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Old 01-17-2013, 08:24 PM   #1
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31 volts Neutral to Ground


Hi
I have an APC backup UPS for my computer. The red wiring fault light is on when plugged into an outlet in my office. I took some readings with my dvm and am getting 118.9v hot to neutral. I get 93.5v hot to ground. When i check neutral to ground i am getting 31.4v.
The wiring in the receptacle is bx without a dedicated ground wire.(The house was built in 1954).
I recently installed a subpanel for my woodshop and tested the UPS on one of the circuits and I do not get any wiring fault light on the unit.
What might be the problem and how do I go about fixing it?

Thank You Much..
Bob

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Old 01-17-2013, 08:44 PM   #2
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31 volts Neutral to Ground


Is there a grounding strip in your BX? Likely not. In that case it shouldn't be used as a ground.

It's steel and the cable wasn't really intended to be an equipment ground due to high resistance. In effect, if there was a short to ground the BX casing could act like the heating element in a toaster.

Updating the wiring may be your best option.

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Old 01-17-2013, 08:49 PM   #3
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31 volts Neutral to Ground


Hot to ground and hot to neutral should be the same. Maybe it's possible that you have a fault somewhere and are getting voltage on your bx that is being used as a ground.
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Old 01-17-2013, 08:51 PM   #4
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31 volts Neutral to Ground


Potential Neutral issue either at the Sub panel or at the main panel. Check other circuits to see what they are showing.
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Old 01-18-2013, 04:58 AM   #5
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31 volts Neutral to Ground


All the receptacles on that circuit show the same readings. I have bx on other circuits that are ok, so I'm sure the little grounding strip is on the armor. When i wired up a ceiling fan a long time ago, I added romex to a junction box in the attic on that circuit, but i grounded the wire to the junction box. I had the panel box open not too long ago and all the neutral wires seemed attached well.
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Old 01-18-2013, 07:28 AM   #6
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31 volts Neutral to Ground


You need an Electrician to come in before you have bigger issues.
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Old 01-18-2013, 08:29 AM   #7
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31 volts Neutral to Ground


Conduit can develop high resistance at one or more of the numerous joints at the clamps and fittings at the various outlet boxes due to oxidation over time. This can result in measuring noticeable voltage between neutral and ground in some locations around your house when you actually put a load, such as an incandescent lamp for testing, between hot and ground.

Ground strips or wires running the full length of metal spiral flex conduit and touching the inside surface are required for the conduit itself to be a good and sufficient equipment grounding conductor but the ground strips do not have to extend into metal boxes and be wire nutted to other ground wires entering the box. But if oxidation at the clamps impairs the grounding path via conduit, an inspector may or may not catch that. If the ground strips do extend into the boxes and are wire nutted as if the boxes were plastic or the conduit was PVC then oxidation problems should be extremely rare.

Getting the full 115 to 120 volts from hot to neutral with normal use of the circuit (including using a hair dryer) means that the hot and neutral are okay.
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Old 01-18-2013, 04:08 PM   #8
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31 volts Neutral to Ground


Could be one or both of these things -

Not bonded properly !

Too much resistance in earth line !


Check main panel and see if there is a single connecttion
between the neutral bar and the earth bar.

Check all earth wire connecttions in problem circuit,
one or more could be loose / dirty / not done at all.
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Old 01-19-2013, 04:59 PM   #9
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31 volts Neutral to Ground


I did some investigating by opening the panel. One of the 15amp breakers had two hots on it and I fixed that. I tightened up all the neutrals and got the neutral to ground voltage down to 20v.
I turned that circuit off and checked the receptacles on the 200k setting and got these results-30 ohms neutral to ground....0 ohms hot to ground..29.6ohms hot to ground. I have four surge supressors on that circuit to power all the electronics in my office. Could that make a difference?
The GFCI outlet in the bathroom on that circuit reads normal when probing it..
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Old 01-19-2013, 06:05 PM   #10
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31 volts Neutral to Ground


You need to also go check the Sub-Panel.
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Old 01-19-2013, 11:58 PM   #11
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31 volts Neutral to Ground


30 ohms between neutral and ground is too high !
If your system is bonded correcttily then it should be
less than 2 ohms !
In Australia it is mandated that it be no more than 2 ohms,
In the USA it could be different, but 30 ohms to too much !
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Old 01-26-2013, 04:59 AM   #12
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31 volts Neutral to Ground


We are getting a little closer to tracking this down. I unplugged everything on the circuit and made sure all the lights were off. Neutral to ground with all unplugged came to 6 volts. I then plugged in the UPC and i get 16.9 volts.
An electrician is coming today to look for a neutral problem in a junction box in the attic. I'll keep you posted...
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Old 01-26-2013, 06:09 PM   #13
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31 volts Neutral to Ground


Problem solved!!!!
My Electrician found a bad ground in the bathroom vanity box. He also went up to the attic and found a bx junction box that was not terminated right. Neutral to ground is now .2 volts and I am a very happy camper.
He also added 2 circuits for my office to take some load off the problem circuit we were working on.
Thanks to all for the input..
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Old 01-26-2013, 08:52 PM   #14
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31 volts Neutral to Ground


Quote:
Originally Posted by packman123 View Post
Problem solved!!!!
My Electrician found a bad ground in the bathroom vanity box. He also went up to the attic and found a bx junction box that was not terminated right. Neutral to ground is now .2 volts and I am a very happy camper.
He also added 2 circuits for my office to take some load off the problem circuit we were working on.
Thanks to all for the input..
That's great. Good to see a competent pro step in and fix things and save money.

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