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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.

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  • 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
    Commented Apr 7, 2015 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
    Commented Apr 7, 2015 at 14:23
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    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
    Commented Apr 7, 2015 at 16:19
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    Motor capacitor on the same circuit? Is the leaking juice AC or transient? Commented Apr 7, 2015 at 23:36
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    'mains power' = 'line voltage' I.e., a (1) circuit breaker. Should the title read: Why did I get shocked after turning off the ONE breaker that was labeled 'outlets'? -Does this panel have a main (no 's') breaker?
    – Mazura
    Commented Apr 8, 2015 at 3:39

4 Answers 4

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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
    Commented Apr 8, 2015 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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If you turned off the main switch, that should be double pole and isolates everything from the supply. The circuit breakers for individual circuits are often single pole, so the neutral is still connected. There can been some residual voltage on the neutral, especially on TT earthing where they aren’t directly connected together, depending on an earth rod.

So the other reason for a shock could be inlet capacitors in equipment. They are across the supply to smooth out power supply noise, but that means they can hold charge. There may be a bleed resistor across them, but that will take time to bleed away the charge.

Next time you switch off the supply, (carefully) measure the voltage with a multimeter afterwards. Something else you could try is an incandescent lightbulb to drain away any stored charge on the circuit. Just use it in a light fitting or a desk lamp with a mechanical switch, no electronics/dimmer. Then confirm with the multimeter on both AC and DC ranges that the line-neutral voltage is zero.

If the lightbulb does not cause the voltage to drop away, or stays lit, something is wrong with your wiring and I’d call an electrician.

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If a neighbor's house is a short distance from yours, say 40 to 50 feet or similar pulling a heavy load on line 1 OR line 2 and has a poor neutral from pole to the panel. Then the current might try to go to your panel from his neutral to his cold water pipe, then to your pipe, and to the bonding jumper that connects your cold water pipe to your neutral and then the service neutral.

The reason I say Line 1 OR Line 2 is that if you have 40 amps on one line and 30 amps on the other, only 10 amps will be on the neutral. Also, the transformer on the pole or on the ground will have the neutral which gives you 120 to each hot leg or ungrounded conductor and it is grounded. So with a neighbor having a bad neutral connection, the current would flow to the cold water pipe and utility's ground and through the city waterline to your place through the ground up to the neutral bar and back to the utility on your service neutral.

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    They don't have 120+120V in the UK.
    – Martin
    Commented Apr 24, 2023 at 11:41

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