old metal conduits - do they need earthing?

[vv2806]

Hi,

I am refurbishing my house and changing the old switches as part of the refurbishment. They are plastic surface mounted, I want to put flash switches in metal back boxes.

It turned out there are back boxes behind the switches but they are old type - smaller than the ones I bought from Screwfix, no earth clamp, and there are no lugs, so I can't use them. The cables run through metal conduits, connected to the boxes with clamps (or brackets?). From what I found in the loft, the house has been rewired at least ones, there are earth clamps attached to the top of the conduits and something that looks like steel wire but it is cut near the clamps. I guess those are the remainings of the old earthing.

I replaced the old metal boxes. Can I leave the metal conduit in place with the cable inside, provided the conduit is not attached to the box in any way. If I leave them, do they have to be earthed? If so, can I run earth wires from the old clamps in the loft inside the conduits and connect them to the earth clamps in the back boxes? Or I should connect all the conduits with a single wire which terminates in the CU as it used to be when the house was built?

 

[Steve]

In a true metal conduit system, the conduit continuity provides the earth - there are no breaks in the conduit and therefore it carries the earth from the DB to the socket / switch.

It sounds like someone has bodged earth clamps onto the conduit. AFAIK, this is not a recognised way of earthing conduit, especially with bits of steel wire of unknown CSA. Are the old backboxes round? These are rare nowadays.

As long as the inner cables contain an earth wire (should have if the house has been rewired in the last 20 years), and it is terminated onto the switchplates (if metal - if not, terminate to the backbox), you can do as you wish with the conduit end. It doesn't need to be earthed, but it is nice to do. It would be accomplished (properly) by securing the conduit end onto the new backbox with the right connectors, but this would be dificult with such old conduit, its probably rusted up and it being buried in plaster wont help you.

 

[vv2806]

Thank you Steve!

The old backboxes are square but the overall dimensions are smaller than the backboxes we use today.

The house has been rewired not long before I bought it, I was told the local council did the job for the old gentleman who owned it. It is not possible to screw a nowadays switch to the old backboxes because they are too small and the screwing holes don't match the holes in the switch. So whoever did the job used the conduits to run the cables and put plastic surface mounted switches on top of the old backboxes.

I don't think it is possible to connect the conduits to the new backboxes, the conduits are oval, not round and their size is different from the size of the holes in the backboxes. Probably such connectors don't exist.

The top part of the conduits is exposed in the loft (about 6 inches). I removed the old earth clamps and the bits of wire and slide longer pieces of plastic conduit over the metal ones. Hope it is enough to pass a PIR if I decide to sell the house.