Lighting circuit earth at 120V

[Martin Brook]

Hi all,

The other day I bought a new screwdriver set from B&Q and decided to try it out on the nearest available screw. It turned out to be one of the screws in the light switch in our kitchen. I noticed a strange tingling sensation when I touched the side of the screwdriver... very mysterious...

On further investigation it seems that the earth conductors in all of my light switches are at 120V (with respect to neutral or live - tested with multimeter set to AC mode). However at the consumer unit, all appears to be fine. I have a functioning RCD and it's not tripped. All the lights work. No (obvious) dodgy wiring in any of the switches.

Before I spend weeks lifting floorboards and crawling around in the loft, can anyone suggest a reason why this might be going on? And is there any way to systematically track down the source of the problem? And why is it at exactly half mains voltage?

And, as an aside, could this have anything to do with the fact that my energy-saving lightbulbs seem to flicker every few seconds when switched off?

Any advice would be appreciated... my electrical experience is still fairly limited...

Thanks Martin

[Tim Mitchell]

In article , Martin Brook writes

Sounds like the earth wire on your lighting circuit is not connected to earth, which is a cause for concern.

You often get 120V on a disconnected earth when some type of surge arrestor is connected to the circuit. (It's very very high impedance, so you don't feel a shock when you touch it, just a tingle. If you measured it with an analogue meter rather than a digital one, the voltage would be much lower or would disappear).

I am not sure how a surge arrestor would get onto a lighting circuit however, unless there is a wider earthing problem in your house and the surge arrestor equipment is on a ring main; the ring main earth is connected to the lighting earth, but neither is connected to real earth.

[Scott M]

Have you checked to see if all the earths on the other circuits are doing the same thing (even money they are as all earths should be commoned, natch)? If so, then I'd guess you've got a faulty earthed appliance on one of the ring mains. Go round unplugging things while someone keeps an eye on the multimeter. My first stop would be any television sets as although not usually earthed directly, they often have floating chassis (there was a discussion a while back on this) which might be electrifying the aerial cable and, from there, the earth circuit of the house.

Can't see how as earth is really a non-used connection in lighting ccts. Again, I think this has been discussed before and could be down to the neutral side of the light being switched or a particularly high floating neutral.

[Andy Wade]

Incoming "conductive things" like water pipes should of course be firmly bonded to the PME terminal (main bonding). Network faults apart - and these would usually clear in a matter of seconds - the PME earth shouldn't be more than a volt or three adrift of the local 'true' ground.

It is at ~120 V _because_ the connection to the main earth terminal is either absent or broken. (There's a lot to be said for Occam's razor, you know.)

No need to postulate a filter: the capacitance in the wiring is quite enough to produce the effect described. T&E cable will have roughly equal capacitance between L and E and N and E. If the earth wire (CPC, strictly) is left floating then a high-impedance meter will measure 120 V AC on it, due to the capacitive divider principle that you describe. The few microamps of leakage through your body (and its own capacitance to earth) that you'd get when touching the CPC in these circumstances is quite enough to produce a tingle.

On a lighting circuit like this I'd expect to see almost exactly half-mains voltage when all the lights are switched off, with the voltage rising as lights are switched on (because in the switch-drops sections both L and SL conductors are 'live' when the light is on and all the capacitance in such section is now between L and E, IYSWIM).

Could this whole thing be a simple case of incompetent extension(s) to an old unearthed lighting circuit - somebody adding new wiring and accessories and having nothing to connect the CPC to?

[Huge]

Errr, I thought one shouldn't run the house earth into an outbuilding? Perhaps that's just for {mumble} (I'd call it PME, but WTF) installations, like what we've got.

[Iain]

replying to Andy Wade, Iain wrote: I had same problem. Different voltage readings. But nonetheless the solution was the same. Reconnect the broken earth connection in one of the ceiling roses. Thanks for the ideas here. Sorted in half hour after reading your post. Spent hours writing down values before.