Settle this Argument.

[telectrix]

metal faceplate 1363 socket. obviously needs to be earthed. argument is whether the back box should be earthed just because it's a metal faceplate. ( i know it's considered good practice to fit a fly lead anyway, but that's not the argument. the argument is with the faceplate being metal, does that make any difference. my argument is No, other party says Yes. Who is right, going by BS7671?)

 

[sparksburnout]

If the backbox has one fixed lug, not required ( but I'd do it anyway). Two adjustable lugs, fly lead required, IMHO irrespective of plastic or metal accessories.

I don't get this. Whats the difference between fixed and adjustable lugs, as regards earthing of the back-box via the accessory? If it's securely fixed, then it is earthed. If it's not securely fixed, then it shouldn't be live. They don't just become insecure in the night do they??Anyway the back-box doesn't need earthing as it shouldn't be accessible if the socket is live, should it???

 

[littlespark]

"Live touches back box, creating a danger."
How??? Kindly explain.

A live conductor, touching a metal box, which isn't earthed because the cpc is also disconnected. Sounds dangerous to me

 

[Andy78]

A live conductor, touching a metal box, which isn't earthed because the cpc is also disconnected. Sounds dangerous to me

It's also dangerous even with the backbox separately earthed because in this imaginary scenario where conductors fall from terminals, that one fell out too.

 

[polo1]

I don't get this. Whats the difference between fixed and adjustable lugs, as regards earthing of the back-box via the accessory? .......

By its very nature, an adjustable lug does not provide a reliable path to earth (via cpc, accessory earth and accessory screws), whereas a fixed lug does.

 

[sparksburnout]

A live conductor, touching a metal box, which isn't earthed because the cpc is also disconnected. Sounds dangerous to me

Why? How can this imaginary scenario exist unless someone has tunnelled through from the other side of the wall???

 

[sparksburnout]

By its very nature, an adjustable lug does not provide a reliable path to earth (via cpc, accessory earth and accessory screws), whereas a fixed lug does.

Eh? So the accessory is fixed via 2 adjustable lugs (???) and they have both come free, very unlikely and in which case said accessory is then hanging off the wall, AND the live has ALSO become disconnected and is touching the back-box? Come on FFS.
Has anybody got a picture of these back-boxes with 2 adjustable lugs cos I can't remember seeing one???

 

[MJPD29]

Back box does not need earthed. Earthed by screws from faceplate, If its not secured properly or opened to work at it, it should be dead anyway and locked off .

 

[polo1]

Eh? So the accessory is fixed via 2 adjustable lugs (???) and they have both come free, very unlikely and in which case said accessory is then hanging off the wall, AND the live has ALSO become disconnected and is touching the back-box? Come on FFS.
Has anybody got a picture of these back-boxes with 2 adjustable lugs cos I can't remember seeing one???

I wasn't commenting on the likelihood of a particular set of circumstances occurring. The question posed was about the need to earth a metal back box viz. run an earth from the b/box to the accessory. My understanding is that, so long as the back box has one of the two lugs fixed, a separate earth is not essential as earthing is achieved via the accessory screw. Others may have a different opinion. Agree/don't agree, I won't loose any sleep either way ;>)

 

[littlespark]

Back box becomes live, therefore screws and faceplate (if metal) becomes live. No tunnelling involved.
If a backbox is designed with an earthing terminal, shouldn’t it be used?

 

[MJPD29]

Back box becomes live, therefore screws and faceplate (if metal) becomes live. No tunnelling involved.
If a backbox is designed with an earthing terminal, shouldn’t it be used?

how is this possible? if its a metal socket, it is earthed through the earth terminal on the faceplate which is connected to back box through terminal screw to fixed lug, if it is a plastic or any other type of socket the screws are attached to the earth bar on the back of the socket, therefore again earthing the back box secured by terminal screws to the fixed lug of the box. If the socket is opened and the earth link from screw to lug is broken, the socket should be dead anyway. your not supposed to be working live.

 

[littlespark]

a metal back box, surface or buried in the wall is an extraneous conductive part. Something that MAY become live in the instance of a fault condition (something like a live cable dropping out a termination, perhaps)
A surface metal box is of course capable of being touched. A buried box in the wall is the same thing due to the fact there are metal screws connected to it, and exposed to the front of the faceplate, which can be touched.

 

[telectrix]

a metal back box, surface or buried in the wall is an extraneous conductive part. Something that MAY become live in the instance of a fault condition (something like a live cable dropping out a termination, perhaps)
A surface metal box is of course capable of being touched. A buried box in the wall is the same thing due to the fact there are metal screws connected to it, and exposed to the front of the faceplate, which can be touched.

BOLLOX.
back box is not extraneous. it's part of the electrical installation and is therefore, by definition an exposed conductive part of the installation.

the faceplate screws are earthed to the faceplate, irrespective of whether the back box has a fly lead or relies on the lugs and screws for it's earthing.

 

[littlespark]

I maybe got my terminology wrong there. (extraneous/exposed??)

My OPINION is that a buried box should be treated the same as a surface box.

 

[telectrix]

agreed. the issue is this.

when faceplate is in position, the back box is earthed via the screws and lugs, so therefore complies with BS7671.

however, if the faceplate is unscrewed, as when decorating, then the back box is no longer earthed. unless a fly lead has been installed. some would say that the circuit should be isolated , but then try wallpapering in the dark.

 

[Deleted member 26818]

a metal back box, surface or buried in the wall is an extraneous conductive part. Something that MAY become live in the instance of a fault condition (something like a live cable dropping out a termination, perhaps)
A surface metal box is of course capable of being touched. A buried box in the wall is the same thing due to the fact there are metal screws connected to it, and exposed to the front of the faceplate, which can be touched.

Extraneous is something that is nothing to do with the wiring system and brings into the building, a difference in Earth potential.
Exposed is something that could become live in the event of a fault.
Don't believe anyone has suggested not Earthing anything.
The point is whether both the back box and the faceplate require earthing via a fly lead, or will the 3.5mm faceplate screws be sufficient.