GE Industrial Solutions AKD-8 User Manual

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GENERAL

Pulsing Current High Resistance Neutral Grounding
System

GEI-72116 Installation/Operation Instructions

TYPE AKD-8
Low-Voltage Switchgear

High Resistance Pulsing Ground Detection
System

General Applications, Vol. IGA- 1 No. 5, Sept./Oct. 1965
under High Resistance Grounding.

The following paragraphs describe a high resistance neutral
grounding system supplied with GE AKD-8 Low voltage
Switchgear. The system features an alarm system and utilizes
a pulsing current to locate the first ground fault. The system
has the following basic features:

1. Grounds the neutral of an ungrounded 3-phase power

system, utilizing the "high resistance" method. This
allows the system to operate as an "ungrounded" system
but eliminates the danger of high transient over-voltage
during certain types of ground faults.

2. Gives an immediate warning when the first ground fault

occurs.

3. Provides a method for quickly locating and removing the

fault before another fault develops on another phase,
thereby preventing circuit outages due to double line to
ground faults.

Reference: The high resistance grounding method is fully
described in IEEE Transactions on Industry and

EQUIPMENT DESCRIPTION AND
GROUNDING SYSTEM OPERATION

During normal conditions, with no ground fault on the
system, no voltage appears across the system and no voltage
appears across the grounding resistors. (Refer to Fig. 1).

When a ground fault occurs, the resistors limit the ground
current to a low value, adjustable between 0.9 and 3.6
amperes.

The voltage appearing across the resistors will be sensed by
the voltage coil of the meter relay. Auxiliary contacts

Fault Locating

The success of operating a high resistance grounded system
relies heavily on a dependable method of searching out the
ground fault location to allow the faulty system member to be
removed with the least delay.

The two important innovations introduced in this equipment
to take the uncertainty out of signal tracing include:

1. Use of repeating, cyclic, switching sequence by which the

tracer current can be rhythmically switched to a slightly
higher value. This gives unique character to the tracer
signal by which it can be distinguished from steady state
background noise.

2. Use of an extremely sensitive ground-fault-current

sensing transformer such that extraneous noise will be
ignored and only the current-carrying member which
passes through the window will be registered on the
meter.

of the meter relay, operated by the voltage coil, may be used
for remote indications and annunciation.

A green indicating light on the equipment indicates that
control voltage is available and that the system is normal.
When a fault develops, a red indicating light on the
equipment will remain lighted until the ground fault is
removed. If no remote annunciator is available to notify the
operator, an audible signal may be added to the equipment as
an option. A rotating red signal beacon may be used in noisy
or remote installations.

EQUIPMENT DESCRIPTION AND

GROUNDING SYSTEM OPERATION
(CONT.)

To trace the ground fault, the operator turns the selector
switch to the "pulse" position. This initiates cycle timing
alternately energizing and deenergizing a shorting contactor
at the secondary resistor, resulting in a rhythmic fluctuation
in the magnitude of the ground current.

An optional current transformer and ammeter at the panel
will allow the operator to determine the current fluctuations
on the system. A portable hook-on detector is then used to
trace the fluctuations in ground current through the system to
the point of fault. After the ground point has been located
and removed from the system, the operator then resets the
selector switch to deenergize pulse cycle timing circuits, and
depresses a reset button to reset indicating lights and
annunciators.

Taps are provided on the resistor to adjust the magnitude of
the ground current in the range of 0.9 to 7.3A., depending on
the size of the system, so that the current through the resistor
to a ground fault will be slightly , greater than the system's
natural capacitance charging current.

The capacitance charging current of most 480 volt systems is
below one ampere, so that the resistor is usually set at 0.9
ampere, with an increase to 3.6 ampere during pulsing. For
unusually large systems, taps are available for an initial
ground current of 3.6 amps with pulsing to 7.3 amperes.

Refer to Fig. 2 for typical system circuit elementary.

CONTROL

TRANSFORMER

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EQUIPMENT DESCRIPTION AND

Figure 1. Basic System Operation

GROUNDING SYSTEM OPERATION

(CONT.)
Grounding Resistors

Since ground current is limited by the resistors, the last
section must never be shorted out by the shorting con-tactor
during pulsing.

The resistor must be adjusted at installation so that the
ground current with one ground fault is equal to or slightly
greater than the maximum system capacitance charging
current. (The value obtained with maximum connected load
of the system). If maximum system capacitance current is not
known, refer to "System Capacitance Charging Current"
below for methods to determine the value.

Operation of Equipment

The following instructions will appear on an engraved
nameplate on the equipment door:

Normal

1 - Green light indicates normal condition and equip-ment

energized.

2 - Red light indicates ground fault on system.

Ground Detection

1 - Press reset button. If red light goes out, there is no

ground.

2 - Plug in portable pulsing unit.

3 - Trace pulsing ground current with portable de-

tector.

1 - Press test button to simulate ground and to operate alarm

and control circuits. Hold for five seconds or until red
light is lit.

2 - Press reset button after test to return to normal.

Equipment Included In Standard 480V WYE
System With System Neutral Available

1 - Neutral current transformer (optional) (CT)
1 - Meter relay (MR)
1 - Pulsing contactor, set to produce approx. 40 cur-

rent pulsations per minute (fixed or portable) (PC)
1 - Neutral grounding resistors. (RES)
1 - Relay for pulsing contactor. (PCR)
1 - Holding relay, with extra interlocks for remote

alarm. (HR)
1 - Neutral ammeter (optional). (AM)
1 - "Ground Fault" red indicating light. (LT/GRD FLT)
1 - "Normal" green indicating light. (LT/NORM)
1 - "Reset" pushbutton. (PB/RESET)
1 - "Test" pushbutton. (PB/TEST)
1 - Instruction plate on door.
1 - Portable clamp on detector (optional)

System Capacitance Charging Current

The maximum system capacitance charging current must be
known before the high resistance grounding system resistors
can be set for the correct ground current during a fault. If the
maximum system charging current is not known, the value
may be established by the procedures detailed on pages 5 or 6,
whichever is appropriate.

4 - Remove ground.

5 · Press reset button to remove fault indication.

6 - Unplug portable pulser.
Test

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