Earthing metallic switch - earth wiring order

[rshof]

Hi

I've had some metallic switch faceplates installed, replacing old plastic ones. However I'm a bit concerned that they've done it wrong!

At the moment there is an earth coming from the wall (mains earth?) which is connected to the metal backbox earth terminal and then there is a separate earth wire connecting the faceplate to the backbox.

Searching around though it seems that the wall earth should be connected to the faceplate itself rather than the backbox? Which way is correct, or does it not matter even?

Thanks.

 

[PBC_1966]

It doesn't matter, either way results in both box and switchplate being securely earthed.

 

[JohnD]

When you are replacing old sockets, the amount of slack wire may be insufficient to reach to the terminal on the new socket, which may be in a different place to the terminal on the old one. In which case running it to the terminal in the backbox is better, because it avoids an extra joint, and it removes the risk that a taught wire might snap or be pulled out.

 

[chivers67]

If one of the lugs is fixed the socket should "not require" this extra wire but it is good practice to run one and it sounds like your electrician is using good methods.

However if they are both loose An earthing tail must be fitted between a socket-outlet and a back box having two adjustable lugs. The need for this has been demonstrated where socket-outlets have suffered overheating and burning around the fixing holes due to a line-to-earth fault in metal boxes which had not been provided with an earthing tail. Because the adjustable lugs had become corroded, they presented a high resistance to the earth fault current, and consequently the circuit overcurrent protective device did not operate within the maximum time permitted by BS 7671. This resulted in a risk of electric shock as well as the thermal effects already mentioned

 

[PBC_1966]

Relying on the lugs is not good practice in my opinion, even if one or more of them is fixed. Apart from anything else, if you remove a switch or socket for some sort of live testing, it results in the earth connection between the device and the box being lost. Or you simply end up with a less-than-ideal earth connection if somebody loosens the fixing screws slightly to adjust the position, or for decorating around the edge.

For the sake of a few extra seconds time and few inches of conductor (which will almost certainly be salvaged from the scrap offcuts of cable which would otherwise just be thrown away anyway), I can never understand why anyone would omit the link.

 

[rshof]

Thanks for the advice.

I'm assuming you are right that it doesn't matter much either way, but why does there seem to be a preference from others for going direct from mains to faceplate, less distance?

 

[JohnW2]

I'm assuming you are right that it doesn't matter much either way, but why does there seem to be a preference from others for going direct from mains to faceplate, less distance?

I presume that preference results from the fact that it is the earthing of the faceplate which normally matters most, so that it is safest to have the 'incoming earth' connected directly to the faceplate, with a secondary conductor from there to the backbox. The true ideal is probably to have a single conductor which goes first to one (box or faceplate) and then to the other in one unbroken run.

Kind Regards, John

 

[stillp]

why does there seem to be a preference from others for going direct from mains to faceplate, less distance?

The faceplate is more likely to be touched than the backbox.