP ( 500)
NF630-SP ( 600)
NF1000-SS ( 900)
NF1600-SS (1400)

Decay time constant

(Hz)

4
4
4
4
4
5
5
5
6
6
8
9

3 phase 400V
NF30-SP ( 30)
NF50-HP ( 40)
NF50-HP ( 50)
NF100-SP ( 50)
NF100-SP ( 60)
NF100-SP ( 100)
NF250-SP ( 150)
NF250-SP ( 175)
NF250-SP ( 225)
NF400-SP ( 300)
NF400-SP ( 350)
NF630-SP ( 600)
NF1000-SS ( 900)

60

Core saturation

1000 P = 3 VI = 2πfCV

2

1000 P = VI = 2πfCV

2

V

t

V

c

C

V

t

V

c

V

c

i

V

c

t

1

t

2

t

3

E

m

E

m

Vc

= –

3Em

V

t

Vc = Em

V

c

= –

7E

m

Vc = 5E

m

Circuit
opening

flux

φ

r

P

Fig. 7.7 Excitation Surge Effects

2φ

m + φR

Transient flux

Surge current

Voltage

Normal flux

In MC CB selection for 400V, 50kVA transformer-

primary used, rated RMS current is:

Capacity (kVA) x 10

I

= = 72.2A

3 x Voltage (V)

3

50 x 10

=

3

3 x 400

From Table 7.2, the peak value of the excitation surge
current Iφ is 23 times that of the rated current, hence:

Iφ =23 x 2 I = 23 x 2 x 72.2A = 2348A

Thus the MCCB selected should have instantaneous
trip current of no less than 2348A. The Type NF250­SP 150A MCCB, with:

I

inst

= 2 x 150 x 11.2 = 2376A

satisfies the above condition. Thus the 3-pole version
of this type is suitable for this application.

Examples of MCCBs selected in this way are shown
in Table 7.3; it is necessary to confirm that the short­circuit capacities of the breakers given are adequate
for the possible primary-side short-circuit current in
each case.

existent; however, some MCCBs do not make and
break so rapidly, and in such cases, if the load ca­pacitance is large enough, they will not discharge
quickly, and if the arc extinguishes near the peak of
the reverse-going oscillation voltage, the capacitor
voltage will be maintained in the region of –3Em by
the first restriking of the arc; at the second restrike it
will become 5Em, on the third –7Em, etc., ultimately
leading to breakdown of the capacitor. Thus, rapid
switching is essential in leading power-factor circuits.

In selecting an MCCB, first consider the surge cur­rent. If the supply voltage is V volts, the capacitor C
farads, the frequency f Hertz and the current Ι amp,
the kVA rating (P) becomes:

For a three-phase system:

For a single-phase system:

Fig. 7.8 Capacitor Circuit

7.6 MCCBs for Use in Capacitor (PF Cor­rection) Circuits

The major surge tendency results from circuit open­ing due to the leading current. If the capacitor circuit
of Fig. 7.8 is opened at time t1 in Fig. 7.8, arc extinc­tion will occur at time t2, the zero-point of the leading
current (i). Subsequently the supply-side voltage (Vt)
will vary normally, but the load-side voltage (Vc) will
be maintained at the capacitor charge value. The po­tential difference (Vc-Vt) will appear across the MCCB
contacts and at time t3, approximately 1/2-cycle after
t2, will become about twice the peak value of the sup­ply voltage (Em). If the MCCB contacts are not suffi­ciently open, an arc will reappear across the gap, re­sulting in an oscillatory capacitor discharge (at a fre­quency determined by the circuit reactance, includ­ing the capacitor) to an initial peak-to-peak amplitude
of 4Em. When the arc extinguishes, Vc will once again
be maintained at a potential of –Em and the potential
difference across the MCCB contacts will increase
again. This cycle will repeat until the gap between the
contacts becomes too great, and the interruption will
be completed.

contact separation, repetitive arcing is virtually non-

Since Mitsubishi MCCBs exhibit extremely rapid

Fig. 7.9 Circuit-Opening Conditions

(q=CV) must be instantaneously supplied to equal the

Fig. 7.10 Accumulative Capacitor Charge

When the switch (Fig. 7.11) is closed, a charge

61

instantaneous supply voltage (V), according to the
phase angle at the instant of circuit closure. This
charge results in a large surge current. If the circuit is
closed at the peak (Em) of the supply voltage (V), the
surge current (i), according to transient phenomena
theory, is:

i =

2 E

4L

C

m

– R

R

t

–

2L

ε

2

sin

4L

C

2L

– R

2

t

From Fig. 7.12, the maximum value (im) is:

under the application of the above current, but selec­tion of an MCCB with an instantaneous-trip current of
greater than

6200

= 4400A is recommended for an

M2

adequate safety margin. Such an MCCB will be rated
at 600A. Accordingly, in this example the Type NF630­SP, rated at 600A, is selected. Table 7.4 is a basis for
selection, but since, in cases where the short-circuit
capacity of the circuit is considerably higher than that
of the MCCB, spurious tripping due to the switching
surge may occur, it is also necessary to make calcu­lations along the lines of the above example.

4L

– R

C

R

4L

C

R

– R

arctan

2

E

i

=

m

–

m

ε

L

C

and appears at time t = t0 where:

4L

t

0

2L

=

4L

C

– R

arctan

2

2

– R

C

R

Although V is not constant, τ0 is extremely small, so
that V = Em can be assumed for the transient dura­tion; similarly, the conduction time can be assumed
as 2τ0. Thus, an MCCB for use in a capacitive circuit
must have an instantaneous-trip current of greater
than im x 2τ0.

Example: MCCB selection for a 3-phase 230V 50Hz

150 kVA capacitor circuit.

From Table 7.4, C = 0.9026 x 10–2 (F) and I =

377(A).

The values of R and L in the circuit must be esti­mated, and for this purpose it is assumed that the
short-circuit current is approximately 100 times the
circuit capacity – i.e., 50,000A.

Z = R2 + (2πfL)2 ∴ 50,000 =

thus: Z = = 2.66 x 10

and assuming:

230

3 x 50,000

2πfL

R

= 5

V

3 Z

–3

then: 2πfL = 2.60 x 10–3 Ω
thus: R = 5.21 x 10–4 Ω L = 8.29 x 10–6 (H)

since: Em =

2

V = 188, im and τ0 can be

3

obtained from their respective formulas as,

im =6200A

τ0 = 4.27 x 10–4 (s).

Since current-flow duration is approximately 2τ0,
an MCCB is selected with a latching time of 0.001
seconds at 6200A. The Type NF630-SP is suitable,
having a latching time of 0.0029 seconds at 10,000A.
Even with a shorter latching time, tripping is unlikely

i

τ

o

iL R

V

c

V

c

C

2

E

m

Fig. 7.11 PF Correction Capacitor

i

m

FIg. 7.12 Currents and Voltages

7.7 MCCBs for Thyristor Circuits

Both overcurrent and overvoltage protection must be
provided for these elements. MCCBs can be used
effectively for overcurrent, although application de­mands vary widely, and selection must be made care­fully in each case. Overvoltage protection must be
provided separately; devices currently in use include
lightning arresters, dischargers, RC filters and oth­ers.

