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I have an APC battery backup and surge protector for my computer. It has a light, labeled "Building wiring fault." This light is on, and my computer will not start. I am unemployed and my landlord is useless, so I bought a multi-meter (Etekcity MSR-C600). Checking voltages at the outlet, I found the following:

  • Hot to neutral: 117.5
  • Hot to ground: 60.8
  • Neutral to ground: 55.9

I then checked the voltage several other outlets. All of them have the same hot to neutral voltage, but the hot to ground and neutral to ground vary considerably. For example, another socket had:

  • Hot to ground: 77.2
  • Neutral to ground: 36.5

Is this sufficient to tell me the most likely cause of the problem? I'm guessing that hot and neutral are cross-wired or shorted in some outlet. Is that probably correct? This circuit is rather overloaded, but it does not seem to me like that should cause the low hot to ground voltage and the high neutral to ground voltage that I observe, unless there is a partial short in some appliance.

All the outlets in my apartment are on a single circuit. How do I go about locating and identifying the actual wiring problem?

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    \$\begingroup\$ I'm not going to add another answer that says "this is a big problem" , but it is a big problem. Even with everything unplugged, there's shock hazard and fire hazard. Do you have any neighbors that are friendly enough to let you measure their outlets? If the problem is bigger than just your apartment's circuit, it's even more important that the landlord and utility company know right away. \$\endgroup\$
    – bitsmack
    Commented May 18, 2016 at 1:00
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    \$\begingroup\$ If neutral is not bonded to ground at the service entrance or breaker box (as it should be) then you will measure basically random voltages between neutral and ground. You have identified the problem, please report it. Even before 3 wire systems, the neutral side was bonded to ground. Your AC supply is "floating", which it should never do. Another fault could conduct the 10,000 volt supply in to your apartment, and you. \$\endgroup\$
    – user56384
    Commented May 18, 2016 at 1:10
  • \$\begingroup\$ Guys, you did forgot that in US they have 2 hots + ground. Not neccessarily it has the neutral conductor. \$\endgroup\$
    – Marko Buršič
    Commented May 18, 2016 at 8:56
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    \$\begingroup\$ Hi @MarkoBuršič! In the US, our standard outlets have Line, Neutral, and ground (with 120V L-N). A home will usually have one or two special outlets with two Lines and a ground (no Neutral), with 240V between the lines. These are for appliances, e.g. ovens or electric clothes dryers. But we don't have any standard residential consumer goods that run on 240V. \$\endgroup\$
    – bitsmack
    Commented May 18, 2016 at 14:13
  • \$\begingroup\$ For anyone following this topic, there is now a part 2 here \$\endgroup\$
    – SamGibson ♦
    Commented May 22, 2016 at 23:31

4 Answers 4

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The normal (if a bit optimistic) values you'd be looking for are 120V Hot->Neutral, 120V Hot->GND, and 0V Neutral->GND. It sounds like you pretty much get that already.

I'd be strongly inclined to believe the fault is not in the socket itself, but rather either in a connected appliance or in the wiring itself. The wiring TO the socket could be at fault, especially in a socket that had been habitually overloaded (and heated up as a result).

You might start by unplugging everything in your apartment and testing again. If that clears the fault, plug half of them back in, see if it reoccurs, then step-wise in that binary fashion narrow it down to a single appliance.

If that doesn't clear the fault, I would head to the breaker box and take measurements with your circuit's breaker in the OFF position. But this is probably where you should start looking for someone like an electrician, because this gets you into the (more) dangerous zone, likely having to remove the cover plate from the breaker box. But a measurement here would show whether the fault is inside or outside your apartment.

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    \$\begingroup\$ +1 This is good advice. But, scanny, how would a connected appliance cause the ground line to be energized? I think it must be a problem with the incoming wiring. \$\endgroup\$
    – bitsmack
    Commented May 18, 2016 at 0:54
  • \$\begingroup\$ That's a very good point @bitsmack, I've been puzzling over that starting about 15 minutes after my post. I'm inclined to agree with no comprende below that this is a likely dangerous site-wide wiring problem and not just the washing machine having developed a short. I'm not quite clear why a floating neutral would affect the Hot->Ground potential, but I find your and no comprende's interpretations compelling. I vote for checking the neighbor's outlet and reporting the overall findings to the landlord and probably the utility as well just in case the landlord doesn't move quickly :) \$\endgroup\$
    – scanny
    Commented May 18, 2016 at 2:32
  • \$\begingroup\$ These answers are all good. I accepted this one because it seemed to have the most useful information. I'll get right on this. \$\endgroup\$
    – andrewH
    Commented May 18, 2016 at 14:56
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Yes, you have a problem. Basically, your "ground" is floating with respect to neutral. This suggests a failed connection, and this is potentially life-threatening.

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  • \$\begingroup\$ +1. Older US homes had ungrounded outlets. It is quite possible that a grounded outlet has been used to replace an ungrounded outlet without connecting the ground (code violation) or downstream of a GFCI (AFAIK OK if labelled appropriately). It is also possible the ground connection (e.g. outer metal sheath of armored cable) has failed. \$\endgroup\$
    – RedGrittyBrick
    Commented May 18, 2016 at 14:03
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You have a SERIOUS PROBLEM. Potentially life-threatening and structure-endangering. As @scanny suggested unplug EVERYTHING in your apartment to make the differential diagnosis whether it is the building wiring or some appliance of yours that is causing the problem. If the problem still exists with everything unplugged, then the building owner must be notified ASAP. Else their property is at risk of fire. If the landlord does not respond appropriately, report the problem to local authorities as this is a violation of safety codes and laws. Your backup gadget has possibly saved your life and property.

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    \$\begingroup\$ If the neutral and ground are connected together at the service entrance (or breaker box), as they are supposed to be, then a faulty appliance would cause the breaker to trip, not cause neutral to float above ground. This is the point of grounded outlets. Therefore, the problem IS that the neutral point is not grounded correctly, so this is a safety issue that the landlord must get fixed immediately. \$\endgroup\$
    – user56384
    Commented May 18, 2016 at 1:07
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Nothing is faulty. The UPS has probably the transformer at the output, so what you get is two hot phases that are floating. It is really not much different when you have the two hots and ground, like 240VAC (mostly in US). What you are measuring gives you the half voltage at each leg vs. gound: \$V_{live}\$ vs. \$V_{gnd} = V_{out}/2\$, and \$V_{neutr}\$ vs. \$V_{gnd} = V_{out}/2\$. We could say that neautral can't be named like neatral, rather live, so you have live_1 and live_2 at the output.

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    \$\begingroup\$ You may have misunderstood the question. The problem is with the building wiring, not the UPS. \$\endgroup\$
    – Richard Crowley
    Commented May 18, 2016 at 11:38

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