Ground Fault Protection Tripping Main

[strap89]

Hello,

I'm having a main trip intermittently, presumably the ground fault protection is the culprit. Service is sized more than adequately. This is a 2500A service with a ground fault trip setting of 200A. My first instinct was that a load imbalance was being mistaken for a ground fault condition, but I am starting to think that this may not be a possibility. It's my understanding that the vectoral sum of the phase currents will equal zero unless regardless of which sensing method used. Is this a correct assumption?

My thoughts are possibly an actual ground fault, or maybe an incorrectly wired CT? Any thoughts or input would be appreciated.

 

[strap89]

More specifically on the incorrectly wired CT hypothesis, I think the service is using a source ground sensing configuration. I would assume the ground sensing CT if wired backwards would read a positive ground fault condition even with an imbalanced load?

 

[ron]

If the GFP is sensing all ungrounded and grounded conductors, then the sum will be 0 unless the current is going on the grounding system/conductor somewhere. Some GFP only senses the ungrounded conductor, so a sever imbalance could be misinterpreted as a GF.

 

[jim dungar]

You should check the main GF setting with the phase setting of your downstream breakers. Many people forget to do this.

Based on a 200A setting I would guess your main will trip before a 30A branch breaker. The result is that a miswired or faulted duct heater could trip your GF now that heating season is upon us.

If you only have GF at the main it will be likely that you can only coordinate up to about a 70A branch.

 

[don_resqcapt19]

You should check the main GF setting with the phase setting of your downstream breakers. Many people forget to do this.

Based on a 200A setting I would guess your main will trip before a 30A branch breaker. The result is that a miswired or faulted duct heater could trip your GF now that heating season is upon us.

If you only have GF at the main it will be likely that you can only coordinate up to about a 70A branch.

Seen it happen more than once with 277 volt ballasts, where the ground fault setting for the main was left at the factory minimum because the designers had never specified the correct setting. Typically this would be long after the project had been completed, when the ballasts started to fail.

 

[jim dungar]

Seen it happen more than once with 277 volt ballasts,

I always had my engineers show a common 30A breaker on the same TCC plot as any GF. I felt this was typically the maximum size circuit that people tended to work on while energized, like when changing ballasts or switches.

For some reason our industry shies away from GF whenever possible, usually resulting in a single level of protection and these main breaker "nuisance" trips. It must be more profitable to troubleshoot these trips than it is to prevent them in the first place.

 

[hillbilly1]

I always had my engineers show a common 30A breaker on the same TCC plot as any GF. I felt this was typically the maximum size circuit that people tended to work on while energized, like when changing ballasts or switches.

For some reason our industry shies away from GF whenever possible, usually resulting in a single level of protection and these main breaker "nuisance" trips. It must be more profitable to troubleshoot these trips than it is to prevent them in the first place.

Probably liability. If a fire happens because of a ground fault, and it’s not at minimum, the finger gets pointed at the electrician, or if it’s in the specs, the engineer.

 

[hillbilly1]

Most of the time it’s doing its job, ground fault in the parking lot lighting, bad second stage A/C compressor or heat strips. The big box store maintenance departments would want us to turn it all the way up.

 

[jim dungar]

Probably liability. If a fire happens because of a ground fault, and it’s not at minimum, the finger gets pointed at the electrician, or if it’s in the specs, the engineer.

I agree, you should not just 'click and go', someone needs to make a conscious decision which settings are appropriate.

I would think a professional contractor would still be liable for blindly following an unqualified customer that says 'crank 'er to the maximum'.