Earthing and lighting circuit

[jono5000]

Hi all

Just some advice needed. Recently had fuse box updated in my 1980s purpose built flat. I have just bought some metal light switches to change over, but on removing the old plastic ones with metal screws, they are just red wires (L1, L2 and common) going up a metal pipe (attached to metal back box). No other colours. Also in the ceiling rose, there are just red and black wires, no earth wire. At the fuse box, there are loads of metal pipes passing through the fuse box.

On changing the fuse box, the electrician tested it and ticked the continuity check box on the certificate. He also recorded 0.87 for the maximum measured earth fault loop for the lighting circuit.
Does this mean the lighting system is earthed via the metal pipes and is sound? If so, would I simply require an small flex of earth wire to attach to a terminal on the metal back box and the back of the metal light switch plate?

 

[RF Lighting]

It sounds like you may be on a steel conduit system, which would normally supply the earth via the conduit, but you MUST verify that this is the case before you fit the switches.

You need to have an electrician do an earth loop test at each switch to make sure.

There is a chance that the conduit is not earthed, or a joint has failed and and the earth has been lost.

An earth wire is drawn into earthed steel conduit these days as it is not the most reliable method of earthing.

 

[hairyben]

Conduit earthing on a 1980's flat is naughty, but not uncommon on ex local authority housing.

Athough it's not uncommon to use the conduit for an earth, and in most cases fine, in time steel conduit can corrode, especially bad joints, and the earth path fail.

The ideal solution would be to rewire, so long as all the conduits are good this is incredibly easy as it's simply a case of pulling new wires in on old. I'm surprised the electrician didn't mention this before changing the consumer unit.

 

[jono5000]

The ring main has earthing cables which is strange. A while back I fitted metal halogen downlighter plates, which were supplied with earthing cable. This was screwed to the fixing bracket which in turn was connected to the metal box encased in concrete in the ceiling with the other wires going up the metal conduit.

After reading all the postings, kind of worries me a bit. So I'm going to call out another electrician to see if its all earthed properly and give me a quote for updating the earthing for the lighting if necessary. Southern Electric provide free visual inspections for wiring, is it worth seeing what they say?

 

[hairyben]

The ring main has earthing cables which is strange. A while back I fitted metal halogen downlighter plates, which were supplied with earthing cable. This was screwed to the fixing bracket which in turn was connected to the metal box encased in concrete in the ceiling with the other wires going up the metal conduit.

After reading all the postings, kind of worries me a bit. So I'm going to call out another electrician to see if its all earthed properly and give me a quote for updating the earthing for the lighting if necessary. Southern Electric provide free visual inspections for wiring, is it worth seeing what they say?

visual inspections a bit of a joke IMO, and it won't include earth loop tests at lighting points.

Ring circuits need to supply an earth to equipment, lighting circuits with plastic fittings don't, thats why they haven't bothered. I wouldn't stress it, but a periodic inspection to confirm the conduits are providing an earth is a good idea, this should have been done as part of the CU upgrade (installation cert), a rewire is the ideal long term solution.

If you have an installation cert for the CU, what are the "max earth fault loop impedance" readings for the lighting circuits(s)?

 

[flameport]

Southern Electric provide free visual inspections for wiring, is it worth seeing what they say?

No - a visual inspection won't reveal anything that you haven't already seen with your own eyes.

 

[jono5000]

So should I not install the metal light switches, even connecting the earth to the metal back box, until I'm certain of continuity?

 

[hairyben]

So should I not install the metal light switches, even connecting the earth to the metal back box, until I'm certain of continuity?

As an electrician, the question I have to ask myself if doing the above is:

"Can I guarantee the conduit will provide a satisfactory earth for the life of the installation?"

Given that "the life" could easily be 30+ years, and I have no way of knowing the condition of buried conduit, the answer has to be no.