A–10 489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER A: APPENDIX
Should a fault occur during the first 50-60ms after closing the breaker, the corresponding
relay would trip instantly. Before closing the breaker the corresponding relay too applies a
delay. However, once the load/inrush current exceeds 5% of CT nominal, its timer is by-
passed and instantaneous protection is provided.
Should a fault occur during generator start-up with both breakers opened, both relays
would operate after the extra time delay of 130ms. This delay is acceptable under such
conditions. Even this delay will be eliminated if the fault is heavy enough to draw more
than 5 times CT nominal from the neutral-side of the generator.
For proper implementation, the internal timer is cleared each time the 87 function
becomes enabled (so that a partial time out from the previous “enabled” period does not
affect the intended operation).
Courtesy of NationalSwitchgear.com
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