Earthing of Lighting Circuit in Concrete Ceiling

1 Apr 2007
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United Kingdom
I've just replaced an old pendant bathroom light with a new flush-mounted unit for a friend. Ceiling was solid concrete. On removing the old rose I found the following wires inside a metal-lined conduit box recessed within the concrete:
2 black neutrals.
1 earth from cable (probably the switch cable, but as outer sheathing not visible I can't be sure).
1 earth wire to a crimped connector which screws into the top of the conduit box.
4 red lives (one of which turned out to be the switch return-but it was not tagged red, but red all the way back).
So by my reckoning we're short of one earth wire?
This property is a maisonette was built around 1970, all the walls are solid concrete. Is it likely that the lighting circuit is run in metal conduit within the concrete, and the metal forms the earth return? If so, why one earth wire to the switch? I thought all lighting circuits were earthed from the 60's onwards?
The new light and circuit are working fine, but just slightly concerned about the reliability of the earthing, as the new light fitting is metal backed and there is at least one metal dimmer switch on the same circuit. Should I advise friend to get this circuit properly tested?
As an afterthought, how the hell can you ever rewire a house built like this without chasing out into solid concrete? It took me half an hour just to drill 2 holes into the ceiling to mount the light fitting!
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Very common, used a lot in housing association/local authority houses.

The metal conduit and conduit boxes were used as the earth path but the integrity of the earth should be checked by somone with a low reading ohm meter from the light fitting back to the fuseboard. You never know what some muppet has done in the last 40 years.

Yes, concrete is very hard. Its easy to drill in to if you have the right tools. A good quality hammer SDS drill will make short work of hole drilling.
Thanks for that. Yes, it is an ex-local authority property. I have a standard multimeter, can I check the earthing for them, if so what is the procedure please?
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A standard multimeter wont do it. You need one that will accurately measure very low resistances. A high street multimeter doesnt do that.

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