4000 amp main GFCI

[james wuebker]

Quick question. Installing a new 4000 amp main in at a business. 277/480v. This will have a GFCI on the main. The lighting at this company now is just coming off a breaker to a sub panel and using 1 line and a neutral(277v). If I install the new main with the GFCI main breaker will I have to do something with the lighting panel. If I just hook up and feed that lighting panel and have a short on the lighting will it trip the main. Do I need a transformer to feed the lighting panel now because of the GFCI main? Some people say no but I think Yes. Any ideas?
Thanks!
Jim

 

[ron]

Re: 4000 amp main GFCI

What type of GFI is being installed at the 4000A main? Does the GFI CT's add the currents of the three phases and the neutral, or just the three phases?
Are the three phases relatively balanced? How balanced?
What will be the setting of the GFI? 1200A?

 

[steve66]

Re: 4000 amp main GFCI

I don't see any problem. A GFP circuit senses the current flowing through the ground, not the neutral. So a load on a single phase shouldn't cause any problems.

As for a short on a lighting circuit tripping the main GFP, you can set the GFP for a 3000A setting at one second delay. The 1200A setting can even have a longer delay (see 230.95 - this happened to be one of the questions on my PE exam). Thus, the branch circuit breaker should trip long before that.

STeve

 

[mclain]

Re: 4000 amp main GFCI

Steve
The GFIs I have worked with didn?t sense current flow through the ground. They sensed the current flow from all three phases and neutral, as long as your currents are balanced you have no ground fault. If a ground fault occurred there would be an imbalance in the current since part of the current is now returning to source by a ground fault path. If this imbalance reached the set point of the GFI then it will trip the system off line.

Don McLain

 

[jgcruz]

Re: 4000 amp main GFCI

Information I have on ground fault system testing preparation guide, shows me three types of sensor locations. One for Zero sequence ground system, residual ground system and neutral ground strap.

 

[ron]

Re: 4000 amp main GFCI

Many ways to skin the GFI cat, thus my questions.

 

[dnbob]

Re: 4000 amp main GFCI

I ran into this a few years ago as we were doing a T.I. in the same building as the AZ dept. of Revenue was in, just a different suite. One of my guys used mc cable for a dead end switch leg and must have forgot what he did and tied the white into the neutrals etc., so when the switch was turned on, the 3000 amp 480/277 v. gfi main tripped. Not too many happy people that day! We didn't install the service, so maybe the gfi was set too sensitive? One more quick story, a backhoe hit a parking lot lighting conduit (277 v.) and the lights happened to be on at the time ...it tripped the building main gfi (1600 amp). That gfi was set specifically by the engineer, after the incident, he still would not let us make any adjustments.

 

[ron]

Re: 4000 amp main GFCI

dnbob,
The gFI was set at 1600A or the main breaker (service size) was 1600A?

 

[dnbob]

Re: 4000 amp main GFCI

Ron. the main was 1600 amps, sorry.

 

[steve66]

Re: 4000 amp main GFCI

Don:

Maybe I'm oversimplifying, but sensing the unbalance in the 3 phases and the neutral is pretty much the same as sensing the current on the ground. Any unbalance must be flowing through a ground fault path. The equipment and parts are different than placing a current transformer around the ground, but the effect is basically the same. Either way, I don't see a problem with feeding a 277V lighting circuit.

Steve

 

[templdl]

Re: 4000 amp main GFCI

I would understand that you are complying with 230.95 (A), Setting. As such the breaker has been provided with a 1200a GF pickup. If should be equipped with a neutral sensor for 3ph4w GF protection. It should be adjustable from a low of commonly 20% of the 1200a to 1200a. The ground fault feature may also be provided with a time delay.
If you don't have any GF protection on any down stream devices your out of luck if a ground fault is seen by the main enough to cause a trip, it will shut down the entire facility.
Coordination with ground fault is the only option which is often omitted to keep down cost. Coordination is often done by a consultant or another qualified person in which the characteristics of the GF as provided in the main is reviewed and compared with that of the GF features that would be provided with a feeder or branch breaker that also has GF. By correctly setting the pickups values and delays of the main, feeder, and/or branch breakers you are able to provide better isolation a ground fault.
Remember that the adjustable pickup and delay setting feature must available of the breaker which can be options and must be specified and ordered correctly.