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Assume you have a metal device box, correctly installed with 14/3 cable incoming and a three-way ungrounded light switch installed. Assume that the box is correctly grounded using the ground wire attached to the ground screw of the box.

No other connections exist in the box.

What should happen if one of the hot screw terminals gets close enough to the side of the box — say you were pushing the switch back into the box while it was energized and the side of the switch got close enough to the metal box to make contact?

Would you expect an arc? Would it trip the circuit breaker for that circuit back in the panel?

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    yes and yes....
    – dandavis
    Feb 16, 2022 at 18:02
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    An arc/spark(If not push together very fast) at least, more than likely a breaker trip also, unless very fast to separate them. It is a good reason not to work on live power. Just imagine the fun if it was your hand touching hot and grounded box.
    – crip659
    Feb 16, 2022 at 18:03
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    Yes to both as the other commenters have said, but at least you will have figured out which breaker controls power to this device!
    – Glen Yates
    Feb 16, 2022 at 18:16
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    Absolutely nothing, because you had the breaker turned off while doing that work?
    – Harper - Reinstate Monica
    Feb 16, 2022 at 18:59
  • @Harper-ReinstateMonica of course!
    – RibaldEddie
    Feb 16, 2022 at 19:02

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If the contact was quick enough, "brushing" the two side by side, you'd have an arc and the breaker might trip, it might not.

If the contact was more definite, you pushed the switch into the side of the box making hard contact, you probably wouldn't see an arc but the two surfaces would weld together, you'd hear a pop and the breaker would/should trip.

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    Does the breaker flavor (e.g., AFCI, GFCI) affect the expected behavior of the system?
    – Rob
    Feb 16, 2022 at 21:58
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    Yes, both those breakers if installed would operate much faster that a normal breaker and would reduce any arcs that might occur. If they were individual outlets, they would trip before the circuit breaker.
    – JACK
    Feb 16, 2022 at 22:31

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