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Discussion Starter · #1 ·
I recently went out to a customer and the call was that they lost power to the building. Before I arrived, they already found the problem breaker, turned it off, and reset the 1200A GFCI main. The problem was a 70A RTU branch ckt. A compressor had a ground fault that caused the main to trip. There is a breaker at the panel and in the RTU, and the RTU has 70A fuses in series with the breaker in the unit, of which A phase was blown. I'm assuming it was the GF sensor that tripped the main, and this is not the first time they have had this problem at the semi-upscale restaurant. The adjustment dials for the ground fault sensor are set at max sensitivity as far as I can tell. I've always heard that an engineer is the only person qualified to adjust these dials. Seems to me that they should be adjusted to avoid this nuisance tripping that has happened at least twice now. Any thoughts?
 

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I recently went out to a customer and the call was that they lost power to the building. Before I arrived, they already found the problem breaker, turned it off, and reset the 1200A GFCI main. The problem was a 70A RTU branch ckt. A compressor had a ground fault that caused the main to trip. There is a breaker at the panel and in the RTU, and the RTU has 70A fuses in series with the breaker in the unit, of which A phase was blown. I'm assuming it was the GF sensor that tripped the main, and this is not the first time they have had this problem at the semi-upscale restaurant. The adjustment dials for the ground fault sensor are set at max sensitivity as far as I can tell. I've always heard that an engineer is the only person qualified to adjust these dials. Seems to me that they should be adjusted to avoid this nuisance tripping that has happened at least twice now. Any thoughts?
Yes, they should, but likely will not pay an engineer to do a coordination study to determine the correct settings.

We refuse to touch the settings without an engineers involvement.
 

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I recently went out to a customer and the call was that they lost power to the building. Before I arrived, they already found the problem breaker, turned it off, and reset the 1200A GFCI main. The problem was a 70A RTU branch ckt. A compressor had a ground fault that caused the main to trip. There is a breaker at the panel and in the RTU, and the RTU has 70A fuses in series with the breaker in the unit, of which A phase was blown. I'm assuming it was the GF sensor that tripped the main, and this is not the first time they have had this problem at the semi-upscale restaurant. The adjustment dials for the ground fault sensor are set at max sensitivity as far as I can tell. I've always heard that an engineer is the only person qualified to adjust these dials. Seems to me that they should be adjusted to avoid this nuisance tripping that has happened at least twice now. Any thoughts?
GFCI and GFP are 2 very different things. And this wasn't a "nuisance trip", there was a ground fault and it did exactly what is was supposed to do.

Unless there are other breakers with ground fault protection in the system there is nothing to coordinate with (Often a cost savings design to meet code minimum) so they don't have many options. Leave the settings where they are if that is the only GFP in the system.
 

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Unless there are other breakers with ground fault protection in the system there is nothing to coordinate with (Often a cost savings design to meet code minimum) so they don't have many options. Leave the settings where they are if that is the only GFP in the system.
You know I disagree.

All breakers and fuses on grounded systems provide ground fault protection of some type and at a known value.

The main breaker has to be set up to let the branch or feeder breakers open first.
 

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You know I disagree.

All breakers and fuses on grounded systems provide ground fault protection of some type and at a known value.

The main breaker has to be set up to let the branch or feeder breakers open first.
There is a tiny bit of truth to that but much easier said than done. Lets look at the senario above. Would be easier to discuss in detail if we knew what type of GFP system they have but I would guess zero sequence.

If the 1200A breaker/switch has it's GFP setting to minimum it is most likely set at 100A. The 70A breaker protecting the compressor that faulted wll have an INST or magnetic trip somewhere between 420-700A (LT or thermal trip will never beat the mains GFP).

So in theory you could crank that GFP all the way up, if it even goes high enough, and your idea would work, however the damages done to the equipment would be much more severe, which is the whole point of GFP.

Also keep in mind if this facility has done an arc flash study adjusting the settings can have adverse effects.
 

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Discussion Starter · #6 ·
GFCI and GFP are 2 very different things. And this wasn't a "nuisance trip", there was a ground fault and it did exactly what is was supposed to do.

Unless there are other breakers with ground fault protection in the system there is nothing to coordinate with (Often a cost savings design to meet code minimum) so they don't have many options. Leave the settings where they are if that is the only GFP in the system.
I see your point that it did what it was supposed to do, but in a way I do view it as a "nuisance trip" because the whole building shouldn't be shutting down because of a ground fault in a RTU... The delays are set at the minimum on the 2 ground fault settings, and I think I agree with BBQ that it does need to be adjusted. I have contacted Square D to see if we can get their input. This place should have the money to get an engineer involved. That's probably the right course of action. Thanks for the input! I'm new to the forum, please forgive any improper etiquette.
 

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1200A breaker for a restaurant? That must be some restaurant. We did a 1600A once, but it was for a restaurant/bar/wine celler/saddlemaker shop all rolled into one huge old historic building.
 

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The code specifically permits the GFP to be set at 1200 amps. Yes that would result in more damage than a lower setting but the code says that is ok.
 
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