Lighting circuit & Ring Main

[Dulwich]

We had major work done on the house a couple of years ago. This included, amongst other things, a loft conversion. When the electrical wiring was being done we had a new consumer unit installed. The builder used various sub-contractors including an electrician - in whom we did not have a lot of confidence. This scepticism has been borne out by a legacy of problems.

A new electrical problem surfaced yesterday and I wanted to air it here to get your views before I take issue.

A light bulb in a ceiling fitting in the loft blew. This is in one of two light fittings on the same circuit. I replaced the bad bulb and switched on the wall switch. The was slight click and neither fitting lit up. I went to the consumer unit and turned off the supply on the circuit for the lights in the loft. I came back up and to be safe, I put a voltage detector to the fitting and was surprised to hear it beep. I toggled the light switch and in both positions and the detector still registered voltage.

I thought then that maybe the circuit was mis-labeled. I then turned off various circuits and found that the voltage disappeared only when the circuit for the 13 amp sockets in the loft and the loft lighting circuit were both turned off at the same time.

I kept the circuit for the sockets off then put two new bulbs in the light fittings. I switched on the wall switch. Only one came on. The original bad one was still not working.

I'm not an expert in electrics but something here just isn't right - to put it mildly.

What should I do next to check things and to try to get to the bottom of the mystery?

Thanks

[tinnews]

That isn't necessarily an indication of anything wrong/faulty. It depends how the voltage detector works. Many such devices will 'detect voltage' due to capacitive coupling between wires.

That fits with the above diagnosis, remove *all* power from adjacent wiring and the voltage detector 'sees' nothing.

I suspect that things may actually be alright (in that the lights are on the right circuit etc.) and what has happened is that the light bulb that has failed also has a faulty/worn out holder and that the new bulb is not working for that reason.

First get a reliable way of detecting the presence of mains, an analogue meter is pretty good. Then check out the lamp holder where the non-working lamp is.

[Jim Alexander]

Between the 13A socket cct and the attic lighting circuit there are 4 possible MCB combinations. You have 1 good attic light. Plug a known good appliance in an attic socket. Try all 4 combinations of MCB and report back.

Jim A

[John Rumm]

The "not right" bit may just be your voltage detector.

Use an appliance plugged into a socket and a real bulb on the lighting circuit to prove to yourself that the MCBs do in fact control the appropriate circuits. Once that is done it is a case of finding the fault (probably) in the non working light fitting.

[Dulwich]

I borrowed a digital multimeter. With the circuit breaker for the lights off and with the socket circuit switched on, it displayed 28 volts ac. This fell away over a period of about 10 seconds down to zero.

From what you said, I presume that it was induced capacitance in the circuit.

Many thanks for your help - all of you.

[tinnews]

Yes, digital multimeters are sensitive enough to do this, that's why I suggested an analogue meter. However the digital meter you used seems to have confirmed what I and other said anyway.

[John Rumm]

Yup. a DMM is very sensitive and places a very low load on a circuit - so it will take a little while to discharge the wires.