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I have the following:

  • a NEMA 5-20 receptable (typical 20amp grounded receptacle) mounted in an exposed work cover on a metal box fed with 12ga conductors through EMF.
  • a typical string of holiday lights with a non-polarized plug.

I accidentally misaligned the plug while trying to insert it into the receptacle. One blade entered the ungrounded conductor ("hot") slot of the receptacle, and the other pressed against the metal exposed work cover.

The lights lit, and the 20 amp breaker did not trip. Should it have?

My present understanding is that since everything was still going through the lights (i.e., there was no fault to ground, but only this connection through the resistance provided by the string of lights), and since the breaker is simple overcurrent protection (it's not a GFCI or AFCI breaker), the breaker should not have tripped.

However, if it were a GFCI breaker, then because current returned through the equipment grounding conductor ("ground") rather than the grounded conductor ("neutral"), the imbalance between hot and neutral would cause a trip. (And because the ad hoc connection was pretty loose, there could have been arcs, which would have tripped an AFCI.)

Is my understanding correct and I just need to exercise more care when plugging things in, or do I have a wiring situation that needs to be fixed?

There have been some comments about whether the box has a cover, and whether this requires bending the plug blades, and the like. It doesn't, as illustrated in this picture (taken with the breaker off):

plug with one blade in hot and other touching box cover

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    It may be my European brain, but this setup scares the shit out of me. If the pin was touching your finger instead of the faceplate, you would be touching a live circuit, with all the risks associated. Just by plugging in a plug slightly too far to the right.
    – Pelle
    Commented Mar 12, 2023 at 17:35
  • @Pelle Just goes to show that "up to code" isn't necessarily "right" Perhaps an insulated plastic-fronted mounting plate would be better than all-metal faceplate.
    – Criggie
    Commented Mar 12, 2023 at 23:06
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    @Criggie What's scary here is not due to the faceplate, though. It's the fact that live can be reached while the other pin is still touchable. Actually, the all-metal faceplate seems better than plastic, because at least you might be marginally safer accidentally touching the pin when it also touches the grounded faceplate due to it presenting a better path than your body.
    – JoL
    Commented Mar 13, 2023 at 4:19
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    @Pelle Don't touch the prongs while you plug stuff in. How often do you touch the insulated pins on a Europlug while plugging? In fact, it's a self-reinforcing problem: insulated pins are longer to accommodate the insulated section while maintaining necessary current-carrying capacity, which makes the pins easier to touch.
    – user71659
    Commented Mar 13, 2023 at 22:40
  • @user71659 the difference is the plugs in developed countries are designed so you can't accidentally touch them while plugging! Commented Mar 14, 2023 at 1:58

2 Answers 2

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A normal breaker will not notice this particular error. You aren't drawing an unreasonable amount of current, it's just being returned along the wrong path.

A GFCI will trip.

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    Kesham's answer is correct, but here's a bit more detail. Assuming the outlet is wired correctly, both the ground and the neutral wire run back to the breaker panel. There they connect, usually, to a ground bus bar and a neutral bus bar (though in some panels they may be the same). Those two bus bars are then tied together. So the return current can flow though either the neutral or the ground wire, though it is supposed to flow through the neutral only.
    – SteveSh
    Commented Mar 12, 2023 at 11:40
  • @SteveSh thats true, but in the case of ground, it also flows along every piece of exposed metal that is required to be grounded: boxes, faceplates, conduits, cladding. And in the case of a subpanel (since neutral is only bonded at the service) the subpanel itself and any cladding back to the main panel. Thats a lot of places you could accidentally touch. I only wouldn't be concerned with this if I knew this circuit was just romex and plastic boxes all the way back to the main panel
    – Steve Cox
    Commented Mar 14, 2023 at 14:05
  • There is no voltage on ground under normal circumstances. It's there for safety; touching it is safe. This is why ground is separate from neutral, even if they are bonded at the main panel.
    – keshlam
    Commented Mar 14, 2023 at 14:10
  • @keshlamisn't OP specifically talking about one of those abnormal circumstances? (using ground as a return path; "grounded" in code vs "grounding")
    – Steve Cox
    Commented Mar 14, 2023 at 15:04
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    @SteveCox If the lights lighted up at normal intensity, then that means the ground path was good. Even if he had a full 20-amp load, with 12 guage wire back to the panel 100 feet away, that should only be a couple volts of potential difference between local ground and "true" ground; enough to measure, likely not enough to feel. The time to worry would be if he does this and the lights don't light up, or light very dimly.
    – david
    Commented Mar 14, 2023 at 16:05
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It seems to be right.

One of the no-no hacks for a broken neutral is to use the non insulated ground wire to make the circuit complete.

A lousy test for a breaker is to place a piece of metal(wire) touching hot and ground with no resistance/working(lights) circuit. The breaker should trip if you are lucky. This sometimes happens by accident when placing a switch or receptacle in a metal box, but the hot screw/s end up touching the box. Why some people add tape around switches/receptacles.

For AC circuits polarized or non polarized does not really make a difference. The difference is that hot goes to certain path that should be further away from fingers/skin. Lights and most stuff don't care. With lights the centre post is always the hot with polarize plugs, and away from fingers when replacing light bulbs.

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    The reason I mentioned that the plug is unpolarized is that it simply wouldn't have fit in had it been polarized. I would have been putting the neutral blade into the hot slot, but it wouldn't have fit. ... Actually, now that I say that, I absolutely could have done the same thing with a polarized plug, just by rotating it 180°. Commented Mar 12, 2023 at 0:45
  • @JoshuaTaylor Polarized plugs are just a minor safety factor. It helps, but if you stick your tongue in a light socket with a polarize plug, I still get to make pickle tongue( I am weird).
    – crip659
    Commented Mar 12, 2023 at 0:54
  • This is an exposed work cover, not an open box. They're common when the work is exposed, common in workspaces, garages, basements, and so on. Commented Mar 12, 2023 at 6:16
  • My mistake then. I have removed that edit. I am still wondering how you could put the prong in far enough to connect. Usually the hot connection in the receptacle is in a bit and not that close to the surface. Would almost need to bend the prong out enough to touch hot and ground.
    – crip659
    Commented Mar 12, 2023 at 10:04
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    @crip659 For these kinds of plates where you mount the device to the plate rather than the box itself, this kind of protrusion is pretty normal.
    – Logarr
    Commented Mar 13, 2023 at 4:21

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