Cooper Bussmann Electronics Short-Circuit Current Rating User Manual

Simplified Guide To

Understanding

Short-Circuit Current Rating

FIND IT, FIX IT, FORGET IT

What Is A Short-Circuit Current Rating

(SCCR)?

SCCRs on components and equipment represent the

maximum level of short-circuit current that the

component or equipment can withstand and is used for

determining compliance with NEC

®

110.10. This rating

can be marked on individual components or

assemblies. The new SCCRs represent the maximum

amount of fault current that the assembly

can withstand

under fault conditions. Assembly ratings take into

account all components contained within the equipment

rather than just the main overcurrent protective device.

A common mistake is to assume that the interrupting

rating of the overcurrent protective device protecting

the circuit represents the SCCR for the entire circuit.

Interrupting ratings, used for compliance with NEC

®

110.9, apply solely to the overcurrent protective device.

It is the characteristics of the overcurrent protective

device (e.g. opening time, let-through energy) that

need to be used in determining compliance with NEC

®

110.10, not the interrupting rating.

Who Is Affected By The SCCR Markings

The 2005 NEC®has requirements for anyone building

equipment listed to the 2005 NEC

®

or to UL508A and

requires the following to be marked with an SCCR:

• Industrial Control Panels [409.110]

• Industrial Machinery Electrical Panels [670.3(A)]

• Certain HVAC Equipment [440.4(B)]

• Meter Disconnect Switches [230.82(3)]

• Certain Motor Controllers [430.8]

Effective April, 2006, all equipment listed to UL508A will

be required to be marked with an assembly SCCR.

Inspectors and installers need this information in order

to ensure compliance with NEC

®

110.10. Equipment

installed where fault current levels exceed their short-

circuit current limit can be hazardous to persons and

property. SCCRs marked on components and

equipment make it easier to verify proper protection for

components and equipment for specific applications —

whether it be the initial installation or relocation of

equipment.

How Is SCCR Determined?

For meter disconnect switches and motor controllers,

this withstand level, or SCCR, is often determined by

product testing. For assemblies, the marking can be

determined through product listing or by an approved

method. With the release of the UL508A Industrial

Control Panel standard, an industry method is now

available. Any method used, whether UL508A or

another approved method, should be based on the

“weakest link” approach. In other words, the assembly

should be limited to installation where fault levels do

not exceed the withstand rating of devices with the

lowest SCCR. The marking determined should

represent the limits of the assembly for a safe

installation. Current-limiting overcurrent protective

devices can be used in the feeder or branch circuits to

increase the assembly SCCR where lower rated

components are used. When current-limiting devices

(that limit fault current levels to within the lower rated

components' ratings) are used, they provide protection

for the assembly — provided the current-limiting

Overcurrent Protective Device (OCPD) is used within

its ratings.

Are You Ready For The New SCCR Marking Requirements?

CAUTION: Short-circuit current ratings (SCCRs) are
different than interrupting ratings marked on
overcurrent protective devices.

Who Is Affected How Are They Affected

Facility/Process Engineers/ Need to specify end use equipment with ratings adequate for available
Consulting Engineers short-circuit current. Need adequate ratings so equipment has flexibility to be

moved around.

Control Panel Builders/ Need to determine and mark the assembly short-circuit current rating on the
Machine Builders/ equipment they build. Need to market equipment with ratings as least as high
HVAC Manufacturers as their competition.

Electrical Contractors Need assurance that the equipment they install is adequate for the available

short-circuit current at the point of installation, to avoid red tags, and lost time
and labor.

Electrical Inspectors Need to assure that the available short-circuit current where equipment is

being installed does not exceed the rating marked on the equipment.

Use this table to initiate the process of identifying the short-circuit current rating of your
components and devices in power circuits. For further information contact your local
Cooper Bussmann sales engineer, Cooper Bussmann Application Engineering,
or visit www.cooperbussmann.com.

