Sudden tripping of 1200amps, 460v, 3ph, impact type circuit breakers

[mnnc]

I'm wondering why our 1200Amps, 460v, 3ph impact type circuit breakers always trips from the main switchgear when there is a ground fault on its load side. The thing here is that the main breaker from where the sorce ground fault originated did not trip instantly eventhough its the nearest disconnecting means. The loads connected are Air condition handling units (AHU).

 

[EEJaime]

If the load distribution breakers downstream of the 1200A main circuit breaker are not equipped with ground fault protection, they are not tripping because they are not seeing an overload or short circuit condition which is what they are protecting against. It would appear that you have a ground fault on the system which is tripping the main which is required by code to have ground fault protection. Either that, or there is something else happening for which you did not include sufficient information to determine. Good luck.

 

[dpc]

This is not unusual behavior. Molded case circuit breakers in series will often mis-coordinate for high amp faults. The addition of ground fault protection to upstream breakers just makes it worse.

The NEC requires ground fault on 1000 A breaker and above, so smaller breakers often do not have ground fault.

To improve coordination (but decrease protection), you can increase the pickup and time delay on the 1200 A breaker's ground fault element (if you haven't already done this )

 

[davidbeach]

Sounds like you have a system in dire need of a coordination study. I've seen too many systems where somebody tried to save a bit of money by not doing a coordination study, but the cost of the outage when the main trips on a branch circuit ground fault is far more than the cost of the coordination study.