1. MCCB Rated Currents
A primary factor determining the rated current of the
MCCB to be used is the question of AC-side or DC­side installation. AC-side installation permits a lower
rating, which is a considerable advantage. Fig. 7.13
shows both AC and DC installation (MCCBs 1 and 2);
Table 7.5 gives a selection of circuit formats and cur­rent configurations; using this table it is possible to
determine the MCCB rating for either MCCB 1 or 2,
as required. The current curve of the thyristor (aver­age current is usually given) and the tripping curve of
the MCCB should be rechecked to ensure that there
is no possibility of overlap.

When an overcurrent is due to a fault in the load,
causing a danger of thermal destruction of the circuit
elements, either AC or DC protection is adequate,
provided the parameters are properly chosen. When

62

Table 7.4 MCCB Selection for Circuits with PF-Correction
a) 230V, 50Hz Circuit

Capacitor rating

kVA

5
10
15
20
25
30
40
50
75

100
150
200
300
400

301
602

903
1203
1504
1805
2407
3009
4513
6017
9026

12034
18052
24069

Single-phase circuit

Capacitor

µF

rated

current

(A)

21.7

43.5

65.2

87.0

108.7

130.4

173.9

217.4

326.1

434.8

652.2

869.6

1304.3

1739.1

MCCB

rated

current

(A)

40

75
100
125
175
200
250
350
500
700

1000
1400
2000
2500

Three-phase circuit

Capacitor

rated

current

(A)

12.6

25.1

37.7

50.2

62.8

75.3

100.4

125.5

188.3

251.0

376.5

502.0

753.1

1004.1

MCCB

rated

current

(A)

20
40
60

75
100
125
150
200
300
400
600
800

1200
1500

c) 400V, 50Hz Circuit

Capacitor rating

Single-phase circuit

Capacitor

kVA

5
10
15
20
25
30
40
50
75

100
150
200
300
400

µF

99
199
298
398
497
597
796
995

1492
1989
2984
3979
5968
7958

rated

current

12.5

25.0

37.5

50.0

62.5

75.0

100.0

125.0

187.5

250.0

375.0

500.0

750.0

1000.0

(A)

MCCB

rated

current

(A)

20
40
60

75
100
125
150
200
300
400
600
800

1200
1500

Three-phase circuit

Capacitor

rated

current

(A)

7.2

14.4

21.7

28.9

36.1

43.3

57.7

72.2

108.3

144.3

216.5

288.7

433.0

577.4

MCCB

rated

current

(A)

15
30
40
50
60

75
100
125
175
225
350
500
700
900

b) 230V, 60Hz Circuit

Capacitor rating

Single-phase circuit

Capacitor

kVA

5
10
15
20
25
30
40
50
75

100
150
200
300
400

µF

251
501

752
1003
1254
1504
2006
2507
3761
5014
7522

10029
15043
20057

rated

current

21.7

43.5

65.2

87.0

108.7

130.4

173.9

217.4

326.1

434.8

652.2

869.6

1304.3

1739.1

(A)

MCCB

rated

current

(A)

40

75
100
125
175
200
250
350
500
700

1000
1400
2000
2500

Three-phase circuit

Capacitor

rated

current

(A)

12.6

25.1

37.7

50.2

62.8

75.3

100.4

125.5

188.3

251.0

376.5

502.0

753.1

1004.1

MCCB

rated

current

(A)

20
40
60

75
100
125
150
200
300
400
600
800

1200
1500

d) 400V, 60Hz Circuit

Capacitor rating

Single-phase circuit

Capacitor

kVA

5
10
15
20
25
30
40
50
75

100
150
200
300
400

µF

83
166
249
332
414
497
663
829

1243
1658
2487
3316
4974
6631

rated

current

12.5

25.0

37.5

50.0

62.5

75.0

100.0

125.0

187.5

250.0

375.0

500.0

750.0

1000.0

(A)

MCCB

rated

current

(A)

20
40
60

75
100
125
150
200
300
400
600
800

1200
1500

Notes: 1. The MCCB rated current should be approx. 150% of the capacitor rated current.

2. The MCCB short-circuit capacity should be adequate for the circuit short-circuit capacity.

the fault is in one of the thyristor elements, resulting
in reverse current, the result is often that other circuit
elements will be destroyed (see Fig. 7.14) if the cir­cuit is not interrupted immediately. In this case AC­side protection or protection in series with each ele­ment is necessary.

2. Tyristor Overcurrent Protection
Total protection of each element is possible in theory,
but in practice overall coordination and the best com­promise for economy are usually demanded. Where
elements are critical, complex combinations of pro­tective devices can be employed, at proportionally
higher cost.

Basically, overcurrent leads to excessive tempera­ture rise of the thyristor junction, resulting in loss of
the control function, and thermal destruction. A fault,
therefore, must be interrupted as quickly as possible,
before the junction temperature rises above its speci­fied limit. In the overcurrent region, designated on the
current-surge withstand curves of the circuit element,
the element can usually withstand the surge for at
least one cycle. The current-surge withstand, gener­ally specified as a peak value, must be converted to
RMS, to select a suitable MCCB.

An overload of short-circuit proportion, either ex­ternal or in a bridge-circuit thyristor element, necessi-

Three-phase circuit

Capacitor

rated

current

(A)

7.2

14.4

21.7

28.9

36.1

43.3

57.7

72.2

108.3

144.3

216.5

288.7

433.0

577.4

MCCB

rated

current

(A)

15
30
40
50
60

75
100
125
175
225
350
500
700
900

63

Load

MCCB1

MCCB2

Fig. 7.13 AC- and DC-side Protectors for Thyristors Fig. 7.14 Fault-Current Flow

Table 7.5 Thyristor Circuits and Current Formats

Circuit No. I Circuit No. II Circuit No. III Circuit No. IV

Fault
element

Load

Circuit diagram

Element average current

(A)

I

F

Element RMS current

(A)

I

e

Average DC current

(A)

I

D

RMS current

I

(A)

B

MCCB1MCCB2

Current
waveform

MCCB1

Load

I

P

π

I

P

2

I

F

π

I

2

or

π

I

2

MCCB1

MCCB1

MCCB2

MCCB2

π

M2

π

2M2

Load

I

P

π

I

P

2

2I

F

I

(6 2.22 IF)

F

or

I

(6 1.11 ID)

D

Load

I

P

π

I

P

2

2I

F

π

F

D

I

F

2

or

π

I

D

4

MCCB1

(6 0.552 IP)

π

(6 2.45 IF)

π

3

MCCB2

Load

I

P

π

14πM3

+

6

3I

F

M3

1

+

3

2π

or

1

+

3

M3
2π

I

P

I

F

I

D

(6 0.817 ID)

I

I

P



I

P

P

I

P

Current flow

RMS current

Current
waveform

I

e

I

(A)

B

or

π

I

D

2

I

P

π

M2

π

2M2

I

F

or

I

D

I

P

Note: Load is assumed resistive, with elements conductive through 180°.

64

π

M2

π

2M2

1

3M3

π

+ I

I

F

2

or

1

I

D

π

2

3

I

P

4π

or

3M3

+ I

4π

6 3I

F

D

6 I

F

D

I

P

tates rapid interruption of the circuit. Normally, such
interruption takes place within one cycle; thus, from
the point of view of element thermal destruction, the
time integral of the current squared must be consid­ered. Quantitatively, the permissible ei2dt of the ele­ment must be greater than the ei2dt of the MCCB cur­rent through interruption, converted to apply to the
element. The latter is influenced by the short-circuit
current magnitude, the interruption time, and the cur­rent-limiting capability of the MCCB.

