metal back box

adwt2004 · 12 Jan 2009

Hi guys,

I am helping an electrican to re-wire my two light circuits due to there being no earths. The metal back boxes are existing and fine to use but they have no earth connector in them. I normally connect to the back box earth screw and then on to the faceplate with no breaks in the earth cable. Is it safe for me to use one of the mounting screws and wrap the earth around that before re-tightening box to wall and then on to the face plate. Otherwise i need to replace about 10 backboxes and damage plaster work etc.

The earth terminals tend to be brass (is that for identification only) or should my back box screws also be brass as well if above idea is allowed?

davy_owen_88 · 12 Jan 2009

So long as the backboxes have one fixed lug, you do not have to install the earth lead to the backbox. The fixed lug and screw do a good enough job on their own. I wouldn't think about wrapping bits of wire around screws as the connection isn't going to be good, and they could come loose later on and start causing faults. If you are really against the idea of replacing the backboxes, and you do not have one fixed lug, then you could solder on an earth lead.

adwt2004 · 12 Jan 2009

Ok, when you say lug are you meaning the two attachment points for the faceplate? There is a fixed lug on the left side, the right hand side slides up and down. So If i just earth the faceplate and screw it to the wall everything including the screws is earthed then?

Just concerned because i have metal light switches.

davy_owen_88 · 12 Jan 2009

Yes, connect the earth conductors from the supply cables to the metal frontplate. The screws holding the frontplate to the backbox will provide an adequate connection to earth should any conductor become loose and make contact with the backbox/screws but not the frontplate.

roberta1000 · 12 Jan 2009

davy_owen_88 said:
So long as the backboxes have one fixed lug, you do not have to install the earth lead to the backbox. The fixed lug and screw do a good enough job on their own.

There was a case quite a few years back where someone was fatally electrocuted by touching a live screwhead while wiping a socket with a wet cloth. IMHO cutting corners by not earthing the backbox was always shoddy and I have always done it as the OP suggests using a continuous conductor from cable to backbox to faceplate in case of corrosion of the screw threads even on a fixed tag. Less an issue with a metal faceplate though! Box terminals can be brass or not depending on the make - brass strips less easily.

flameport · 12 Jan 2009

robertw63 said:
There was a case quite a few years back where someone was fatally electrocuted by touching a live screwhead while wiping a socket with a wet cloth.

Not impossible, but a highly improbable set of circumstances would have to apply:

1. The socket screws would have to be be old steel ones (new ones being of nickel plated brass, which does not rust)
2. The screws must have rusted to such an extent that the resistance between the socket and the screws was in the tens of ohms at least.
3. A fault existed between the metal back box and a live wire (and this must have occured after the screws rusted, as screwing the socket back on would have cleaned any rust away from the threads)
4. The person was standing on a reasonably conductive surface (so probably not wooden floorboards or carpet)
5. There was no RCD on the circuit for the socket.

As for people wiping down sockets with wet cloths - what next, hosing down the CU with a pressure washer?

Lectrician · 13 Jan 2009

What then happens when someone changes the chrome or brass front for a plastic?

In my opinion you need to either carefully change the backbox, or fit a cheese head self tapper, or M4 brass screw into a tapped hole.

I believe this is also the thinking of th NIC.

adwt2004 · 13 Jan 2009

I will do that then. I will drill and plug a hole in a spare hole of the backbox. Insert a brass screw with a wide head and wrap the earth around that and tighten down. I don't want to remove the box, it will ruin plaster and create lots of work. When renovating an entire house that is not favourable!

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metal back box

roberta1000

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