S6 Current Elements; Short Circuit - GE MOTOR MANAGEMENT RELAY 469 Instruction Manual

4.7 S6 CURRENT ELEMENTS

4.7 S6 CURRENT ELEMENTS
S SHORT CIRCUIT TRIP
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4
Care must be taken when turning On this feature. If the interrupting device (contactor or cir-
cuit breaker) is not rated to break the fault current, this feature should be disabled. Alterna-
tively, this feature may be assigned to an auxiliary relay and connected such that it trips an
CAUTION
upstream device that is capable of breaking the fault current.
If turned on, the Short Circuit element functions as follows.
A trip occurs once the magnitude of either Ia, Ib, or Ic exceeds the Pickup Level × Phase CT Primary for a
period of time specified by INTENTIONAL S/C TRIP DELAY. A backup trip feature may also be enabled. The SHORT
CIRCUIT TRIP BACKUP DELAY should be greater than the INTENTIONAL S/C TRIP DELAY plus the breaker clearing
time. If the SHORT CIRCUIT TRIP BACKUP is "On", and a Short Circuit trip has initiated, a second trip occurs if the
motor phase current persists for a period of time exceeding the SHORT CIRCUIT TRIP BACKUP DELAY. It is intended
that this second trip be assigned to R2 or R3 which would be dedicated as an upstream breaker trip relay.
Various situations (e.g. charging a long line to the motor or power factor correction capacitors) may cause tran-
sient inrush currents during motor starting that may exceed the SHORT CIRCUIT TRIP PICKUP level for a very short
period of time. The INTENTIONAL S/C TRIP DELAY is adjustable in 10 ms increments. This delay can be fine tuned
to an application so it still responds very fast but rides through normal operational disturbances. Normally, the
INTENTIONAL S/C TRIP DELAY is set as quick as possible, 0 ms. This time may be increased if nuisance tripping
occurs.
When a motor starts, the starting current (typically 6 × FLA for an induction motor) has an asymmetrical com-
ponent. This asymmetrical current may cause one phase to see as much as 1.6 times the normal RMS starting
current. If the SHORT CIRCUIT TRIP PICKUP was set at 1.25 times the symmetrical starting current, it is probable
that there would be nuisance trips during motor starting. A rule of thumb has been developed over time that
short circuit protection at least 1.6 times the symmetrical starting current value. This allows the motor to start
without nuisance tripping.
The overreach filter removes 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.
4-46
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SHORT CIRCUIT

ENTER
TRIP: Off
ESCAPE
SHORT CIRCUIT TRIP
ESCAPE
OVERREACH FILTER: Off
MESSAGE
ASSIGN TRIP RELAYS:
ESCAPE
Trip
MESSAGE
SHORT CIRCUIT TRIP
ESCAPE
PICKUP: 0.2 x CT
MESSAGE
INTENTIONAL S/C TRIP
ESCAPE
DELAY: 0 ms
MESSAGE
SHORT CIRCUIT TRIP
ESCAPE
BACKUP: Off
MESSAGE
ASSIGN BACKUP
ESCAPE
RELAYS: Auxiliary2
MESSAGE
SHORT CIRCUIT TRIP
ESCAPE
BACKUP DELAY: 200 ms
MESSAGE
469 Motor Management Relay
4 SETPOINT PROGRAMMING
Range: Off, Latched, Unlatched
Range: On, Off
Range: Alarm, Auxiliary2, Auxiliary3, Alarm & Aux2, Alarm &
Aux3, Aux2 & Aux3, Alarm & Aux2 & Aux3
Range: 0.2 to 20.0 x CT, step: 0.1
Range: 0 to 1000 ms, step: 10
Range: On, Off
Range: Auxiliary2, Aux2 & Aux3, Auxiliary3
Range: 10 to 2000 ms, step: 10
4.7.1 SHORT CIRCUIT
GE Power Management