It is important to note that the MCCB interruption
time will be considerably influenced by the short-cir­cuit current rise rate, di/dt, on the load side. In the
short circuit of Figs. 7.15 and 7.16, the current is:

E

i = (1 – ε )

R

–t

L

and the current rise rate di/dt is:

di

( )

dt

t=0

=

E

L

Thus, the inductance of the line, and the smoothing
inductance significantly affect di/dt. Where the poten­tial short-circuit current is very large, the inductance
should be increased, to inhibit the rise rate and assist
the MCCB to interrupt the circuit in safe time. This is
illustrated in Fig. 7.17, for MCCB2 of Fig. 7.15.

The MCCB current during total time (tT) is ei2 dt,
which, converted to the ei2 dt applied to the circuit
element, must be within the limit specified. Having
determined the circuit constants, testing is preferable
to calculation for confirmation of this relationship.

Assuming a large current-rise rate, with an AC-side
short-circuit current i = Ipssin ωt, and an MCCB inter­ruption time of one cycle, the ei2 dt applied to the thy­ristor is as follows:

1. For circuits I, II and III of Table 7.10:

1

ei2dt = e I

2f

0

2

sin2 ωtdt = I

p

4f

1

2

(A2s)

p

2. For circuit IV:

1

ei2dt = 2e I

3f

1

6f

2

sin2 ωtdt =

p

2

I

p

( )

f

3

+(A

614π

2

s)

where Ip is the peak value of the element current and
f is the supply frequency.

If the ei2 dt of the circuit element is known, the per­missible ei2dt for the MCCB can be determined, us­ing the last two equations given above. Provided that
the interruption time is not greater than one cycle, the
MCCB current will be the same as the element cur­rent for circuits I and II, and twice that for circuits III
and IV. This means that the MCCB ei2dt through the
interruption time should be within twice the permis­sible ei2dt of the element.

Diodes are generally stronger against overcurrent
than thyristors, and since diodes can handle larger

2

I

·t, protection is easier.
Fig. 7.17 shows the protection coordination situa-

tion of a selection of devices, plotted together with
the thyristor current-surge withstand curve. AC-side

protection (MCCB1, Fig. 7.15) is presented, but the
DC-protection case (MCCB2) can be plotted in the
same way.

Region 2 in Fig. 7.17 is the area of overcurrent for
which protection is effected by the MCCB. For pro­tection of region 1, an overload relay is effective, and
for region 2, inductance L must be relied on to limit
the fault-current rise rate, or a high-speed current-lim­iting fuse must be used. Practical considerations, in­cluding economy and the actual likelihood of faults in
the regions concerned, may dictate the omission of
the protective devices for regions 1 and 3, in many
cases. The lower the instantaneous-trip setting of the
MCCB, the wider the region 2 coverage becomes.

Smoothing inductance

L

R

MCCB1

E

MCCB2

Short
circuit

Fig. 7.15 Thyristor Short Circuit

Short-circuit current

MCCB

Trip current

q

t

1

t

: Time to MCCB latching

1

: MCCB opening time

t

2

: Time from contact parting to

t

3

current peak value

: Arc duration

t

4

: Total interruption time

t

T

q : Current-rise rate

Fig. 7.16 Thyristor Short-Circuit Interruption

t

3

t

2

t

4

t

T

Arc voltage

Circuit voltage

Load

65

2
1

h

30
20

14
10

8
6
4

2

min

1

30
20

10

5

Tripping time

2
1

s

0.5

0.2

0.1

0.05

0.02

0.01

3-Phase Fullwave Rectification
MCCB: Mag-Only

Overcurrent-relay

High-speed current-limiting fuse

MCCB tripping

Thyristor current-surge withstand

Region 1

Region 3

Region 2

125 200 300 400500 600700 1000 1500 2000 3000 4000

100

Current (% of rating)

Fig. 7.17 Thyristor and Protector Operating Curves

3. Element Breakdown in Thyristor-Leonard Systems
In this system of DC motor control, if power outage or
commutation failure due to a thyristor control-circuit
fault occurs during inversion (while motor regenera­tive power is being returned to the AC supply), the
DC motor, acting as a generator while coasting, will
be connected to a short-circuit path, as in Fig. 7.18.
For thyristor protection, MCCBs must be placed in the
DC side, as shown.

A Mag-Only MCCB with a tripping current of about
3 times the rated current is employed, either 3- or 4­pole, series-connected as shown in Fig. 7.20. Since
the element short-circuit current is the same as the
MCCB current, circuit protection is effected provided
that the ei2dt limit for the element is larger than that
for the MCCB interruption duration. This must be es­tablished by test.

Commutation element failure

M

Short-circuit path in a power outage

Short-circuit path in a commutation element failure

Fig. 7.18 Ward-Leonard Thyristor Protection

High-speed fuses

AC
supply

M

M

M

b) 4-pole MCCB a) 3-pole MCCB

Fig. 7.19 High-Speed Fuses for Thyristor-Circuit Protection Fig. 7.20 Series Connection of MCCB Poles

66

Fig. 7.19 shows connection of high-speed fuses
for protection against thyristor breakdown that would
otherwise result in short-circuit flow from the AC sup­ply side.

4. MCCBs for Lamp Mercury-Lamp Circuits
The ballasts (stabilizers) used in this type of lamp
cover a variety of types and characteristics. For 200V
applications (typical), choke-coil ballasts are used. For
100V applications a leakage-transformer ballast is
employed. Normal ballasts come in low power-factor
versions and high power-factor versions, with correc­tion capacitors. More sophisticated types include the
constant-power (or constant-output) type, which main­tains constant lamp current both in starting and nor­mal running, and flickerless types, which minimize the
flicker attendant on the supply frequency.

In selecting an MCCB where normal (high or low
PF) ballasts are to be used, the determining factor is
the starting current, which is about 170% of the stable
running current. In the cases of constant-power or
flickerless types, the determining factor is the normal
running current, which is higher than the starting cur-

rent. For MCCB selection, the latter types can be re­garded as lighting and heating general loads, as pre­viously discussed.

For selection of MCCBs for regular ballasts, the
170% starting current is assumed to endure for a
maximum of 5 minutes. MCCBs of 100A or less frame
size have a tripping value very close to rating for over­loads of duration of this order, so that the MCCB rat­ing should be the nearest standard value above 170%
of the stable running current. MCCBs of above 100A
frame size can handle a current of around 120% of
the rating for 5 minutes without tripping; thus the near­est standard MCCB rating above

1.7

= 1.4 times the

1.2

stable-running current of the lamp load is the suitable
protector.

As an example, consider MCCB selection for 10
units of 100W, 100V, 50Hz general-purpose high
power-factor mercury lamps. The stable-running cur­rent per lamp is 1.35A. Thus:

1.35 x 10 x 1.7 = 23A, and the selection becomes

NF30-SP, 30A rated.

