Current on grounding conductor

[faw]

I'm trying to determine the source of current on a grounding conductor. I have a single phase 400 amp metered CT configuration feeding 2, 200 amp disconnects. These in turn feed 2, 200 amp panel boards. These panels are side by side. On panel "A" I am seeing approx. .03 amps on ground. On panel "B", which is feeding most of the buildings mechanical devices (HVAC, Compressors, Vacuum pumps) I see anywhere from .74 amps to as high as 3 amps depending on how many devices are running. The HVAC's cause the largest changes. The neutral and grounding buses are isolated from each other at the panel boards. A grounding electrode is installed at the metered CT. I don't know if this is related by I also have 3 volt between line/neutral and line/ground when a circuit is turned off. Any ideas on how to eliminate the current from ground? If you need more information I will gladly provide.

 

[GG]

Re: Current on grounding conductor

What I would do is get my reading on my meter showing me the current flowing on the GC and then flip each breaker off 1 at a time and see if I can isolate the circuit that is causing current to flow on your GC. Hopefully if you do that you will find the circuit causing the problem, if the source is even from 1 of the 2 panels. It could even be current from a neighboring service that has lost its neutral, not likely but possible. Happy hunting.

 

[hurk27]

Re: Current on grounding conductor

3 Amps on a grounding electrode is not very much and if the electrode is a commonly connected water main then it's common to even have more. This is because current takes all paths back to source not just the one with the least resistance. A water main will be in parallel with with your service neutral as well as your neighbors service neutral and you will see current from both services flowing on the GEC, A grounding electrode can pass as much as 4.8 amps if it has the NEC minimum of 25 ohms resistance to Earth, but this would be proportional to the amount of unbalanced current on the service neutral and could indicate a week or lost neutral. The third possibility is the transformer supplying your service or one just down the road has lost it's primary neutral (MGN) and is now getting it's return path through your service grounding electrode system, in this latter case it should be mention that if you opened your grounding electrode connections at your service you could find yourself with primary voltage on your service grounding so you should never remove all grounding electrode connections if you suspect a bad primary neutral. The utility will come out and check the neutral for you by seeing if there is any current on their primary neutral, if there is not any current on it, then they know that it's getting a return from somewhere else, If you have both a water pipe electrode and a rod or other type of electrode removing the water pipe connection should cause the current and voltage to Earth to rise, if this happens then call the utility immediately! Of course put the connection back ASAP. And wear HV gloves when doing this test. Under no reason disconnect the other type of electrode! as it will be the only thing keeping the voltage at bay. Also turn off all the single pole breakers on one leg of the panel so if there is a lost neutral it can't fry anything but it will still apply a load to the transformer.