GE MOTOR MANAGEMENT RELAY 469 Instruction Manual page 101

4 SETPOINT PROGRAMMING
Care must be taken when turning On this feature. If the interrupting device (contactor or cir-
cuit 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 auxil-
NOTE
iary 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 adjust-
able 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 pos-
sible, 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.
GE Power Management 469 Motor Management Relay
4.7 S6 CURRENT ELEMENTS
4
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