Why are light switches not earthed?

[R D S]

When using plastic switches and a dry lining back box there's no earth.

How come?

Ta.

[GB]

What were you planning to earth?

Or do you just want to have a terminal to connect two earth wires?

[R D S]

Metal switches have an earth terminal, as do metal back boxes.

I'm just a bit surprised that plastic switch/boxes have no earth protection, after all there is a live wire in there and a pair of screws that could be 'fingered'.

[GB]

Aren't the screws shrouded in plastic on the inside?

[bert]

In message , R D S writes

Presumably because they are non-conducting

[R D S]

Yeah, but the screws are metal and i'm pretty sure there's a metal track between them. If the live wire came unattached from it's terminal it could potentially cause danger. Not that i'm concerned, just surprised.

[R D S]

In a possible answer to my own question...

The screws appear well contained and i'm guessing that modern switches don't have the conductive track between the screw holes.

[Dave Plowman (News)]

Nothing to earth? But the dry lining box should have a terminal to make off the earth to - there for later use if the switch is changed to metal.

[John Rumm]

cos they are plastic ;-) (the chances of them becoming live due to fault conditions are limited to slim and fat)

(the earth will still be present in the cables - that should be terminated somewhere to preserve continuity and keep it out of the way)

[John Rumm]

Many shallow plastic pattresses include a terminal for "parking" the earth.

There should be limited scope for the screws becoming live - and even if they were they pose a limited shock risk since you can't easily get a hold of them!

[The Natural Philosopher]

No they dont

[F Murtz]

In Australia the screws are recessed such that a "standard finger" can not reach them.

[Johny B Good]

Nevertheless, afair, the plastic backboxes have an earth terminal post to allow metal switch plates to be earthed[1]. The plastic switch plates don't need this protection and the screws are quite well isolated from possible contact with the live once fitted.

There might be a slight risk when removing the plate but the lighting circuit fuse should be pulled out when doing this sort of work anyway so even this risk doesn't matter. If you're going to test whilst the switch plate is unscrewed from the backplate, the screws will be absent and you'll be mindful of other safety concerns anyway (like not poking your fingers into the wiring).

In short, there's absolutely no need to earth the fixing screws on a plastic switchplate (and an earth terminal post is available to connect a metal switch plate to the safety earth in accordance to the regulations).

[1] It's been a while since I last looked at the backside of a plastic backbox but I think I've seen some designs were the faceplate screw thread inserts are rivetted to a track which joins to the earth terminal post (which would guarantee that a metal switchplate, once installed, will be automatically earthed via the fixing screws without the need for a seperate earth strap connection).

[charles]

Certainly the one I fiited on Monday had one.

[charles]

the 2A socket I fitted on Monday (for a motion sensing outdoor light) had recessed screw holes and there were plastic "bungs" to cover the screw heads, too.

[charles]

When I re-wired my house (new to me) in 1965, the Electricity Board checked the earth on all screws including light swtches.

[GB]

Were the bungs for looks or for weather-proofing?

[charles]

Certainly not weatherproofing - it's an indoor fitting. Looks, possibly, but more likely to deter unauthorised tampering.

[Brian_Gaff]

Well maybe you are not supposed to use the plastic boxes? I suppose if you really wanted to earth them you could achieve it with a bit ofwire and a tag. I would not use a plastic box for a dimmer, I've come a cropper that way too many times in the past when the metal plate got shorted internally. I guess plastic dimmers are much safer!

Brian

[Andy Wade]

Yes, although dry-lining boxes are usually sold without this. There is a place for an earth terminal in the moulding though, and the terminals are (I presume) available separately.

And it's not just for parking - you may need to join the CPCs of two or more cables - e.g. where you have a 2+E 'switch drop' and a 3+E cable going off to a second 2-way switch.

Exactly so. There is a specific exemption in the wiring regs (BS 7671):

410.3.9 The provision for fault protection may be omitted for the following equipment: [...]

(iii) Exposed-conductive-parts which, owing to their reduced dimensions (approximate maximum of 50 mm x 50 mm) or their disposition, cannot be gripped or come into significant contact with a part of the human body and provided that connection with a protective conductor could only be made with difficulty or would be unreliable.

NOTE: This exemption applies, for example, to bolts, rivets, nameplates, cable clips, screws and other fixings.