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Bit of a mystery here.

We've got an outlet in our 2nd floor bedroom that I assumed was dead or disconnected. I poked it with my AC voltage detector to confirm — yep, nothing. I was wondering if maybe the outlet itself was mis-wired, so I went to the breaker box and turned off the 2nd floor lights (15 amp AFCI breaker) so I could safely open up the outlet and take a look.

Good thing I thought to double-check with the voltage meter before sticking my fingers in there. Turns out, now that the breaker for the rest of the floor was off, suddenly I'm getting current to this outlet! Confirmed that plugging a floor lamp in works, which it didn't before. Confirmed that turning the AFCI breaker back on renders the outlet dead again.

...WHAT?

Any insights on what could be going on here?

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  • What make and model are your breakers? – ThreePhaseEel Dec 29 '16 at 23:22
  • The AFCI in question is a Murray Type MP-AT. – Geordie Kaytes Dec 29 '16 at 23:33
  • UPDATE: figured it out. See my answer below. AFCI had nothing to do with it. – Geordie Kaytes Jan 9 '17 at 13:27
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Mystery solved. We had a bit of a Car Allergic to Vanilla Ice Cream problem. Turns out the bedroom outlet is on the same switch as the basement lights. Whenever I went down to the basement and flipped a breaker, I left the basement lights on when I came back upstairs, since I was planning on going back down to flip the breaker back. So, because the basement lights were on, the outlet in the room was powered when I got back upstairs. Not the most intelligent wiring decision in the world, but not rampantly dangerous either.

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Sounds like your outlet is mis-wired so that it's connected between the hot wires on two separate circuits. When both circuits are powered, there's no voltage across the socket's prongs (both the "hot" and "neutral" will be at 120VAC). If you turn off one, then that circuit's hot will be pulled down by the other loads on the line, and bingo: you see 120VAC across the socket.

You should pick up an outlet tester. When both circuits are on, it will probably register that outlet as "Hot and Ground Reversed"; when the "Second Floor Lights" breaker is off, it will probably register that outlet as either "Correct" or "Hot and Neutral Reversed".

Be very, very careful with this; if I'm right, and this portion of your house wiring is wrong, there's a whole lot of other things that might also be wrong.

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  • Terrifyingly plausible. Will pick up an outlet tester tomorrow and report back. – Geordie Kaytes Dec 30 '16 at 3:56
  • @GeordieKaytes ... any news? Would love to hear... – Daniel Griscom Jan 1 '17 at 0:53
  • Haven't had a chance to play with the breakers yet, but the first pass outlet test with all the breakers on didn't light up any of the indicators at all — as if the test tool wasn't even plugged in. Trying today to flip the breakers and see what it indicates when the outlet is functional. – Geordie Kaytes Jan 1 '17 at 16:52
  • That would be true if the ground were disconnected, which would make this even more entertainingly dangerous. And, this would be confirmed if throwing the breaker makes the outlet read "open ground". Again: be careful. (Or, if not, have someone else videoing you so that they can post the resulting carnage on Youtube and get big hits.) – Daniel Griscom Jan 1 '17 at 16:56
  • Mystery solved. We had a bit of a Car Allergic to Vanilla Ice Cream problem ( cgl.uwaterloo.ca/smann/IceCream/humor.html ). Turns out the bedroom outlet is on the same switch as the basement lights. Whenever I went down to the basement and flipped a breaker, I left the basement lights on when I came back upstairs, since I was planning on going back down to flip the breaker back. So, because the basement lights were on, the outlet in the room was powered when I got back upstairs. Not the most intelligent wiring decision in the world, but not rampantly dangerous either. – Geordie Kaytes Jan 9 '17 at 2:09

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