67

7.8 Selection of MCCBs in inverter circuit

7.8.1 Cause of distorted-wave current

Distorted-wave current is caused by factors such as the CVCF device of a computer power unit, various recti­fiers, induction motor control VVVF device corresponding to more recent energy-saving techniques, etc, wherein
thyristor and transistor are used. Any of these devices generates DC power utilizing the switching function of a
semiconductor and, in addition, transforms the generated DC power into intended AC power. Generally, a large
capacity capacitor is connected on its downstream side from the rectification circuit for smoothing the rectifica­tion, so that the charged current for the capacitor flows in pulse form into the power circuit. Because voltage is
chopped at high frequency in AC to DC transforming process, load current to which high frequency current was
superimposed by chopping basic frequency flows into the load line. This paragraph describes the VVVF in­verter, of these devices, which will develop further as main control methods for induction motors currently in
broad use in various fields . Fig. 7.27 illustrates an example of MCCBs application to inverter circuit. Two
control methods of PAM (Pulse Amplitude Modulation) and PWM (Pulse Wide Modulation) are available for the
VVVF inverter and generating higher harmonic wave components differs depending on the difference between
the control methods. As seen from Tables 7.9 and 7.10, this harmonic wave component of input current can be
made smaller (improved) by inputting DC reactor (DCL) or AC reactor (ACL). Further, in the case of the output
current waveform in Fig. 7.29, the PWM generates higher harmonic wave components than that of the PAM.

Inverter

Induction motor

M

MCCB

Fig.7.27 Example of MCCBs Application to Inverter Circuit

7.8.2 Selection of MCCBs

MCCBs characteristic variations and temperature rises dependent on distortion of the current wave must be
considered when selecting MCCBs for application to an inverter circuit (power circuit). The relation of rated
current I

I

MCCB

MCCB

Q K x I

to load current I of MCCBs is selected as follows from the MCCBs tripping system.

Thermal-magnetic type (bimetal system) and electronic type (RMS value detection) are both RMS current
detection systems which enable exact overload protection even under distorted-wave current. Due to the above
explanation, it is advantageous to select RMS current detection type MCCBs.

Table 7.8 Reduction Rate

MCCBs tripping system

Thermal-magnetic (bimetal system)

(Note 2) Thermal-magnetic (CT system)

(Note 1) Hydraulic-magnetic

Electronic (RMS value detection)

(Note 3) Electronic (Peak value detection)

Reduction

rate K

1.4

2

1.4

1.4

2

This table is subject to the current which meets the
following requirements.

RMS value of total harmonic

q

Distortion factor

w Peak factor =

e Higher harmonic wave components are mainly No.7 or a lower

harmonic wave.

=

wave component

RMS value of basic frequency

Peak value

RMS value

x 100 q 100% or less

q 3 or less

Notes: 1. The characteristics of hydraulic-magnetic type MCCBs vary significantly depending on wave distor-

tion. Therefore, use of thermal-magnetic type MCCBs is recommended.

2. NF2000-S, NF2500-S, NF3200-S, NF4000-S

3. NFE2000-S, NFE3000-S, NFE4000-S

68

Table 7.9 Data of High Harmonic Wave Current Content in Inverter Power Circuit (Example)

High harmonic wave current content (%)

High harmonic

wave degree

Basic

2
3
4
5
6
7
8

9
10
11
12
13

No ACL (Standard)

81.6
_

3.7
_

49.6
_

27.4
_

_
_

7.6
_

6.7

P W M

With power factor modifying ACL With power factor modifying ACL

97.0
_

_
_

21.9
_

7.1
_

_
_

3.9
_

2.8

With standard ACL

83.6
_

2.5
_

48.3
_

23.7
_

_
_

6.2
_

4.7

P A M

Note: No DCL Output frequency 60Hz , subject to 100% load

Table 7.10 Peak Factor of Inverter Input Current

97.2
_

_
_

21.7
_

7.0
_

_
_

3.7
_

2.6

Input current

Circuit

V

with ACL

Large ACL Small

With
DCL

V

ACL

DCL

d

E

d

E

Power factor

Below 58.7

58.7%

58.7–83.5%

83.5%

83.5–95.3%

Waveform factor Peak factor

Above 1.99

1.99

1.99–1.27

1.27

1.27–1.23

95.3% 1.23 1.28

Power factor = (DC voltage x DC) /( 3 x AC RMS voltage x AC RMS current)
Waveform factor = (RMS value) /(Mean value)
Peak factor = (Max value) /(RMS value)

Above 2.16

2.16

2.16–1.71

1.71

1.71–1.28

Waveform

(half wave portion)

I

t

(a) PAM system (b) PWM system

(a) PAM system (b) Equal-value PWM system

Fig.7.28 Inverter Input Current Fig.7.29 Inverter Output Current

69

8. ENVIRONMENTAL CHARACTERISTICS

8.1 Atmospheric Environment

Abnormal environments may adversely affect perfor­mance, service life, insulation and other aspects of
MCCB quality. Where service conditions differ sub­stantially from the specified range as below, derating
of performance levels may result.

1.

Ambient temperature range

2. Relative humidity 85% max. with no dewing

3. Altitude 2,000m max.

4. Ambient No excessive water or oil

Table 8.1 Abnormal Environments, and Countermeasures

Environment

High temperature

–10˚C~+40˚C (Average
temperature for 24 hours,
however, shall not be
higher than 35˚C.)

vapour, smoke, dust, salt
content, corrosive sub­stance, vibration, and im­pact
Expected service life
(MTTF) under the above
conditions is 15 years.

Trouble

8.1.1 High Temperature Application

To comply with relevant standards, all circuit break­ers are calibrated at 40˚C. If the circuit breaker is to
be used in an environment where the ambient tem­perature is likely to exceed 40˚C please apply the de­rating factor shown in table 8.2.
For example: To select a circuit breaker for use on a
system where the full load current is 70A in an ambi­ent temperature at 50˚C then from table 8.2

70A

= 77.8A

0.9

Select a circuit breaker with a trip unit adjustable from
80-100A or fixed at 100A.

Table 8.2 MCCB Derating

Ambient Temperature (°C)

50
55
60

Derating factor

0.9

0.8

0.7

Countermeasures

Low temperature

High humidity

High altitude

Dirt and dust

5:

1. Nuisance tripping

2. Insulation deterioration

1. Condensation and freezing

2. Low-temperature fragility in shipping
(around –40˚C)

1. Insulation resistance loss

2. Corrosion

1. Reduced temperature, otherwise no
problem up to 2,000m

1. Contact discontinuity

2. Impaired mechanism movement

3. Insulation resistance loss

1. Reduce load current (derate).

2. Avoid ambients above 60˚C.

1. Install heater for defrosting and drying.

2. Ship tripped, or if not possible, OFF.

1. Use MCCB enclosure such as Type W.

2. Inspect frequently, or install high­corrosion-resistant MCCBs.

1. See “Low temperature”, above.

1. Use Type I MCCB enclosure.

Corrosive gas, salt air

70

1. Corrosion

1. Use Type W MCCB enclosure or install
high-corrosion-resistant MCCBs.

Altitude

3000m
4000m
5000m
6000m

Rated current

0.98

0.96

0.94

0.92

Rated voltage

0.91

0.82

0.73

0.65

8.1.2 Low Temperature Application

In conditions where temperatures reach as low as
–5˚C special MCCBs are usually required. Mitsubishi,
however, have tested their standard MCCBs to tem­peratures as low as –10°C without any detrimental
effects.

For conditions where temperatures drop below

–10˚C special MCCBs must be used.

If standard MCCBs experience a sudden change
from high temperature, high humidity conditions to low
temperature conditions, there is a possibility of ice
forming inside the mechanism. In such conditions we
recommend that some form of heating be made avail­able to prevent mal-operation.

