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I was changing a socket outlet yesterday. I switched off the mains power at the consumer unit (the big red switch) so thought all was safe. I disconnected the old socket, but when I went to fit the new one, I got a quite powerful electric shock from the wires coming out the wall. I didn't flip the other contact breakers in the consumer unit as I assumed (wrongly) that everything would be isolated.

The house is in the UK, built 1992, and has a consumer unit with built in RCD and 4 contact breakers (lights up/down, sockets, cooker).

Any ideas how this could have possibly happened? Any help would be appreciated.

  • Any chance that your building may have wiring which crosses occupancies? Not likely in a freestanding house, more likely in a house that is part of a row of houses, if there's some old wiring that was run through multiple units...? But that seems unlikely for 1992, doesn't it? Anyway, I always lead with a voltmeter, even when I "Know" the circuit is off. – Ecnerwal Apr 7 '15 at 13:38
  • I very much doubt it. This is a modern house built by a national housebuilder. I was wondering if maybe there is a fault in the consumer unit switch, allowing some current to trickle past? Or maybe an earth or neutral short somewhere? – Graham T Apr 7 '15 at 14:23
  • Any other ideas - PLEASE? – Graham T Apr 7 '15 at 16:12
  • Sorry - other side of the Atlantic, no detailed understanding of many of the oddities (from our POV) of UK wiring. I'm sure ours has oddities from your POV. Might call the power provider in, given that OFF not being OFF is a fairly serious error. – Ecnerwal Apr 7 '15 at 16:19
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    Motor capacitor on the same circuit? Is the leaking juice AC or transient? – Wayfaring Stranger Apr 7 '15 at 23:36
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How could this have happened? By:

If I understand it right, RCDs are the UK version of AFCIs, so if it was coming off a RCD, you shouldn't have gotten bit so bad: this breaker is fail; use the test button (or this was simply the wrong breaker).

In the US, code requires AFCIs in all bedrooms but not for convenience outlets in say, a hallway. It is very common to steal power from the 'hallway lights' for 'that-one-outlet-under-the-stairs' (no one would label it as such). This may or may not be code but I assure you it is done.

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    +1. This is why a non-contact voltage tester or multimeter should always be used to verify assumptions. – Hank Apr 8 '15 at 3:21
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You probably have 230V AC, which is admittedly a little stronger than the 110V AC across the pond. I've had quite a few 230V shocks, the result of an experimental childhood, and as you have seen, these aren't pretty and have some slightly lasting impacts.

Please recruit an electrician to see what's going on. Internet strangers, kind though we are, really can't help beyond asking you to make sure your mains are really off (they are), or to use a voltmeter or equivalent device to really make sure there's no power.

For all we know, a previous homeowner has stashed a decent capacitor away somewhere in your wiring.

Good luck!

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