Lighting Shocker !

[jangler]

I was recently working on a friends new (old) house lighting circuit, changing a light switch.

I isolated the lighting circuit, tested for voltage at the switch, circuit was dead. I removed the switch circuit, then removed the earth, when I received a shock from the earth!!!!!

She knew there was a fault on the circuit and had a discount on the property price to get it fixed. The fault has now been corrected.

Any clues how this could have happened and how would I test for a potential on the lighting switch earth in future?

 

[fubar]

Any clues how this could have happened and how would I test for a potential on the lighting switch earth in future?

One possibility is that the earth wire wasn't connected to the earthing terminal block at the consumer unit. The cpc then acts as a centre-tap capacitor (as it is sandwiched between neutral, which is probably referenced to ground, and live). There could be anything between 20V and 120V induced on the cpc, but normally it would read around 50V.

Measure the voltage between each of the conductors and a known, good electrical earthing point or follow the sequence given in IEE Guidance Note 3.

--
Michael

 

[ban-all-sheds]

One possibility is that the earth wire wasn't connected to the earthing terminal block at the consumer unit. The cpc then acts as a centre-tap capacitor (as it is sandwiched between neutral, which is probably referenced to ground, and live). There could be anything between 20V and 120V induced on the cpc, but normally it would read around 50V.

But he'd disconnected the circuit - there was no voltage in the cable

 

[fubar]

But he'd disconnected the circuit - there was no voltage in the cable

Missed that Voltage could still be induced by running parrallel to energised cables, though. Another possibility is that the cpc from an energised cable is connected to the cpc for this lighting circuit but not to an effective earth - again with voltage induced in the cpc.

--
Michael

 

[phhut]

Then the earth was connected somewhere else in the house circuit to 'live' possibly. The earth is pretty common along ALL circuits regardless of switching off a particular one (pulled fuse, mcb off ...). Probably would have got the same shock on an earth somewhere else in the house.

 

[jangler]

'Then the earth was connected somewhere else in the house circuit to 'live' possibly' - phhut

Strange thing, is that I had touched the metal casing of the switch, I only got a shock when I disconnected the earth cpc from the metal casing, touching the cpc.

I originally thought that the earth could have been crossed with a neutral somwhere in the house?

 

[Damocles]

Were you touching the neutral when you touched the earth again? Neutral would still have provided a good return to earth and you could have completed the circuit from the floating earth?