In conditions of low temperature MCCBs should
be stored in either the tripped or OFF position.

Low Temperature MCCBs

Special low temperature MCCBs are available that
can withstand conditions where temperatures fall to
as low as –40˚C. These special MCCBs are available
in sizes up to 1200A in the standard series and above
50A in the compact series.

8.1.3 High Humidity

In conditions of high humidity the insulation resistance
to earth will be reduced as will the electrical life.

For applications where the relative humidity ex­ceeds 85% the MCCB must be specially prepared or
special enclosures used. Special preparation includes
plating all metal parts to avoid corrosion and special
painting of insulating parts to avoid the build up of
mildew.

There are two degrees of tropicalisation:

Treatment 1- painting of insulating material to avoid

build up of mildew plus special plating
of metal parts to avoid corrosion.

Treatment 2- painting of insulating material to avoid

build up of mildew only.

8.1.4 Corrosive Atmospheres

In the environment containing much corrosive gas, it
is advisable to use MCCB of added corrosion resis­tive specifications.

For the breakers of added corrosionproof type,
corrosion-proof plating is applied to the metal parts.

Where concentration of corrosive gas exceeds the
level stated below, it is necessary to use MCCB of
added corrosion resistive type being enclosed in a
water-proof type enclosure or in any enclosure of pro­tective structure.
Allowable containment for corrosive gas.

H2S 0.01ppm SO20.05ppm
NH30.25ppm

8.1.5 Affecting of Altitude

When MCCBs are used at altitudes exceeding 2000m
above sea level, the effects of a drop in pressure and
drop in temperature will affect the operating perfor­mance of the MCCBs. At an altitude of 2200m, the air
pressure will drop to 80% and it drops to 50% at

5500m, however interrupting capacity is unaffected.
The derating factors that are applicable for high alti­tude applications are shown in table 8.3. (According
to ANSI C 37.29-1970)

Table 8.3 Derating Factors for High Altitude Appli-

cations

For example: NF800-SEP on 4000m

1. Voltage
The rated operating voltage is AC690V. You should

derate by 690x0.82=565.8V. It means that you can
use this NF800-SEP up to AC565.8V rated voltage.

2. Current
The rated current is 800A. You should derate by

800x0.96=768A. It means that you can use this
NF800-SEP up to 768A rated current.

8.2 Vibration-Withstand Characteristics

8.2.1 The Condition of Test

1. Installation position and Direction of vibration

• Every vertical and horizontal at vertical installed

(as shown in Fig. 8.1)

2. The position of MCCBs and vibration time
Forty minutes in each position (ON, OFF and TRIP)

3. Vibration criteria

• Frequency 10~100Hz

• Vibration acceleration 22 m/s

• Period 10min./cycle

8.2.2 The Result of Test

The samples must show no damage and no change
of operating characteristic (200% release), and must
not be tripped or switched off by the vibration.

Horizontal

Vertical

Wire
connection

Fig. 8.1 Applied Vibration

2

71

8.3 Shock-Withstand Characteristics

8.3.1 The Condition of Test

1. MCCBs are drop-tested, as described in Fig. 8.2.
The arrows show the drop direction.

2. The samples are set to ON, with no current flow­ing.

8.3.2 The Result of Test (as Shown in Table 8.4)

The samples must show no physical damage, and
the switched condition must not be changed by the
drop in any of the drop-attitudes tested.

Line terminals

Line terminals

The judgment of failure:

• A case the switched condition changed from ON

to OFF

• A case the switched condition changed from ON

to Trip

• A case the sample shows physical damage

Table 8.4 Shock-Withstand Characteristics of Mitsubishi MCCB

Series

BH

BH-K BH-P, BH-S, BH-PS, BH-D

Type

MB30-CS

MB

MB30-SP MB50-CP MB50-SP
MB100-SP MB225-SP

NF30-SP NF50-HP
NF50-HRP NF60-HP NF100-SP NF100-SEP
NF100-HP NF100-HEP NF160-SP NF160-HP
NF250-SP NF250-SEP NF250-HP NF250-HEP
NF400-SP NF400-SEP NF400-HEP NF400-REP

S

NF630-SP NF630-SEP NF630-HEP NF630-REP
NF800-SDP NF800-SEP NF800-HEP NF800-REP
NF1000-SS NF1250-SS
NF1600-SS NF2000-S NF2500-S NF3200-S
NF4000-S NFE2000-S NFE3000-S NFE4000-S

NF

NF30-CS

Fig. 8.2 Drop-Test Attitudes

No tripped

2

)

(m/s

147
147

196

196

147

No damage

(m/s2)

490

72

C

NF50-CP NF60-CP
NF100-CP NF250-CP NF400-CP NF630-CP

NF800-CEP

196

196

NF100-UP NF100-RP NF225-UP

U

NF225-RP NF400-UEP NF630-UEP

196

NF800-UEP NF1250-UR

9. SHORT-CIRCUIT CURRENT CALCULATIONS

9.1 Purpose

Japanese and international standards require, in sum­mary, that an overcurrent protector must be capable
of interrupting the short-circuit current that may flow
at the location of the protector. Thus it is necessary to
establish practical methods for calculating short-cir­cuit currents for various circuit configurations in low­voltage systems.

9.2 Definitions

1. % Impedance
The voltage drop resulting from the reference current,
as a percentage of the reference voltage (used for
short-circuit current calculations by the % impedance
method).

% impedance =

(Reference voltage:3-phase – phase voltage)

2. Reference Capacity
The capacity determined from the rated current and
voltage used for computing the % impedance (nor­mally 1000kVA is used).

3. Per-Unit Impedance
The % impedance expressed as a decimal (used for
short-circuit current calculations by the per-unit
method).

4. Power Supply Short-Circuit Capacity
3-phase supply (MVA) = kl3 x rated voltage (kV) x

5. Power Supply Impedance
Impedance computed from the short-circuit capacity
of the supply (normally indicated by the electric power
company; if not known, it is defined, together with the
X/R ratio, as 1000MVA and X/R=25 for a 3-phase
supply (from NEMA.AB1).

6. Motor contribution Current
While a motor is rotating it acts as generator; in the
event of a short circuit it contributes to increase the
total short-circuit current. (Motor current contribution
must be included when measuring 3-phase circuit
short-circuit current).

7. Motor Impedance
The internal impedance of a contributing motor. (A
contributing motor equal to the capacity of the trans­former is assumed to be in the same position as the
transformer, and its % impedance and X/R value are
assumed as 25% and 6 (from NEMA.AB1).

8. Power Supply Overall Impedance
The impedance vector sum of the supply (ZL), the
transformer (ZT) and the motor (ZM).
Overall impedance of 3-phase supply

(Zs) = (%Ω)

9. Short-Circuit Current Measurement Locations
In determining the interruption capacity required of

voltage drop at capacity load

reference voltage

short circuit current (kA)

(ZL + ZT) • Z

ZL + ZT + Z

M

M

x 100 (%)

the MCCB, generally, the short-circuit current is cal­culated from the impedance on the supply side of the
breaker.
Fig. 9.1 represents a summary of Japanese standards.

9.3 Impedances and Equivalent Circuits of
Circuit Components

In computing low-voltage short-circuit current, all im­pedances from the generator (motor) to the short-cir­cuit point must be included; also, the current contrib­uted by the motor operating as a load. The method is
outlined below.

