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A fixture, installed in a 1995 storage facility, suddenly stopped working. The bulb tested good, the fixture was remove and tested on a workbench: good. When good / tested bulb and fixture were reconnected to the white (neutral and black (hot) wires, the bulb did not illuminate.

However when the fixture's white wire was connected connected to the metal as shown in the photo, the bulb illuminates:

enter image description here

UPDATED 5:08PM Voltmeter shows:

  1. 40 volts difference between the hot and neutral wires.
  2. 120 significant voltage between the hot and metal fixture.

I would imagine that this is not a new problem (not sure what it is called or I would have Googled it by its name). What is the likely cause of the problem?

It was originally thought to be an open neutral, however, because there is 40V between the neutral and the hot, I suspect something else is afoot.

Bonus question: Why is it unsafe to simply connect the white fixture wire to the metal housing? It is not obvious that the metal box is pinned into a sheet metal wall.

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  • Your bonus question actually has quite a few answers here. This is one that got a reasonable amount of attention, though it's not exactly the same as yours. diy.stackexchange.com/q/236756/18078
    – Ecnerwal
    Nov 26 '21 at 16:56
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"Open Neutral" is the usual term. Regarding your update, with a typical digital multimeter and no load on the wire, 80V of induced voltage (thus, 40V difference) is entirely unremarkable.

There's a bad connection (thus, "open" not connected; rather than "closed," connected; in "switch" terms) somewhere on that wire run.

Often the other end of the same wire, sometimes further than that, sometimes but rarely inconveniently in the wall.

If your house uses "backstabbed" switches, often those. Alternatively, given it's a neutral and should not be going through a switch, a poorly done wirenut, or a rodent chewed a wire. Or the power for the light circuit might be coming off a "backstabbed" outlet.

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It is unsafe because while the metal fixture itself is out of reach and therefore relatively safe (not to current code, but "relatively"), you now have current going over the ground wires everywhere that is normally on neutral. It can cause a lot of problems, including risk to life & limb, depending on a number of other possible failures.

As a test, this was reasonable. It tells you your ground is good (which is a very good thing) and it tells you the problem is definitely the neutral wire. But you can't leave it that way.

The next step is to track down the neutral wire problem. I'd start with the switch box, and if that is good then you have to check every box on the same circuit because the problem could be almost anywhere. You know a neutral is good, in any particular location if (a) neutral to ground wire (or to metal box if grounded metal box, like this fixture) is 0V and low resistance (close to 0 Ohms) and (b) when the circuit is on, neutral to hot is ~ 120V.

Could be a backstab gone bad, loose wire nut, loose screw, etc. Small chance it is a problem inside a wall - nail or screw through a wire or rodent chewed through wire. Hopefully it is in a junction box, as those problems are much easier to fix.

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