GE Industrial Solutions AKD-8 - Manual (Page 6)

Testing For System Capacitance Charging
Current, 480 Volt WYE
WARNING: Testing for capacitance charging cur-
rent requires that the entire system be energized;
hence, take all the necessary safety measures and
employ all safety codes and practices.
The following instruction describes a high resistance
neutral ground system supplied with Type AKD-8
Low Voltage Switchgear. The system features an
alarm, and utilizes a pulsing current to locate the first
ground fault.
This system is designed to:
1. De-energize grounding equipment by opening
main circuit breaker, and by disconnecting hard
wire connection to neutral bus. Be sure no other
ground is connected to system. Remove neutral
ground link, if supplied.
2. Disconnect or open-circuit the resistors during
test, and insert a 6-amp, 600V current limiting
fuse as shown in Figure 1.
3. Connect a Variable Voltage Auto Transformer
rated five (5) amps or more between equipment
and ground as shown. Initially set at zero volts.
4. Connect a voltmeter as shown.
5. Use portable signal detector if furnished to read
ground current. (1-amp scale for most systems.)
Wire can be passed through detector window
several times to produce greater sensitivity. (Test
quality clamp-on ammeter may be used if
portable signal detector is not supplied).
6. With maximum operating load equipment
connected on system, re-energize the grounding
equipment by re-connecting hard wire connection
to neutral bus and closing main breaker. (Do Not
reinstall neutral-ground link, if supplied).
7. Energize the variable voltage auto transformer
from an ungrounded, 110V, 60 Hz. single-phase
test source. (Use 1:1 isolation transformer rated
(5) amps or more to obtain ungrounded voltage).
8. With maximum operating load equipment
connected on the system, bring the voltage up
slowly, recording neutral displacement voltage
and current at selected test points until a linear
variation of current with voltage is obtained.
9. Calculate system capacitance charging current,
IC, at rated voltage by extrapolation as follows:
I
c
= E x I
a
= Amps system charging
√ 3 V
current, where
E = Rated system voltage, line to line
(usually 480V)
V =Measured test voltage. (Neutral
displacement).
I
a
= Measured ground current corresponding to
V.
10. Plot various test values to verify that linear
relationship of current to voltage is being
attained as system neutral is gradually displaced
from ground potential.