Ground fault

[jsilva]

Hey guys, at where I work we have 3000Amps at 480V main power coming in. The ground fault for the 3000amp main is not working, but as for the individuals distribution disconnects on the main swith gear that feeds the MCC's in the building are all working fine. The power to all 480/277 and 208/120 circuit panels comes from the MCC's. I have notice that most of times when there is a short on the 277v lighting or 120v circuit, the breakers won't trip under the ground fault condition, they will trip under the overload, but NOT on the ground fault. What could be the problem here? Thanks.

 

[bphgravity]

I don't completely follow your post, however if a circuit has a ground fault and the overcurrent device protecting that circuit does not open, you do not have an effective fault path as provided by a code required equipment grounding and bonding means.

 

[petersonra]

Hey guys, at where I work we have 3000Amps at 480V main power coming in. The ground fault for the 3000amp main is not working, but as for the individuals distribution disconnects on the main swath gear that feeds the Mac's in the building are all working fine. The power to all 480/277 and 208/120 circuit panels comes from the Mac's. I have notice that most of times when there is a short on the 277v lighting or 120v circuit, the breakers won't trip under the ground fault condition, they will trip under the overload, but NOT on the ground fault. What could be the problem here? Thanks.

I am a little confused. Do you have a ground fault relay on the mains that is not tripping? Or are you saying a branch circuit breaker doesn't trip on a ground fault.

In the first case it sounds like the ground fault relay is just bad.

If the second case is true, it may be that as another poster suggested, that you do not have an effective ground fault path. If you did, there would be an adequate amount of current to trip the BC breaker.

If you have a ground fault for an extended period of time does the BC breaker eventually trip?

If they are not tripping, how do you even know you have a ground fault?

 

[charlie b]

I don't follow your post either. Specific questions:

• What do you mean by saying that the "ground fault for the 3000 amp is not working"? How do you know? Are you saying that you think it should have tripped, but it did not trip?
• How often do you get short circuit conditions on your 277V lighting or 120V power circuits?
• Are you saying that when you have a short circuit, the breaker does trip, but it is the overload feature, and not the ground fault feature, that causes the trip? If so, how do you know this?
• When you say that "the breakers won't trip," what breakers are you talking about? The branch circuit breaker (on the MCC) that feeds the particular load? The MCC's own main feeder breaker? The breaker on the main panel that feeds the MCC?

 

[jsilva]

Ground Fault

Ground Fault

Sorry for making this hard to understand, I'll try to make it a little easier. (1) The ground fault trip relay on the main is not working, but we are in process of fixing it. (2) As for the other ground fault relays on the main switch gear which are overload protection for the MCC's does trip. (3) The other question is how do I know that branch circuit breakers does not trip? When we have to troubleshoot a circuit that is not working, we sometimes find the hot conductor is shorted to gound, the c/b is still on close position, the hot conductor burnt and still hot.
(4) If there is an overload on the circuit, the branch circuit breaker will open, which that is a good thing.
Brian suggested the we don't have an effective fault path to ground to cause the branch circuit breaker to trip and he might be right, we will investigate this problem. Thanks Guys, you all stay safe.