9.3.1 Impedances

1. Power Supply Impedance (ZL)

The impedance from the power supply to the trans­former-primary terminals can be calculated from the
short-circuit capacity specified by the power company,
if known.
Otherwise it should be defined, together with X/R, as
1000MVA and X/R=25 for a 3-phase supply. Note that
it can be ignored completely if significantly smaller
than the remaining circuit impedance.

2. Transformer Impedance (ZT)

Together with the line impedance, this is the largest
factor in determining the short-circuit current magni­tude. Transformer impedance is designated as a per­centage for the transformer capacity; thus it must be
converted into a reference-capacity value (or if using
Ohm’s law, into an ohmic value).
Tables 9.1 show typical impedance values for trans­formers, which can be used when the transformer
impedance is not known.

3. Motor Contribution Current and Impedance (ZM)

The additional current contributed by one or more
motors must be included, in considering the total 3­phase short-circuit current. Motor impedance depends
on the type and capacity, etc.; however, for typical
induction motors, % impedance can be taken as 25%
and X/R as 6. The short-circuit current will thus in­crease according to the motor capacity, and the im­pedance up to the short-circuit point. The following
assumptions can normally be made.
a. The total current contribution can be considered

as a single motor, positioned at the transformer
location.

b. The total input (VA) of motor contribution can be

considered as equal to the capacity of the trans­former (even though in practice it is usually larger).
Also, both the power factor and efficiency can be
assumed to be 0.9; thus the resultant motor contri­bution output is approximately 80% of the trans­former capacity.

c. The % impedance of the single motor can be con-

sidered as 25% and the X/R as 6.

73

MCCB

Load

Supply side

Load terminal in the case of
insulated line (the line
impedance on the MCCB
load side can be added.)

MCCB load terminals in the case of bare line (the line
impedance on the MCCB load side may not be added).

Fig. 9.1 Short-Circuit Locations for Current Calculations

Table 9.1 Impedances of 3-Phase Transformers

Transformer

capacity (kVA)

50

75
100
150
200
300
500
750

1000
1500
2000

Impedance (%)

%R

1.81

1.78

1.73

1.61

1.63

1.50

1.25

1.31

1.17

1.23

1.13

%X

1.31

1.73

1.74

1.91

2.60

2.82

4.06

4.92

4.94

5.41

5.89

4. Line and Bus-Duct Impedance (ZW, ZB)
Table 9.2 gives unit impedances for various configu­rations of wiring, and Table 9.3 gives values for duct­ing.
Since the tables give ohmic values, they must be con­verted, if the %-impedance method is employed.

Table 9.2 Wiring Impedance

Cable size

(mm2)

1.5

2.5

4.0

6.0

10.0

16.0

25.0

35.0

50.0

70.0

95.0

120.0

150.0

185.0

240.0

300.0

400.0

500.0

630.0

Resistance

(mΩ/m)

12.10

7.41

4.61

3.08

1.83

1.15

0.727

0.524

0.387

0.268

0.193

0.153

0.124

0.0991

0.0754

0.0601

0.0470

0.0366

0.0283

2-or 3-core

cables

0.1076

0.1032

0.0992

0.0935

0.0873

0.0799

0.0793

0.0762

0.0760

0.0737

0.0735

0.0720

0.0721

0.0720

0.0716

0.0712
–
–
–

50Hz

1-core cables

(close-spaced)

0.1576

0.1496

0.1390

0.1299

0.1211

0.1043

0.1014

0.0964

0.0924

0.0893

0.0867

0.0838

0.0797

0.0806

0.0818

0.0790

0.0777

0.0702

0.0691

5. Other Impedances
Other impedances in the path to the short-circuit point
include such items as CTs, MCCBs, control devices,
and so on. Where known, these are taken into con­sideration, but generally they are small enough to be
ignored.

Reactance(mW/m)

60Hz

1-core cables

(6cm-spaced)

0.2963

0.2803

0.2656

0.2527

0.2369

0.2138

0.2000

0.1879

0.1774

0.1669

0.1573

0.1498

0.1427

0.1356

0.1275

0.1195

0.1116

0.1043

0.0964

2-or 3-core

cables

0.1292

0.1238

0.1191

0.1122

0.1048

0.0959

0.0952

0.0915

0.0912

0.0884

0.0882

0.0864

0.0865

0.0864

0.0859

0.0854
–
–
–

1-core cables

(close-spaced)

0.1891

0.1796

0.1668

0.1559

0.1453

0.1251

0.1217

0.1157

0.1109

0.1072

0.1040

0.1006

0.0956

0.0967

0.0982

0.0948

0.0932

0.0843

0.0829

1-core cables
(6cm-spaced)

0.3555

0.3363

0.3187

0.3033

0.2843

0.2565

0.2400

0.2254

0.2129

0.2001

0.1888

0.1798

0.1712

0.1627

0.1530

0.1434

0.1339

0.1252

0.1157

Notes: 1. Resistance values per IEC 60228

2.

Reactance per the equation: L(mH/km) = 0.05 + 0.4605log10D/r(D=core separation, r=conductor radius)

3. Close-spaced reactance values are used.

Table 9.3 Bus-Duct Impedance

Rated

current (A)

400
600
800

1000
1200
1500

2000
2500
3000

74

Resistance
(mΩ/m) at 20°C

0.125

0.114

0.0839

0.0637

0.0397

0.0328

0.0244

0.0192

0.0162

Reactance (mΩ/m)

50Hz

0.0250

0.0231

0.0179

0.0139

0.0191

0.0158

0.0118

0.0092

0.0077

0.0300

0.0278

0.0215

0.0167

0.0230

0.0190

0.0141

0.0110

0.0092

60Hz

9.3.2 Equivalent Circuits

1. Three-Phase
Based on the foregoing assumptions for motors, the
equivalent circuits of Fig. 9.2 can be used for calcu­lating 3-phase short-circuit current. The motor imped­ance (ZM) can be considered as shunting the series
string consisting of the supply (ZL) and transformer
(ZT) impedances, by busbars of infinite short-circuit
capacity. When the three impedances are summed,
the total impedance and the resistive and reactive
components are given as:

(ZL + ZT) · Z

ZS = = RS + j X

ZL + ZT + Z

M

M

S

(RL + RT + RM) {RM(RL + RT) – XM(XL + XT)}

[

+ (XL + XT + XM) {XM(RL + RT) + RM(XL + XT)}

RS =

(RL + RT + RM)2 + (XL + XT + XM)

(RL + RT + RM) {XM(RL + RT) + RM(XL + XT)}

[

– (XL + XT + XM) {RM(RL + RT) – XM(XL + XT)}

XS =

(RL + RT + RM)2 + (XL + XT + XM)

L

Z

L

Thus, when calculating the short-circuit current at
various points in a load system, if the value ZS is first
computed, it is a simple matter to add the various wire
or bus-duct impedances. Table 9.4 gives values of

]

2

total supply impedance (ZS), using transformer imped­ance per Table 9.1, power-supply short-circuit capacity
of 1000MVA, and X/R of 25.

