Earthing a metal backbox

If the back box has fixed lugs for the screw holes and the screw holes in the switch are earthed, that is an adequate system of earthing. Adjustable lugs cannot be relied upon to make a safe connection though.

back boxes tend to have one adjustable and one fixed lug.

Modern ones do. None of the ones in my house do. However, so long as you have one fixed lug and the hole in the faceplate the screw to that goes through is earthed, a separate earth is merely recommended.

Ensure there is no paint around the connection and use a star washer for a good contact.

Thanks. It's probably my lack of knowledge about metal faceplate light switches. Do they have an earth contact that has the CPC of the T+E connected directly to it, or does it rely on earthing through the mounting screws to the earthed backbox?

The only metal faceplates I have used were industrial ones and those were on surface mounted boxes with an earth terminal. However, the plastic switches I use at home have the earth connected to metal plates on the back of the switch that carry it to the screw holes. They also have metal grommets in the hole, to ensure that the screw connects to the earth.

It's not a problem that I've had to solve (because the back boxes here are very shallow so I'm replacing them with deeper modern ones as I work through the house) but I would add a crimp tag and star washer to one of the fixing screws if I had to solve it without replacing the box. I have always earthed back boxed but many people seem to rely on the faceplate mounting screw.

I installed one of Toolstation's finest ('Axiom') white light switches yesterday. No earth terminal on the switch at all, I had to screw the CPC into the backbox (which was plastic and had an earth terminal that went nowhere). So those screws weren't earthed.

Theo

Although it's not unknown to have the backbox earthed via contact with the accessory via the screws, it's not at all usual for the faceplate to rely on contact with an earthed backbox to provide it's earthing.

Many plastic light switches won't provide any termination for an earth wire at all.

Metal ones that are Class I will do, and those will include the annulus of the screw holes as normal.

Note however there are a number of Class II (i.e. double insulated) metal clad switches that do not require earthing. These may lack continuity between screw hole and backbox.

I have never seen that on plastic light switches. Sure on FCUs, 20A DP switches etc.

Are you sure?

They were installed over 25 years ago and I've not looked at the back of them since. I prefer not to disturb wiring if I don't have to. However, loosening one screw does show a metal grommet behind it, so I'm probably sure :-).