Ground Fault relay tripping off the MAIN intermittently - Any immediate or temporary help to offer? 1

[bdn2004]

There's a 480Y/277V utility feed coming into a plant's switchboard. It divides between two 1000A Pringle Switches that serve as the Mains to the distribution panels.
They are coupled together. There's a ground relay set to 300A, .4 seconds on each 1000A main. Both are tripping - sporadically but at the same time when it happens.
It's tripped off once last month, once this month. It's reset and held.

There's obviously a ground fault, but who knows where.

Is there any advice we could offer, to try to make sure until they figure out exactly what's the issue, that it won't trip off ?

 

[bdn2004]

Not a residual type relay.

 

[davidbeach]

Verify that all conductors of each phase and neutral go through the CT and verify that there are no neutral-ground connections downstream of that CT. Either could increase the sensitivity of the system, particularly adding sensitivity to utility faults (as mentioned further up).

 

[jghrist]

I think rcwilson has the answer. In the diagram below, the left feeder has 2000A more current than the other phases that has to return to the service neutral by some path. The two feeder neutrals are grounded after the relays and allow the return current to split between the feeders and ground return to the service neutral. This makes an imbalance in each ground relay. 2000-1000=1000A in the left feeder and 700A in the right relay. Only the unbalanced portion of loads are shown.

 

[bdn2004]

Thanks ... that makes sense. The only thing is we’ve confirmed that nothing has changed to the downstream loads over the last few months. Of course it is winter time - and that’s a change in what gets utilized. Maybe something went bad ?

So what would be a confirmation of this theory ? An ammeter on the equipment ground at the service entrance ?

 

[Bill West]

Recapping that the plant is decades old but the problem is recent I think we can conclude that it's not a fundamental design or construction flaw. That makes the question of extraneous grounds on the neutral one of "is there anywhere the insulation on the neutral has failed or been bridged?". Measuring at the padmount transformer would be great as basic indicator but presumably the neutral ground is inside a utility locked compartment. So as you ask, if there is a utility ground conductor coming into your first panel then that would be the next best basic check.

After that, probing each enclosure's ground with a tong ammeter would be a way to localize the problem. The difficulty of course is that in each enclosure most of the cables are metallic or have a ground wire making it hard to gather the whole of any ground current flow. All I can suggest is some thought to which grounds come from upstream and which are likely have the lowest resistance back to home.

Another way you might find any ground conductors that are carrying neutral current would be to do an infrared scan of them.

Bill