Supply Disconnect and OCPD

Feeder Circuit OCPD and Components

Branch Circuit OCPD and Components

*Class CC, J, T, R, and L UL248 Branch Circuit Type Fuses.
**When used in conjunction with Class CC, J, T, R, and L UL248 Branch Circuit Type Fuses.
***50HP or less 480V

+Combination Rating is used for Sweep 1.
For Sweep 2 IR=5kA (Default Value)

UL98 Non-Fusible

Disconnect

Switches

UL1429 Pullout

Molded Case Switch

UL98 Fusible

Disconnect

Switches

UL489 Circuit

Breakers

Branch Circuit

Fuses

UL98 Non-Fusible

Disconnect

Switches

Power Distribution

Blocks

Fuse Holder

UL98 Fusible

Disconnect

Switches

UL489 Circuit

Breakers

UL98 Fusible

Disconnect

Switches

Branch Circuit

Fuses

Branch Circuit

Fuses

UL98 Non-Fusible

Disconnect

Switches

200kA*

UL489 Circuit

Breakers

5kA 200kA* 10kA**

Magnetic Motor

Starter

5kA***

Type E Self

Protected

Starter

5kA*** 5kA

Instantaneous Trip

Circuit

Breakers

+

Molded Case

Switch

200kA* 5kA 200kA* 10kA** 10kA 200kA*

200kA* 5kA 200kA* 10kA** 10kA 5kA

Fuse Holder

Busbar

Molded Case Switch

200kA*

UL508 Manual

Motor Starter

5kA***

UL508 Switch

5kA***

Drives

5kA***

Contactors

5kA***

Receptacle

10kA

GFCI Receptacle

2kA

10kA 5kA

Default
Value

Your
Value

Glossary of Terms

Industrial Control Panel - An assembly of a

systematic and standard arrangement of two or more

components such as motor controllers, overload relays,

fused disconnect switches, fuses, and circuit breakers

and related control devices such as pushbutton

stations, selector switches, timers, switches, and

control relays with associated wiring, terminal blocks,

pilot lights and similar components.

Interrupting Rating - The highest current at rated

voltage that a device is intended to interrupt under

standard test conditions.

Short-Circuit Current Rating – The prospective

symmetrical fault current at a nominal voltage to which

an apparatus or system is able to be connected without

sustaining damage exceeding the defined acceptance

criteria

Control Circuit – A circuit that carries the electric

signals directing the performance of a controller, and

which does not carry the main power circuit. A control

circuit is, in most cases, limited to 15 amp.

Power Circuit – Conductors and components of

supply, branch, and feeder circuits, supplying main line

power to loads such as heaters, motors, lighting,

receptacles, power transformers, and appliances.

FIND IT – Look for... the Weakest Link

The SCCR marking for the assembly will be based

upon the lowest short-circuit current or withstand rating

of any power circuit component contained within the

assembly — or the “weakest link”. Many components

used in industrial control panels, industrial machinery

and HVAC equipment may be the weakest link,

including:

• motor controllers

• definite purpose contactors

• supplemental protectors

• circuit breakers (with low interrupting ratings)

• power distribution blocks.

FIX IT — How To Get A Higher SCCR So
Equipment Isn’t Limited

By fixing these weakest links, higher SCCRs can be
achieved. In general, there are three options:

1. Use overcurrent protective devices with

higher interrupting ratings — Overcurrent

protective devices with low interrupting ratings often
become the weakest link and limit the SCCR of the
assembly. The interrupting ratings of the overcurrent
protective devices used directly affects the assembly
rating. The only fix is to use devices with higher
interrupting ratings.

2. Use current limitation to fix low rated

components — It only takes one component with a

low SCCR to limit the entire assembly. Many of the
components used in industrial control applications have
low short-circuit current ratings. Fix these low rated
components and the assembly rating by using
current limitation. Since current-limiting fuses reduce high
level short-circuit currents to a minimal level, they allow
the use of these lower rated devices while
providing a higher SCCR.

