Phase Differential - GE 469 Instruction Manual

Note
Note
5.7.7

Phase Differential

PHASE
5–64
MESSAGE
The
GROUND FAULT ALARM PICKUP
units of '× CT' if the Ground CT is programmed as 1 A or 5 A Secondary or in units of 'A' is
the Ground CT is programmed as "50:0.025".
The Ground Fault element functions as follows. Once the ground current magnitude
exceeds the Pickup Level ×
for the time specified by the delay, a trip and/or alarm will occur. There is also a backup trip
feature that can be enabled. If the
trip has initiated, a second trip will occur if the ground current persists longer than the
GROUND FAULT TRIP BACKUP DELAY
2 Auxiliary or 3 Auxiliary, which would be dedicated as an upstream breaker trip relay. The
GROUND FAULT TRIP BACKUP DELAY
time. Whenever the output relay assigned to the backup trip operates, it will stay latched
regardless of how the trip relay is configured for the Ground Fault element.
Care must be taken when turning On this feature. If the interrupting device (contactor or
circuit breaker) is not rated to break ground fault current (low resistance or solidly
grounded systems), the feature should be disabled. Alternately, the feature may be
assigned to an auxiliary relay and connected such that it trips an upstream device that is
capable of breaking the fault current.
Various situations (e.g. contactor bounce) may cause transient ground currents during
motor starting that may exceed the Ground Fault pickup levels for a very short period of
time. The Ground Fault time delays are adjustable in 10 ms increments. The delay can be
fine tuned to an application such that it still responds very fast, but rides through normal
operational disturbances. Normally, the Ground Fault time delays are set as quick as
possible, that is, 0 ms. Time may have to be increased if nuisance tripping occurs.
Special care must be taken when the ground input is wired to the phase CTs in a residual
connection. When a motor starts, the starting current (typically 6 × FLA for an induction
motor) has an asymmetrical component. This asymmetrical current may cause one phase
to see as much as 1.6 times the normal RMS starting current. This momentary DC
component will cause each of the phase CTs to react differently and the net current into
the ground input of the 469 will not be negligible. A 20 ms block of the ground fault
elements when the motor starts enables the 469 to ride through this momentary ground
current signal.
The overreach filter removed the DC component from the asymmetrical current present at
the moment a fault occurs. This results in no overreach whatsoever, however, the response
time slows slightly (10 to 15 ms) but times still remain within specifications.
PATH: SETTINGS S6 CURRENT ELEM.
[
GROUND FAULT TRIP
BACKUP DELAY: 200
and
GROUND FAULT TRIP PICKUP
(see 5.3.1: Current Sensing on page –17)
GROUND CT PRIMARY
GROUND FAULT TRIP BACKUP
. It is intended that this second trip be assigned to
must be set to a time longer than the breaker clearing
PHASE DIFFERENTIAL
PHASE DIFFEREN-
TIAL
469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
Range: 10 to 2000 ms in steps of 10
settings are entered in
is "On", and a Ground Fault
Range: Off, Latched, Unlatched