]

2

M

Short­circuit

T

Z

T

B

Z

M

Z

B

W

Z

W

point

Z

M

Z

L

Z

T

Z

S

Z

Fig. 9.2 3-Phase Equivalent Circuits

Table 9.4 Total Impedances for 3-Phase Power Supplies

Transformer capacity

(kA)

50

75
100
150
200
300
500
750

1000
1500
2000

Impedance based on

1000kVA(%)

33.182 +j26.482

21.229 +j22.583

15.473 +j17.109

9.56 +j12.389

6.977 +j12.15

4.306 +j 8.795

2.089 +j 7.27

1.427 +j 5.736

0.969 +j 4.336

0.671 +j 3.142

0.467 +j 2.544

Z

B

Z

B

Ohmic value (mΩ)

230V

17.553+j 14.009

11.230+j 11.946

8.185+j 9.051

5.057+j 6.554

3.691+j 6.427

2.278+j 4.653

1.105+j 3.846

0.755+j 3.034

0.513+j 2.294

0.355+j 1.662

0.247+j 1.346

Z

W

Z

W

440V

64.240+j 51.269

41.099+j 43.720

29.956+j 33.123

18.508+j 23.985

13.507+j 23.522

8.336 +j17.027

4.044 +j14.074

2.763 +j11.104

1.876 +j 8.394

1.299 +j 6.083

0.904 +j 4.925

Notes: 1. Total power-supply impedance

2. For line voltages (E') other than 230V, multiply the ohmic value by

ZS =

(ZL + ZT)Z

Z

+ ZT + Z

L

M

M

2

E'

()

230

75

9.4 Classification of Short-Circuit Current

A DC current (Fig. 9.3) of magnitude determined by
the voltage phase angle at the instant of short circuit
and-the circuit power factor will be superimposed on
the AC short-circuit current.

This DC component will rapidly decay; however,
where a high-speed circuit-interruption device such
as an MCCB or fuse is employed, the DC component
must be considered. Further, the mechanical stress
of the electric circuit will be affected by the maximum
instantaneous short-circuit current; hence, the short­circuit current is divided, as below.

1. RMS Symmetrical Short-Circuit Current (Is)
This is the value exclusive of the DC component; it is
As/M2 of Fig. 9.3.

2. RMS Asymmetrical Short-Circuit Current (Ias)
This value includes the DC component. It is defined
as:

A

I

= )2 + A

as

s

(

2

Accordingly, when the DC component becomes maxi­mum (i.e., θ – ϕ = ±
at short circuit is θ, and the circuit power factor is cosϕ),

I

will also become maximum

as

circuit occurs, as follows:

I

= Is · 1 + 2e = Is · K1, that is: K1 = 1 + 2e

as

where K1 is the single-phase maximum asymmetrical
coefficient, and Ias can be calculated from the asym­metrical value and the circuit power factor. In a 3­phase circuit, since the voltage phase angle at switch­on differs between phases, Ias will do the same. If the
average of these values is taken
the 3-phase average asymmetrical short-circuit cur­rent, the following relationship is obtained:

2

d

π

, where the voltage phase angle

2

1

cycle after the short

2

2πR

–

x

1

cycle later, to give

2

2πR

–

x

I

= I

· { 1 + 2e e+ 2 1 + } = I

as

s

that is: K

1
3

1

= { 1 + 2e e+ 2 1 + }

3

3

2πR

–

x

2πR

–

x

2πR

1

–

x

2

1
2

· K

s

2πR

–

x

K3 is the asymmetrical coefficient, derived from the
symmetrical value and the circuit power factor.

3. Peak Value of Asymmetrical Short-Circuit Current
This value (Ip in Fig. 9.3) depends upon the phase
angle at short circuit closing and on the circuit power
factor; it is maximum when θ = 0. It will reach peak

value in each case, ωt =

π

+ ϕ after the short circuit

2

occurrence. It can be computed as before, by means
of the circuit power factor and the symmetrical short­circuit current.

I

= I

[1 + sinϕ·e ] = Is · K

p

s

thus: K

= 2 [1 + sinϕ·e ]

p

–( + ϕ)·

x

2

–( + ϕ)·

2πx

p

R

R

π

Kp, the peak asymmetrical short-circuit current coeffi­cient, is also known as the closing-capacity coefficient,
since Ip is called the closing capacity. Thus, in each
case, the asymmetrical coefficients can be derived
from the symmetrical values and the circuit power fac­tor. These coefficients are shown Fig. 9.4.

1

/2 Cycle

A

s

A

d

p

I

A

s

Fig. 9.3 Short-Circuit Current

3

76

:

K

Single-phase maximum

2.0

3.0

1.9

1.8

3

p

1.7

K

K

1

K

2.0

1.0

1.6

1.5

1.4

1.3

1.2

1.1

1.0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

20 10 8 7 6 5 4 3 2.5 2 1.5 1 0.5

K

K

3

Fig. 9.4 Short-Circuit Current Coefficients

K

p

1

1

asymmetrical coefficient

:

3-phase asymmetrical coefficient

K

3

:

Closing capacity coefficient

K

p

Power factor

X
R

9.5 Calculation Procedures

Table 9.5 Necessary Equations

Ohmic method % impedance method Remarks

V

= ..................................Eq. 1

I

S

M3 · Z

I

= K3 · Is.......................Eq. 4

as

Key

I

3-phase short-circuit current (A, sym)

:

s

V

Line-line voltage (V)

3-phaseImpedance

:

Z

Circuit impedance (1-phase component)

:

I

3-phase short-circuit current (A, asym.)

:

as

P

Reference capacity (3-phase component, VA)

:

%Z

% impedance of circuit (single-phase component, %)

:

I

Reference current (A)

:

B

K3

3-phase asymmetrical coefficient

:

1

K

=

3

1 + 2e e+ 2 1 +

{

3

· Z

I

S

2πR

–

x

P

= x 100 .................Eq. 2

M3 · V · %Z

I

B

= x 100 .............................Eq. 3

%Z

2πR

1

–

x

2

}

I

B

%Z = x 100 ........................Eq. 1'

V/M3

P = M3 · V · IB...............................Eq. 2'

Á Eq. 2 is derived from Eqs. 1, 1' and 2'.
Á Eq. 3 is derived from Eqs. 1 and 1'.
Á Because Eq. 1 can be obtained from

Eqs. 2 and 12, it can be seen that I
the % impedance method is not affect­ed by the selection of the reference ca­pacity.

Á The single-phase short-circuit current

in a 3-phase circuit is M3/2 times the 3­phase short-circuit current. Conse­quently, a 3-phase circuit can be exam­ined via the 3-phase short-circuit
current.

s

of

Á Conversion from percentage value to

ohmic value

2

V

Z = · %Z x 10–2Ω........................Eq. 9

P

Where P is the capacity at which %Z
was derived.
Á Power supply impedance seen from

primary side

(primary voltages)

Z = .............Eq. 10

short-circuit capacity

Á Supply impedance seen from second-

ary side

primary-side
power supply x

Z =

impedance

..........................................Eq. 11

2

secondary voltages

()

primary voltage

Á Conversion from ohmic value to per-

centage value

P

%Z = · Z x 100%......................Eq. 12

2

V

Á Conversion to %Z at reference capac-

ity

Power-supply impedance:

reference capacity

%Z = x 100......Eq. 13

short-circuit capacity

Transformer impedance, motor im­pedance:

reference capacity

%Z = x

2

equipment capacity

............................................Eq. 14

9.5.1 Computation Methods

Regardless of method, the aim is to obtain the total
impedance to the short-circuit point. One of two com­mon methods is used, depending upon whether a
percentage or ohmic value is required.