3. Buy more expensive components with

higher ratings — A 65kA circuit breaker will more

than double your cost compared to a 14kA circuit
breaker. The fusible solution provides an interrupting
rating as high as 200kA, at a comparable cost to your
existing design, making it the most economical solution.

FORGET IT — Use Current-Limiting Fuses

Not only can modern current-limiting fusible solutions

fix undesired weak links and obtain a high assembly

short-circuit current rating, they can also provide

enhancements to your design in the areas of flexibility

of installation, increased workplace safety, and reliable

protection through the life of your equipment. These

enhancements take care of the hassle associated with

equipment with limited ratings so you can FORGET IT!

Benefits of Using Cooper Bussmann Current-

Limiting Fuses

Straight Voltage Ratings:

• Straight voltage ratings allow for more installations where
equipment can be installed. Straight voltage rated
equipment can be installed in any type of electrical system.
With a straight voltage rating you can ship it anywhere and
not have to worry.

High Assembly SCCRs:

• High SCCRs allow for more applications where equipment
can be installed. With a high SCCR you can ship it
anywhere and not have to worry.

• A wider range of fault current levels provides assurance that
the equipment will be installed correctly and safely.

• Meets the UL508A and NEC

®

requirements for SCCR

marking.

• Marketing advantage over competitive equipment with a
lower rating.

Increased Flexibility In Panel Use and Installation:

• Valuable time that was spent gathering information for
proper application is drastically reduced with fuses because
modern current-limiting fuses can provide high assembly
SCCRs and straight voltage ratings since they have:

- High Interrupting Ratings of 200,000A or more

- Excellent current limitation to protect low rated
components at high short-circuit current levels

- Straight voltage ratings so they can be installed in any
type of application independent of the grounding scheme
used, unlike devices with a slash voltage rating which are
limited to installation in ONLY a solidly grounded wye
type system

Find It, Fix It, Forget It

Fuse & Motor Starter
Combination

• Fusible disconnect with
Cooper Bussmann LP-CC
fuses marked interrupting
rating 200kA

• Motor starter marked 50kA
when protected by fuses or
circuit breakers

• The SCCR is 50kA

Circuit Breaker &
Motor Starter
Combination

• Three-pole circuit
breaker marked
interrupting rating 14kA

• Motor starter marked
50kA when protected by
fuses or circuit breakers

• The SCCR is 14kA

©2005 Cooper Bussmann • St. Louis, MO 63178

636-394-2877 • www.cooperbussmann.com

Printed in USA

Reorder #7009 11-05-5M

Cooper Bussmann Products And
Technical Support Delivered Worldwide

Customer Assistance

Customer Satisfaction Team

The Cooper Bussmann Customer Satisfaction Team is
available to answer questions regarding Cooper
Bussmann products and services. Calls should be made
between 8:00 a.m. – 4:30 p.m. Central Time
for all US time zones.

The Customer Satisfaction Team can be reached via:

• Phone: [phone]

• Toll-free fax: [phone]

• E-mail: [e-mail]

Emergency and After-Hours Orders

To accommodate time-critical needs, Cooper Bussmann
offers emergency and after-hours service for next flight
out or will call. Customers pay only standard price for
the circuit protection device, rush freight charges and a
modest emergency fee for this service. Emergency and
after-hours orders should be placed through the
Customer Satisfaction Team. Call:

• 8:00 a.m. – 4:30 p.m. Central Time 636-527-3877

• After hours 314-995-1342

Application Engineering

Application Engineering assistance is available to all
customers. The Application Engineering team is staffed
by degreed electrical engineers and available by phone
with technical and application support Monday – Friday,
8:00 a.m. – 5:00 p.m. Central Time.

Application Engineering can be reached via phone, fax
or email:

• Phone: [phone]

• Fax: [phone]

• E-mail: [e-mail]

Online Resources

Visit www

.cooperbussmann.com for the following

services:

• Product cross reference

• Arc-flash calculator

• SCCR calculator

• Training modules

Your Authorized Cooper Bussmann Distributor is: