Two ground fault relays on one feeder. Required?

gtout

Member
Location
Princeton NJ
Greetings all, this is my first post. I'm a design engineer from Princeton NJ with about 30 years experience.

I have an installation of a 750kW PV system. The collector switchgear at the array side is about 1300' away from the utility tie-in point. Since the feeder is rated 1200A at 480VAC we have a GF relay on the main (Siemens HNG3B120L and external GF relay iGard MGFB-1200-ZB). The output is a series of (99) single-phase distributed inverters connected "wye." Note we did not bond the neutral to ground here.

At the tie-in point to the utility we installed a 2nd 1200A breaker (Siemens NNG3B120L with internal trip unit 576) ahead of the tap to the switchgear bus. The tap conductors are downstream of the main 1600A breaker and upstream of the utility C/Ts as required by the utility. We terminated the neutral conductors ahead of the GF sensors on the main and the neutral bonding jumper is right there. So as far as we can tell, there are not multiple paths to ground where GF current might leak around a sensor.

The problem arose when the local AHJ insisted that the 2nd 1200A breaker also be equipped with GF protection. We are getting what we believe to be nuisance GF trips on this 2nd breaker, 3 times in approx 3 months. We had a setting study performed but we're still getting trips. The installer has meggered all cables after each event but the cables are fine. In desperation, we set the relay to its least sensitive setting but we're still tripping out for no apparent reason.

We thought we didn't need the 2nd GF relay and referred the local AHJ to NEC Article 215 in a letter:

"We believe that we are in compliance with NEC Article 215.10 Exception No. 2 in that we have provided GF protection at the PV switchgear at the solar array which is the supply side of this feeder (there are no transformers). This protects the entire feeder right up to the point of connection to the main switchgear including the disconnect in question. The key is that we?re not bonding the neutral at the PV gear. Any potential ground fault current can only find its way back to the utility transformer X[SUB]0[/SUB] point via the single bonding jumper in the main gear. We are unaware of any NEC article that requires more than one GF protection means on a single feeder."

The local AHJ did not agree and required the 2nd GF relay. Any insight or opinions on whether this 2nd GF relay is required or what our problem might be?

Thanks,
Garry Out
 

brian john

Senior Member
Location
Leesburg, VA
To be honest I had trouble following your install and a one line would be NICE.

You have a Solar array that is 480 VAC the CB at this location has GFPE?

Then you have a utility feeder that has a CB this has GFPE?

These two feeders are tied at some point?

The neutrals are tied downstream of the GFPE sensor in the utility switchboard?

The neutral to ground bond is upstream of the utility GFPE Sensor?

Where is the 2nd CB with GFPE at the array? Or at the utility switchboard and this is how the array feeder ties into the utility switchboard?




But

Have you tested the Circuit breaker?
Are you positive the CB has no other features UV Release? Phase Relay?
On the GFP CB that is tripping what are the settings?
Did they megger the neutral for downstream grounds?
Seeing as you have meggered all the downstream feeders I would install a Line Disturbance analyzer, utilizing a 5 channel unit I would utilize the Ground Channel for a zero sequence CT if large enough or install 4 CTs on the phase and neutral capture the ground current that way that way.

Lastly as an engineer I would think by use of your one line and a does of education you could show the AHJ the error of his ways. I find it rare to never that an AHJ has a clue about GFPE.
 
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gtout

Member
Location
Princeton NJ
Additional info Re: Two ground fault relays on one feeder. Required?

Additional info Re: Two ground fault relays on one feeder. Required?

Brian:
Thanks for the timely reply. I wasn't sure if I should immediately start attaching drawings and such to my initial post, but I also appreciate the value of a SLD to understand what's going on. Here it is. This should answer your first 6 questions.

CB-2 is the problem breaker. Are you also of the opinion that the GF protection on CB-2 in not required by code?

Replies to your next questions:

  • The breaker was tested pre-installation back in May '12 and worked for 5-6 months before we started having the problems, so no it hasn't been tested again.
  • I have requested the electrician confirm whether we have a UV feature. I suspect this could actually be the culprit if set incorrectly.
  • CB settings: We had a study done and set the breakers accordingly. We're in process of re-confirming all settings.
  • Yes, the neutral was meggered for downstream grounds. All OK.
  • The testing agent has recommended monitoring with a Hioki 3196. It's being hooked up this week, using 4 C/Ts.
  • Lastly, (sigh) the relationship between my client and the AHJ is delicate one, so we've always stepped lightly. Some will never admit being wrong is all I can say.
Thanks for the insights,

Garry Out
 

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brian john

Senior Member
Location
Leesburg, VA
1st you only need the one GFPE protected circuit breaker. If I am wrong I will apologize but this inspector needs to be schooled on the NEC. It is his job to provide you a reference to his EXPENSIVE request.

Your drawings state that the CB-1 is Zero Sequence (external to the circuit breaker one window CT).
Is CB-2 zero sequence or residual?

We have had some residual GFPE with a CT installed backwards from the factory, only caused issues as the loads increased from construction to full building occupancy.

There have also been some cases where newer CB's had Trip device issues, that showed up after being on line a while.

Do you think the trips happen when you go from the PV system feeding the load and the grid to the utility only feeding the load?

In the PV configuration is there neutral current?

I doubt you have a CT largest enough to encompas all 4-sets of 500kcmil. But I would install 4 large CTs and connect the output to monitor the current (residual setup), so there is no load normally. Not sure on the Hyoki's sample rate.

The other 3 phases I would connect to the phase's.

CB-2 is set at 1200 maps 1.0 seconds? If so I am betting on a defective CB.

On a wild idea you could install a zero sequence window sensor and GFPE relay with a trip indicator (Like Electro Magnetics GFPE relay).
 

zog

Senior Member
Location
Charlotte, NC
Does CB2 use a 3 or 4 wire scheme for the trip unit? If it is 3 wire unbalanced loading on the phases can cause false GF trips.
 
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