3 phase neutral. Where is this current coming from?

[jes25]

So I got involved with this job my company was doing today. The guys were providing some temp power for a bunch of RV's set up for a show. They daisy chained a few single phase temp panels with receptacles and cords off an existing 3 phase panel. Some breakers in the temp panels were tripping because some of the RV's were drawing more than they were told they would. So a few 120V temp cords were also ran from the 3 phase panels to power some of the trailers and motorhomes.

Here's the weird part: I'm going around amp clamping all the circuits to see if they are overloaded. I check the ground and see it has a bunch of current on it. I look around and see that the N and G are bonded in the temp panels. OK, that's wrong and explains the ground current. Then I check the neutral and, of course, it has a little more on it that the ground, which is incorrectly installed in parallel with the N. This is the part I don't get, the sum of the G and N currents exceeds the the current on either one of the hots! Apparently, the RV's have power inverters to run the lights that are 12V Dc, not sure if this makes a difference. I know the basic formula for calculating N current in a 3 phase system, but I don't know how power factor plays in. Could this make the N current exceed the line current? Going from memory, I was getting readings like: A: 9 amps. B: 19 amps C: not used N and G sum: 30 amps. These readings are taken on the temp feeder.

And here's the weirder part: Since I could not understand the excessive N current. I kept checking into this problem. I shut off the 2 pole breaker that feeds the temp panels and checked the N current on the temp feeder. It still read about 25 amps!! I am totally baffled were that current could be coming from. I did drop to zero when I shut off the main three phase panel however.

Any ideas?

 

[GoldDigger]

So I got involved with this job my company was doing today. The guys were providing some temp power for a bunch of RV's set up for a show. They daisy chained a few single phase temp panels with receptacles and cords off an existing 3 phase panel. Some breakers in the temp panels were tripping because some of the RV's were drawing more than they were told they would. So a few 120V temp cords were also ran from the 3 phase panels to power some of the trailers and motorhomes.

Here's the weird part: I'm going around amp clamping all the circuits to see if they are overloaded. I check the ground and see it has a bunch of current on it. I look around and see that the N and G are bonded in the temp panels. OK, that's wrong and explains the ground current. Then I check the neutral and, of course, it has a little more on it that the ground, which is incorrectly installed in parallel with the N. This is the part I don't get, the sum of the G and N currents exceeds the the current on either one of the hots! Apparently, the RV's have power inverters to run the lights that are 12V Dc, not sure if this makes a difference. I know the basic formula for calculating N current in a 3 phase system, but I don't know how power factor plays in. Could this make the N current exceed the line current? Going from memory, I was getting readings like: A: 9 amps. B: 19 amps C: not used N and G sum: 30 amps. These readings are taken on the temp feeder.

And here's the weirder part: Since I could not understand the excessive N current. I kept checking into this problem. I shut off the 2 pole breaker that feeds the temp panels and checked the N current on the temp feeder. It still read about 25 amps!! I am totally baffled were that current could be coming from. I did drop to zero when I shut off the main three phase panel however.

Any ideas?

Look at the two bolded statements above and put them together:

When you opened the 2 pole breaker that feeds the temp panels, did that also disconnect the temp 120 cords run directly from the three phase panel(s)? If you did not and there is an interconnection at the ground and/or neutral point between the equipment powered by cords and the ground of some other equipment (maybe via water line or metallic connection between grounds?), then the return current from those could end up flowing, in part, through the temp feeder if it presents a lower resistance path than the neutral/ground in the (fairly long?) extension cords.

Keep in mind that there may or may not be a ground to neutral bond in the AC panel of each of the RVs, or at the shore power-to-inverter transfer switch if they have one. (The transfer switch may be built into their inverter charger where you can't see it.). Just to be picky, the inverter is not inverting to power the 12 volt accessories in the RV, but the power converter or inverter-charger may be generating 12 volts to the batteries and lights using the AC shore power.

 

[jes25]

Goldigger, What you describe about the interconnection was my first thought. The only problem is I can't possibly imagine where this interconnection might be. There is no common water line. The things are parked close, but I doubt they are making metal to metal contact. I admit it does seem to be the only plausible explanation, but still......

I do think the G and N are bonded in the RV's. I know this because, I found an effectively broken N wire in one of the temp panels (the shop guys built these panels) and the RV was on and working.

 

[Smart $]

.... Could this make the N current exceed the line current? Going from memory, I was getting readings like: A: 9 amps. B: 19 amps C: not used N and G sum: 30 amps. These readings are taken on the temp feeder.

Yes, it is possible. First off, when you have a 120/208V 1? 3W (i.e. two L and N from 208/120V 3? 4W system), the neutral current does not cancel out completely. The neutral current is exactly what it would take from a load connected to the third phase to cancel out completely. At unity power factor, the neutral would be ~16.5A. If A had a power factor of .6 and B 1.0, neutral current would be ~24A. Now if you have additive triplen harmonics from the power converters, the neutral current could easily reach 30A.

And here's the weirder part: Since I could not understand the excessive N current. I kept checking into this problem. I shut off the 2 pole breaker that feeds the temp panels and checked the N current on the temp feeder. It still read about 25 amps!! I am totally baffled were that current could be coming from. I did drop to zero when I shut off the main three phase panel however.

Yes, that is wierd. Is there any common bonding elsewhere (especially N-G bonds)? You need to track down the cause of that current...

 

[GoldDigger]

Goldigger, What you describe about the interconnection was my first thought. The only problem is I can't possibly imagine where this interconnection might be. There is no common water line. The things are parked close, but I doubt they are making metal to metal contact. I admit it does seem to be the only plausible explanation, but still......

I do think the G and N are bonded in the RV's. I know this because, I found an effectively broken N wire in one of the temp panels (the shop guys built these panels) and the RV was on and working.

Well, Smart Money gave a good explanation for unexpectedly high neutral current when loads were turned on. But with the feed to the RVs connected to the temp panel circuits turned off, the additional neutral current will have to be coming from somewhere.

Even if you do not see any metal to metal contact anywhere, there are two things you might not notice:
1. Connection to ground at one of the RVs. Any such deliberate or accidental earth connection would have to be improbably good to carry the amount of current you are seeing, given that it would have to present a lower impedance path than the official extension cord returns.
2. Somebody in one of the RVs on a temp panel has run an extension cord from a grounded appliance in their RV to one of the extension-cord-fed outlets. That would create a ground-to-ground connection between the two systems, and as a result a neutral-to-neutral connection too.

I suspect that if you turned off the temp panel breakers and then unplugged the cords from the three phase panels one at a time you would find that all of the neutral current is coming from one or two of them (and probably ones which are on a different phase than the two that are going to the temp panels!)

If you identify a problem cord, follow it and you will find your answer, I am confident.

PS: Whatever else happens, you may just have to accept for now that there are far too many ground to neutral bonds in play. RVs tend to do that.
And trying to fix the problem on umpteen RVs will not be high on your agenda as finding the crossed over neutrals. Right now you are working with an MWBC from hell.

 

[jes25]

Thanks for the help guys. I was sent on another project today so I don't have any new information, but I'll keep you posted when I do.