1. Percentage Impedance Method
This method is convenient in that the total can be
derived by simply adding the individual impedances,
without the necessity of conversion when a voltage
transformer is used.
Since impedance is not an absolute value, being
based on reference capacity, the reference value must
first be determined. The reference capacity is normally
taken as 1000kVA; thus, the percentage impedance
at the transformer capacity, the percentage imped­ance derived from the power supply short-circuit ca­pacity, and also the motor impedance must be con­verted into values based on 1000kVA (Eqs. 13 and

14). Also, the wiring and bus-duct impedances that
are given in ohmic values must be converted into per­centage impedances (Eq. 12).

2. Ohmic Method
In calculating short-circuit currents for a number of
points in a system, since the wire and bus-duct im-

Á Eqs. 9 and 12 are derived from Eqs. 1'

and 2', and Eqs. 3' and 4'.

Á As the supply impedance is defined as

100% at short circuit capacity, for Eq.
13 conversion to reference capacity is
made.

Á When the supply short-circuit capacity

is unknown, the impedance is taken as

0.0040+j0.0999 (%) for 3-phase sup­ply, and 0.0080+j0.1998 (%) for a 1­phase supply (see Table 9.6).

Á The motor and transformer impedanc-

es are converted from %Z at their

%Z at equip­ment capacity

equipment capacities into %Z at refer­ence capacity, using Eq. 14.

Á Eq. 14 for motor impedance becomes

(4.11 + j24.66) x
(For details see Table 9.6.)

reference capacity

equipment capacity

pedances will be different in each case, it is conve­nient to use Ohm’s law, in that if, for example, the
total supply impedance (Zs) is derived as an ohmic
value, the total impedance up to the short-circuit point
can be obtained by simply adding this value to the
wire and bus-duct impedances, which are in series
with the supply. For total 3-phase supply impedance
(Zs), refer to Table 9.4 (which shows calculations of
Zs based on standard transformers) to eliminate
troublesome calculations attendant to the motor im­pedance being in parallel with Zs.

9.5.2 Calculation Examples

1. 3-phase Circuit
For the short circuit at point S in Fig. 9.5, the equiva­lent circuit will be as shown in Fig. 9.6. The 3-phase
short-circuit current can be obtained by either the %­impedance method or Ohm’s law, as given in Table

9.6.

77

Table 9.6 Calculation Example: 3-Phase Short-Circuit Current

Short-circuit
point S

Z

L

Z

M

Z

W

Z

T

The supply short-circuit capacity, being unknown, is
defined as 1000MVA with XL/RL = 25.

Power supply

impedance

Z

L

Transformer

impedance

Z

T

Motor impedance

Z

M

Total power supply

impedance

Z

S

From Eq. 13, at the 1000kVA reference capacity:

1000 x 10

Z

= x 100 = 0.1 (%)

L

1000 x 10

since XL/RL = 25,

0.1 = R
ZL = RL + jXL = 0.0040 + j0.0999 (%)

From Table 9.1:

= 1.23 + j5.41

Z

T

From Eq. 14, after conversion to reference capacity,
1000kVA:

ZT = (1.23 + j5.41) x

= 0.82 + j3.607 (%)

The total motor capacity, being unknown, is assumed
equal to the transformer capacity, with:

%Z

= 25(%) XM/RM = 6

M

From Eq. 14, at reference capacity, 1000kVA:
ZM = (4.11 + j24.66) x

= 3.42 + j20.55 (%)

(ZL + ZT)Z

ZS =

Z

= 0.671 + j3.142 (%)

(R and X are calculated, per §9.3.2.)

3
6

2

+ (25RL)2 = 25.02R

L

M

+ ZT + Z

L

M

1000 x 10
1500 x 10

1000 x 10

1500 x 10

L

3
3

3

x 0.8

3

Ohmic method% impedance method

The supply short-circuit capacity, being unknown, is
defined as 1000MVA with XL/RL = 25.
From Eq. 10, the supply impedance seen from the
primary sicde:

ZL = = 0.0436 (Ω)
and since X

(6600)

1000 x 10

L/RL

2

6

= 25: ZL = 1.741 + j43.525 (mΩ)

From Eq. 11, supply impedance converted to the
secondary side is:

2

ZL = (1.741 + j43.525) x

440

( )

6600

= 0.00773 + j0.1934 (mΩ)

Note: The supply ohmic impedance can more simply

be derived: since it is 100% at short-circuit ca­pacity, Z
age to ohmic conversion:

ZL =

is obtained from Eq. 9, after percent-

L

2

440

1000 x 10

x 100 x 10–2 x 103 = 0.1936 (mΩ)

6

and since
XL/RL = 25, ZL = 0.0069 + j0.1721 (mΩ)

From Table 9.1:

= 1.23 + j5.41 (%)

Z

T

From Eq. 9, after percentage to ohmic conversion.
ZT = x (1.23 + j5.41) x 10–2 (Ω)

2

440

1500 x 10

3

= 1.2906 + j6.9825 (mΩ)

The total motor capacity, being unknown, is assumed
equal to the transformer capacity, with:

%Z

= 25(%) XM/RM = 6 ZM = 4.11 + j24.66

M

ZM = 4.11 + j24.66 (%)
From Eq. 9, after percentage to ohmic conversion:

ZM = x (4.11 + j24.66) x 10–2 (Ω)

1500 x 10

440

2

3

x 0.8

= 6.6294 + j39.7847 (mΩ)

ZS =

(ZL + ZT)Z

Z

L

+ ZT + Z

M

M

= 1.299 + j6.083 (mΩ)

(R and X are calculated, per §9.3.2.)

Line impedance

Total impedance

3-phase short-circuit

symmetrical current

78

Multiplying the value from Table 9.2 by a wire length
of 10M, and converting to the 1000kVA reference,
from Eq. 12:

Z

W

1000 x 10

Z

= (0.0601 + j0.079) x 10–3 x 10 x 100

W

440

3

2

Multiplying the value from Table 9.2 by a wire length
of 10M.

= (0.0601 + j0.079) x 10

Z

W

= 0.601 + j0.79 (mΩ)

= 0.310 + j0.408 (%)

Z = Z

+ Z

S

W

Z

I

s

= 0.981 + j3.550 = 3.683 (%)

From Eq. 2:

1000 x 10

I

=

s

M3 x 440 x3.683

= 35.622 (A)

3ph 50Hz

6.6kV/440V

Wire
300mm

Short-circuit
point S

10m

2

M

3

x 100

1500kVA

Z = Z

+ Z

S

W

= 1.900 + j6.873 = 7.1307 (mΩ)

From Eq. 1

=

I

s

440

M3 x 7.1307x10

–3

= 35.622 (A)

Fig. 9.6 Equivalent CircuitFig. 9.5 Circuit Configuration

MOULDED CASE CIRCUIT BREAKERS

Safety Tips

Made from recycled paper
Y-0525-C 0007 (ROD) Printed in Japan

:

Be sure to read the instruction manual fully before using this product.

HEAD OFFICE: MITSUBISHI DENKI BLDG., MARUNOUCHI, TOKYO 100-8310. TELEX: J24532 CABLE: MELCO TOKYO

Specifications subject to change without notice.

Revised publication, effective July. 2000