Earthing of light switch mounting box

[riph72]

Hello all,

I changed a couple of old light dimming switches last week for "normal" brass switches. I ensured the new switches were firmly earthed, but I didn't connect an earth to the metal mounting box in the wall.

How big a problem is this, if at all? I figured it was impossible for someone to physically come into contact with the box because it's hidden inside the wall, behind the metal body of the new switch. The connecting screws that attach the new switch to the mounting box are a possible weakness though?

I'm probably going to look again at this anyway and add in a link, but thought I'd see what others think!

Regards,
Richard.

 

[Taylortwocities]

If the mounting box has at least on fixed lug then it is not necessary to earth the box, provided that the switch itself is earthed.

It is, however, considered best practice to provide the earth link beween the box and the switch/socket, etc

 

[riph72]

If the mounting box has at least on fixed lug then it is not necessary to earth the box, provided that the switch itself is earthed.

It is, however, considered best practice to provide the earth link beween the box and the switch/socket, etc

What do you mean by "fixed lug"? Do you mean where the screws (from the switch) attach to the box? If so, I think both were "loose", in the sense that they allowed very small (mm) adjustment of the switch position.

Sounds sensible though, what you say. I will probably put the link in anyway, but I won't make it a high priority.

Thanks,
Richard.

 

[Taylortwocities]

Most boxes these days have one fixed and one moveable lug.

The back box is earthed by the screw to the FIXED lug.

If your box has two moveable lugs then an earth link is necessary.

 

[sparkwright]

By fixed lug this will be metal too.

Older switch boxes circa early 60s had nylon threaded lugs, so in the case of these you should fix an earth connection to the metal box in some way.

 

[radweld]

It is, however, considered best practice to provide the earth link between the box and the switch/socket, etc

I am curious why people consider this best practice. Surely it is rather poor engineering practice to waste time and materials on unnecessary work?

Also, I have never come across a situation where a hazard has arisen because of an unearthed back box. However, I have come across numerous problems caused by the quantity of cabling in a back box, or poor connections where multiple cables are terminated under the same screw. Adding an unnecessary cable doesn't seem like a sensible thing to do.

 

[EFLImpudence]

It is not a requirement. - but...

...following your reasoning, should there be a metal back-box and plastic switch you may as well not attach the cpc or cut it off.

 

[EFLImpudence]

It is not a requirement. - but...

...following your reasoning, should there be a metal back-box and plastic switch you may as well not attach the cpc or cut it off.

Edit - Thought I'd better add that my comment relates to when the switch is taken off - time of greatest danger.
I realise the implications when fixed.

 

[radweld]

It is not a requirement. - but...

...following your reasoning, should there be a metal back-box and plastic switch you may as well not attach the cpc or cut it off.

The back box must be earthed. Where that is achieved satisfactorily from the accessory, I consider it bad practice to introduce superfluous wiring. If the switch does not have a CPC connection, the CPC clearly needs to be terminated on the back box.