Neutral to ground current

[rick hart]

I almost hate to ask this question; I believe I should know the answers but, humbly admit that I need an education.

The scenario:
A 200A 120/208V 3 PH panel was noticed to have an arc between neutral and ground. The electricians measured 25A @180Hz of current, 1.047V between ground and neutral. The panels are served from an 600A riser duct which is fed from a 2000A service entrance switchboard <100' away and a double neutral. We have not checked the neutral connection, preferring to wait for a scheduled shutdown next week. I am not sure how the individual phase currents are loaded.

Question:
1. Is the arching N-G a sign of just the distance between the bonded neutral and the point measured i.e. Is the low voltage current flowing to ground because it is the shorter path?
2. Is the 180 Hz current is a combined frequency of all three phases?
3. Since there is a measurable voltage differential between N/G, would there likely be any negative impacts?
4. Besides checking the neutral for positive connection at the duct riser terminal and the service entrance neutral/ground bond, is their something else that should be checked?
5. Is this normal to begin with?

Thanks in advance for any input and....be gentle .

 

[charlie b]

Re: Neutral to ground current

I'm not quite following your description. I infer this is a "sub-panel." I conclude that its neutral and ground buses should be separated (no connection). I can understand how you could get a voltage reading between the two; indeed I would be surprised if there weren't.

But I do not understand what you mean by the measurement of 25 amps of current. In what wire (or other conductive path) was this current measured?

If the neutral and ground buses are separated, and if you are seeing signs of arcing between the two, then you do have a problem. They should not be close enough to allow an arc, and it would take a far, far higher voltage than your measured 1.07 volts to force the arc to jump the gap.

As to the 180 hertz, that is the third harmonic for a 60 hertz system. Third harmonic currents from all three phases will add together in the neutral. This tells me that you have some amount of non-linear load being supplied from this panel. I can't say whether this is a large amount, or just a normal amount, without further information.

 

[rick hart]

Re: Neutral to ground current

Arcing was probably not a good choice of words; sparks are visible as a screwdriver touched both the can and neutral bar at the same time. No damage is done when this is occurs but a hum is audible. The current was measured off of a temporary jumper between the neutral bar and the ground bar.
The 180Hz- wouldn't I expect to see the currents from A, B and C phase combine in the neutral that way since there are three zero crossings per cycle?
I am starting to feel like I reacted too soon instead of trying to cypher this out.through. :roll:

 

[don_resqcapt19]

Re: Neutral to ground current

Rick,
Assuming that the SDS has the required system bonding jumper at the transformer or the first OCPD, and the arc was at a remote panel, it is just a result of the voltage drop on the grounded conductor. You can get a small arc between the grounded and grounding conductors on most systems at any point on the load side of the system bonding jumper. The 180 hertz indicates that you have a harmonics problem. I have also assumed that the 25 amps of current was measured on the grounded conductor.
Don

 

[charlie b]

Re: Neutral to ground current

Originally posted by don_resqcapt19:
(the arc) is just a result of the voltage drop on the grounded conductor.

That is what I meant, when I said that I would be surprised if there were no measurable voltage between N and G at a sub-panel.

 

[charlie b]

Re: Neutral to ground current

Originally posted by rick hart:The 180Hz- wouldn't I expect to see the currents from A, B and C phase combine in the neutral that way since there are three zero crossings per cycle?

The math is actually much harder than that, but it's not a bad way to think about it.