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I have a switched outlet which, when turned on, reads as correctly wired on one of the three-light plug-in testers.

When turned off, however, I get it weakly lighting as "hot and ground reversed".

I haven't taken a proper meter to it yet, but I am hoping this is just an annoying trickle of back-current/induced current from another circuit... Or perhaps a switch installed in neutral rather than the hot side.

Has anyone seen this particular nuisance before, and if so what's the likely cause?

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  • It can be caused from several things. A lighted switch. Or electronic controlled switch. These use a small amount of power that when the switch is in the off position causes the tester to see things backwards. – Ed Beal Oct 31 '16 at 20:01
  • What kind of switch? Does it have a light? – DoxyLover Oct 31 '16 at 21:50
  • Good questions, @doxylover and edbeal, but no -- no light to allow a trickle current, not is it a smart switch powering itself that way. Just a mechanical switch. – keshlam Oct 31 '16 at 22:01
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I would suspect a switched neutral here. Turn off the breaker for the circuit, then measure continuity between the neutral and ground holes on the receptacle with the switch in each position. If you get continuity with the switch ON but not OFF, there's your culprit -- only fix for that is to rewire the switch so it switches the hot.

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  • OK; guessed that might be the cause but wasn't sure. Will check. – keshlam Oct 31 '16 at 23:01
  • Correct but there was more to it; see my answer. – keshlam Nov 5 '16 at 21:16
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Turned out there were multiple sins.

Theirs: Hot and neutral were reversed at the box before the switch, when going from old fabric-insulated to plastic-insulated (still in conduit).

Mine: I had hot and neutral reversed at the outlet, when trying to figure out what was going on.

The double reversal effectively meant that, yes, the switch was in the neutral line despite the switch itself being correctly wired.

Fixed it all; tester is now happy.

This is why we should always test, rather than assuming it must be correct if it seems to work.

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