In my "datacenter" (read: basement), I've got a rack of various computer and A/V equipment plugged into a UPS, and into a standard 120v outlet on one side of the room. This equipment works normally, and the UPS detects no wiring faults.

On the other side of the room, is another rack of equipment, going into a UPS, going into another standard 120v outlet.

All equipment is directly connected to either the outlet for testing or to the UPS.

Basically, imagine one side of this room as a mirror image of the other side.

Each outlet is connected to a different circuit.

The problem

Attempting to connect equipment using one circuit to equipment using the other circuit (say, a monitor plugged into circuit 1 to a PC plugged into circuit 2) results in a rather sizeable arc and burn marks where the cable touched the port. The lights dim, and the circuit breaker for one side or the other will trip if left for too long.

I have eliminated the equipment as a cause. All gear works fine whether plugged into one circuit or the other. The light show only happens when you try to connect devices that reside on different circuits. (With the exception of networking gear - I can connect computers on circuit 1 to routers on circuit 2 via ethernet with no problems).

The UPSes are also not the cause - the same thing happens when the equipment is plugged in directly.

Basic probing with a multimeter indicates that the outlets are operating properly and have working grounds.

A regular 3 light receptacle tester also shows that both outlets are wired "correctly".

What the heck is going on here, how do I test for it, and most importantly, how do I fix it without burning the place down?

  • Basic probing with a multimeter indicates that the outlets are operating properly and have working grounds. Multimeter to power outlets can be misleading. Sound like a wiring problem to me. Whether in the outlets or whatever you are using to plug all these devices into one outlet remains to be seen. – Trevor_G Mar 15 '17 at 20:21
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    You're sure you don't have an outlet with hot and neutral swapped? – The Photon Mar 15 '17 at 20:21
  • GO buy one of these.. and test your outlets again.. globalindustrial.ca/p/tools/test-measurement/Metrs-HVAC-R/… AND DO NOT PLUG ANYTHING ELSE IN TILL YOU DO ! – Trevor_G Mar 15 '17 at 20:23
  • Quite sure - one of the tests I did was with an outlet tester that looks for exactly this problem (one of the things with the 3 lights on it). All outlets register as wired correctly. @Trevor - that link is exactly the device I used. First light off, next two lights on. – Mikey T.K. Mar 15 '17 at 20:25
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    You plugged something in, it made a huge spark, the lights dimmed, and you left it plugged in? D: – Bryan Boettcher Mar 15 '17 at 20:58

That is very bad. I recommend a comprehensive survey of the relevant parts of the electrical system, starting at your main panel.

Service grounding (earthing) system

This goes from a water pipe, ground rods, etc. to your main service panel. If you have a sub-panel, it goes to the main panel. It is the reference to zero voltage for the entire system. This goes to the ground in your panel.

Don't worry too much if it's not perfect, I'm more concerned with its relationship with the rest of your panel(s).

Ground-neutral bond in your main panel

Throughout the world, mains power is wired as if it's an isolated system with a grounded safety shield. That is to say, ground is never used in any way to carry current, except during a fault condition. But it is not an isolated system: to prevent wild voltages, neutral is intentionally bonded to ground in the main panel only, via a wire, strap or green screw in the neutral bus.

Make sure that exists and looks OK. Without it, a ground fault in equipment will "electrify your grounds". (actually put neutral at a wild voltage compared to ground.)

In the main panel only, it is legal to simply use one bus for all neutrals and grounds. It looks awful; it makes you think neutral and ground are redundant; they're not!

Any sub-panels

If these branch circuits are fed from a panel, which is itself fed from a breaker off the main panel, make sure the feeder has a separate white or gray neutral and green/bare ground wire. Metal conduit can substitute for a ground wire. Make sure the sub-panel has a neutral bus and a ground bus, carefully separated and not bonded.

Also in the panel

Watch out for a ground bus with wires that are not green or bare. Watch out for a neutral bus with wires that are not white or gray.

Now look for circuit breakers which have white, gray, green or bare wires going to them. White/gray to a breaker is OK under a few circumstances:

  • a 2-pole breaker (the breaker itself is double normal width, and obviously has 2 handles tied together), often has 1 black and 1 white
  • A GFCI/AFCI breaker, which has 1 hot and 2 neutrals going to it (one coiled up and going to the neutral bus).

Obviously, pay special attention to your difficult circuits.

Now follow the receptacle chain back to the panel

For each circuit in turn... Unplug everything and turn off one of the breakers. Use a nightlight, circuit tester, plain lamp and some intuition to identify which receptacles and other loads are on that circuit.

Physically check: Now, work forward from the panel to your problem location. Open up each junction box, remove the switches/receptacles from the boxes, and give it the basic eyeball. I bet you'll find problems at one of them.

Electrically check: Voltage testers are hokey and don't work, because they are only comparing the receptacle to itself. Noting that you started on electronics.se and know a thing or two, you can electrically test by running a wire back to that original zero reference. The zero reference won't harm you, but getting between it and a hot voltage could, so I'd add a 20k-50k ohm resistor on the far end of it.

  • Voltage to ground should be <0.1 volts, essentially 0.
  • Voltage to the taller neutral slot should be 0 to 3 volts depending on load on the circuit
  • Voltage to the shorter "hot" slot should be near 120 volts
  • If you add a ~1500W heavy load on the circuit, neutral should increase, and ground shouldn't.

This is no substitute for a physical inspection. You can't reliably detect a bootleg ground or ground-neutral swap, for instance, which are things which would cause this.

I guarantee you'll find a problem before you're done. Possibly 2 or more, since a guy who makes a mistake often repeats it.

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    Thought I'd update this after a long while. I ended up getting a couple of extension cables and measuring across the grounds from the outlets on either side of the room. Turns out.. 120V across the grounding pins. Some contact with an electrician later (sadly, I was a renter in this home and couldn't do my own repairs), and we found a junction box on that circuit with some mouse-eaten wiring (complete with crispy mouse), basically making the entire box (and so the ground for the outlets there) hot. It was replaced, and some GFCI's installed for good measure. Thanks for the excellent answer. – Mikey T.K. Jul 17 '19 at 15:15

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