Shocking electrics

[R D S]

(pun intended)

I'm changing the living room light switch and budging it to one side slightly so needed to remove the back box. Tucked away behind are a pair of earths, I wondered why they were not connected to the back box and instinctively put a tester on them which lit up. This reminded me that this light was once controlled by 2 switches and I recall some years ago discovering a live earth behind (and not connected to) the other switch while doing a different job (decommissioning a couple of wall lights connected to an adjacent switch).

Also of the 2 wires that were connected to the switch, the black one is live and the red one isn't.

It has to have been like this since we moved a doorway about 25 years ago but this can't be my handiwork, if I was that clueless back then to have left things this way I wouldn't have messed in the first place. But I had an aunt who's fella called himself an electrician and he fitted us a CU at around the same time so I wonder if it's his doing.

Reason for this post is, I can't decide what to do about it, I changed the ceiling light a couple of years ago and there are no extra earths connected to anything live there so #### knows where they are being fed from.

Given that it hasn't caused any obvious bother for a quarter of a century I can't decide what length to go to to work out what is going on and disconnect any redundant live wires.

[Dave Plowman (News]

If the earth wire isn't connected at either end it's just possible a very high impedance device like a neon will light when you check it. Inductive or whatever coupling.

[SH]

did the room use to have wall lights or did it once have 2 ceiling lights but now has one? I'm wondering if a lighting circuit within the room has been "decommmissioned" but with cables left in place?

Alternatively did there use to be 2 way switching but then changed to 1 way switching with the bricking up of a door/doorway?

We had this happen to our house during previous occupiers where a garage was converted to a living room. Someone removed and bricked up the garage door and simply removed one of the 2 way light switches next to it and taped up the 3 core and earth for the light switch and plastered overthe back box!

Not only that, they also bricked up teh doorway where the other light switch was and ran 2&e from this lightswitch to a newly opened doorway and put a new switch there. So no chainges at the light fitting itself and the two old light switches still had live wires and all plastered over.

[R D S]

Interesting... I'll stick my finger on it...

[SteveW]

Yes, I get a shock touching connections inside my boiler (such as when changing a fan on the old boiler), despite the circuit being isolated. The boiler is in the kitchen, while the isolator is with the majority of controls, motorised valves and pump, in an airing cupboard, upstairs in a bedroom. A DVM shows 90 volts - dropping to 0 volts on the Neutral when the isolator is turned back on.

[John Rumm]

What kind of tester? A neon screwdriver for example will light even on a disconnected wire that is inductively coupled to an adjacent live one.

Which would be (mostly) acceptable. If you think about it, the two wires are live and switched live. So unless you have bothered to lay in some T&E with 2 red or 2 brown wires, one will have to be neutral coloured.

(mostly because, in an ideal world the neutral should be over sleeved to indicate it is a live - however on a light switch with only two wires it is pretty obvious)

Possibly nowhere. How old is the lighting circuit? There was a time (prior to 1966) when they were not routinely earthed (and later installed carried on the old way for some years after that). So if it has been extended since with T&E cable, there is not always a place to collect an earth from.

(In which case double isolated switches and light fittings, a RCD, and warning notice near the CU are a good idea)

Work out if they really are live first, and then if so, find out where they are connected[1] and decide what to do.

[1] Do a binary chop on the lighting circuit - split it in half at a ceiling rose, and see if they stay live. You have now eliminated half the circuit. Reconnection and then repeat halving the previous half each time. That should find you the segment of circuit in log to the base 2 of the number of lights on the circuit. :-)

[Brian Gaff]

Yeah, I would be tempted to short itand see what happens, only the CU protection should trip. I used to have a 60 wat bulb with a couple of insulated crock clips handy for this sort of thing, as then, if the bulb does light you know its properly live. Brian

[Colin Bignell]

The back of a finger is an excellent piece of electrical test equipment for domestic mains.

[Andrew]

A bit of grass was how we checked if the electric fence was on at my grandfathers farm :-)

[Dave Plowman (News]

Sensible way would be to wire a small mains bulb between it and neutral (or a true earth)

[hgt]

Yes, but it doesn't work with mains voltage.

[Scott]

On Thu, 23 Jun 2022 15:17:02 +0100, R D S [e-mail] wrote: [snip]

On a similar vein, my late father moved into a flat. He asked a friend of mine, an electrical engineer who worked for the South of Scotland Electricity Board at the time, to have a look to see if the electrics were okay.

My friend found that the flat had been rewired but whoever did it left all the old Vulcanised India Rubber cables energised.

[ARW]

+1

[Brian Gaff]

Oh no. That was a recipe for intermittent shorts as many houses of that age had buried metal conduits in walls. I have a friend with a house like that, but he says he can't be bothered to get it done. I pity the person who gets it when he pops his clogs.

In my case it was rewired in the mid 70s, and has thus got pvc cables and normal circuit breakers Obviously the colours have changed, but in the main its in good nick and should not cause any problems. I guess you could put in a more modern CU and up to date protection, but in the main its found. Brian

[Scott]

I think it was a bit worse than you are suggesting. The VIR cables were in addition to the new wiring. Anyone finding a VIR cable anywhere would assume it was disconnected. Surely the disconnection would require nothing more than a screwdriver or even a set of pliers? On another occasion (in our previous house), I had one of these bedlamps that hung over the bed's headboard. I put my hand up and received an electric shock that threw me back into the bed. I then looked and found half the glass was missing. Next morning my brother told me that 'Davie' had slept in my bed the night before when the bulb exploded. They cleared up the glass. I asked why they did not change the bulb. Brother said he assumed that Mrs T (my dad's cleaner) would check the light fittings when she made the beds..

[Scott]

I was told to sweep the finger across the cable keeping it moving so it would not get stuck. To be used as the final test after the neon screwdriver in case the neon screwdriver failed.

[Colin Bignell]

The point is that a shock will cause the muscles to contract. If you use the back of the finger, that takes it out of contact. Anything else and you risk grasping the live wire with no way to let go.

[Scott]

Good point but I think keeping the finger in motion is an added precaution (and ready to karati chop forearm with the other arm if necessary).