Multilin SR469 Instruction Manual Download

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S6 CURRENT ELEMENTS

4. SETPOINT PROGRAMMING

4-44

4.7.6

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FUNCTION:
The Ground Fault element will function as follows: Once the magnitude of ground current exceeds the Pickup Level × Ground CT Pri-
mary (S1 SYSTEM SETUP /CURRENT SENSING) for a period of 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 backup is On, and a Ground Fault trip has initiated, if the ground current persists for
a period of time that exceeds the backup delay, a second trip will occur. It is intended that this second trip be assigned to R2 or R3
which would be dedicated as an upstream breaker trip relay. The Ground Fault Trip Backup delay must be set to a time longer than the
breaker clearing time.

Note: 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 fea-
ture 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 (eg. contactor bounce) may cause transient ground currents during motor starting that may exceed the Ground Fault
Pickup levels only 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 will be set as quick as possible, 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 SR469 will not be negligible. A 20 ms block of the ground fault elements when
the motor starts enables the SR469 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 (10-15ms) but times still remain within specifications.