what happened here?

[jacko57]

i was skimming a small room, the centre light cable was just poked back through the hole made in the ceiling,there was no connectors just the bare ends,when they were pushed back the switch live and neutral were touching,although no current was there as the pull cord switch was removed,and the live and switch wire were taped up separately.

when i skimmed the ceiling first the plaster forced its way into the ceiling hole(normal) when i began the walls i noticed a tingle when i touched the wet plaster,got a mulitmeter out went back into the room and just the probes being in the room gave me 64volts,same slightly more when i put to wet plaster.
obviously tripped the lighting circuit and all well, hooked the cable back down ,separated the cables and choc boxed..

why did this happen when there was no current to the lamp cables ?

 

[Softus]

Probably because the neutral wasn't isolated.

 

[bernardgreen]

Or the live loop was still live in the ceiling ?

 

[Softus]

Or the live loop was still live in the ceiling ?

The 64V live loop?

 

[RF Lighting]

Its possible that the resistance of wet plaster only allowed 64V of the full potential to flow to earth

 

[ricicle]

Its possible that the resistance of wet plaster only allowed 64V of the full potential to flow to earth

Voltage doesn't flow - current does though...

 

[Space cat]

This sounds like our old friend the hidden capacitor again. Somewhere up there it is leaking current from a live wire into that supposedly dead one.

,although no current was there as the pull cord switch was removed,and the live and switch wire were taped up separately.

There was current but it was small, far to small to pop a fuse. With the two ends touching it returned through the neutral wire so there was no voltage. My guess is that when you pushed the plaster into the hole you separated the ends. With no load on it the 'dead' wire could float up to a high enough voltage to give you a tingle.

 

[electronicsuk]

got a mulitmeter out went back into the room and just the probes being in the room gave me 64volts,same slightly more when i put to wet plaster.

Hmm, sounds a little suspect to me. There's no way you'll get 64 volts induced into the test leads just standing in the middle of the room. Is it an autoranging meter? Are you sure you weren't reading millivolts?

 

[Lectrician]

If it is a single T+E from the ceiling:

I put money on the switch switching the neutral, not the live.

The 64v due to the fact he didn't put a probe on a true earth.

He got a tingle from the trowel - thats not capacitive in my opinion.

Sounds like a typical renovation where the lectrics are left as they are and never given a thought to!

 

[Space cat]

Lectrician wrote

I put money on the switch switching the neutral, not the live.

I discounted this idea initially because of this:

-- there was no connectors just the bare ends,when they were pushed back --

Could you really push a bare live wire end up through a ceiling and notice nothing? Maybe, if you were wearing rubber soles at the time.

On the other hand I agree that it would take a lot of capacitive leakage to produce a tingle, more than you would expect across a switch cable with an earth wire in the middle.

This brings us to option three. I remember a post from someone who got tingles from metal light switches. Upon further investigation it was found that all the earth wires in all the lighting cables were floating free because the connection at the CU was broken. That was a lot of capacitance. There's no mention in jacko57's post of an earth wire up in the ceiling but it's probably there, touching wet plaster - and it might just be the source of the tingle. It's definitely worth checking out.

 

[jacko57]

cheers for the replys guys.
yes 1mm T/E at the lamp
all the cable ends were bared back
it was enough of a tingle to pull the float away.
went to the job today and checked junction box supplying and all seems ok.
no problem now all dried out.

 

[Lectrician]

no problem now all dried out.

Ohh - Not sure about that

 

[Space cat]

Ohh - Not sure about that

I agree. It may be 'no problem' right now but something isn't right and it's just waiting to catch somebody out in the future. And that somebody, jacko57, might be you! If I were you I'd get my Sherlock Holmes hat on and get to the bottom of this mystery.