Shared/open ground problem


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Old 09-29-13, 12:13 PM
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Shared/open ground problem

I'd like to start by saying I have no problem hiring a pro if this isn't a DIY fix, but I would like to understand the problem before I do. There is a tl;dr at the bottom if you don't have time to read the whole thing.

I have two circuits with wiring issues and I think the problems are related. Here is what I have discovered so far:

When an outlet tester is used on a receptacle on Circuit #1, both yellow lights come on (correct).

When an outlet tester is used on a receptacle on Circuit #2, both yellow lights come on, and the red one comes on 1/2 way. Google pointed me to a reference describing this situation as "the wiring is FUBAR".

When an outlet tester is used on a different receptacle on #1, all three lights come on (FUBAR). If I plug a UPS into the other outlet on this receptacle, the red light on the tester goes out (correct) and the UPS's "building wiring fault" light comes on. This receptacle has a ground wire joining it to a ground wire from a light on #2.

#2 has no ground to the panel. But, some of it appears to have been upgraded and now has a ground wire. It's unfortunately not possible to run a ground wire to the panel.

#1 and #2 are on different legs and do not have a shared neutral.

tl;dr: Is it a problem to share ground, particularly if the two hots are on different legs?

Thank you in advance to anyone who would care to tell me their opinion.
 
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Old 09-29-13, 12:35 PM
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Outlet testers are fallible. They are good for a quick test everything is okay but they are not reliable for determining what is wrong. For that you need a multimeter (preferably analog). A cheap $8-$15 dollar analog meter is what you need. (A cheap digital can give misleading results.)

Using a multimeter set to voltage on the scale neares to 250 volts you need to make these measurements:
  • Hot (narrow slot) to neutral (wide slot). Should be ~120 volts.
  • Hot to ground (round hole). Should be ~120v.
  • Neutral to ground. Should be about ~0v.
 
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Old 09-29-13, 12:45 PM
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Thank you very much for replying.

First outlet I mentioned: (Circuit #1)
Hot to Neutral 150V
Hot to Ground 150V
Neutral to Ground 0

Second outlet: (Circuit #2, no ground at panel)
Hot to Neutral 150V
Hot to Ground 225V
Neutral to Ground 50V

Third outlet: (Circuit #1, ground wire connected to circuit #2.)
Hot to Neutral 150V
Hot to Ground 150V
Neutral to Ground - needle moves slightly when I tested just now, was 45V a couple of hours ago.

I suspect the meter is out of calibration and the 150V is actually correct.
 
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Old 09-29-13, 12:48 PM
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Those readings don't look correct. If this is digital meter change the battery.
 
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Old 09-29-13, 12:50 PM
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Those were taken with an analog meter.

I would be delighted if you could comment on this question: "Is it a problem to share ground between circuits, particularly if the two hots are on different legs?"

I've opened up about 1/2 the receptacles so far and that was the only thing that appeared majorly out of place.
 
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Old 09-29-13, 12:57 PM
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Do you mean to share neutrals on two hots that are on different legs ?

It already sounds like you have a open neutral in three wire circuit.

I suspect the meter is out of calibration and the 150V is actually correct.
Analog meters don't require user calibration on the VOLT scales.
 
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Old 09-29-13, 01:00 PM
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No, I did in fact meant shared ground.

These are two two-wire circuits, each with its own neutral, not a three-wire circuit.
 
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Old 09-29-13, 01:26 PM
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Grounds need to be connected together when there are multiple circuits in the same box.
 
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Old 09-29-13, 01:40 PM
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Okay, I understand that. And it's not a problem to connect those grounds together, if one of the circuits doesn't have ground going to the panel?

(Beyond the obvious "both circuits should have ground going to the panel".)
 
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Old 09-30-13, 07:19 PM
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"Is it a problem to share ground between circuits, particularly if the two hots are on different legs?"
Consider MWBC or shared neutral --- two different *hot* legs sharing the neutral and grounding conductors.

Those voltage readings sound fishy.
It's almost like you might see with a neutral problem on a MWBC or at the service.

If you are reading 45 volts between the neutral and grounding conductors there might be some neutral-to-ground connections --- possibly at a receptacle(s). Might be leakage current.

There were some misguided people that believe neutal-to-ground connections were ok when connecting grounding receptacles to 2 wire ungrounded circuits.

2 cents worth.
 
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Old 10-01-13, 07:42 AM
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Thanks; that makes sense. I will look elsewhere for the source of the problem.

I hope to get time to inspect the remainder of the receptacles later today.
 
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Old 10-05-13, 09:59 PM
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The verdict: interrupted ground between two outlets, and severely degraded insulation on wires at a ceiling light. Thanks to everyone who responded!
 
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Old 10-05-13, 10:31 PM
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Glad you figured it out, and thanks for letting us know.
 
 

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