Earthing metal back boxes

[V1k_10]

Hi all,

I wanted to ask why metal back boxes need to be earthed? I ask as on all of my sockets, I have an earthing cable leading from the earth terminal on the back of the socket to the back box in the wall.

However, each back box is already earthed via the screws which hold the socket face plate into the back box - so why does the cable also need to be added?

 

[chivers67]

What happens when the screws are taken out?

 

[EFLImpudence]

I wanted to ask why metal back boxes need to be earthed?

They are not actually exposed-conductive-parts so they are not required to be earthed.

However, if you consider the screws to be exposed-c-ps then the only way to earth them when using an unearthed switch is by earthing the back box.

I ask as on all of my sockets, I have an earthing cable leading from the earth terminal on the back of the socket to the back box in the wall.
However, each back box is already earthed via the screws which hold the socket face plate into the back box - so why does the cable also need to be added?

The screws in this case will be earthed by the socket earth so it is not actually required to earth the back box.

 

[blup]

Ask yourself if you would be happy with a back box not being earthed to a metal socket, then ask why it should be different with a plastic socket as both have exposed conductive parts in contact with the back box. I can see an argument where the screws are shrouded with the plastic caps

 

[blup]

And a metal back box with lugs that are fitted with the metal fixing screws is seemingly a single exposed conductive part in its own right

 

[EFLImpudence]

Are you asking me?

Ask yourself if you would be happy with a back box not being earthed to a metal socket,

How can it be not earthed?

then ask why it should be different with a plastic socket as both have exposed conductive parts in contact with the back box.

Plastic sockets have the screw holes riveted to the earth bar.

I can see an argument where the screws are shrouded with the plastic caps

An argument for what?

If you are getting confused with plastic switches which do not earth the screws then I covered that in my reply?

 

[EFLImpudence]

And a metal back box with lugs that are fitted with the metal fixing screws is seemingly a single exposed conductive part in its own right

The back box is quite simply not exposed.

 

[ebee]

This topic is what I often refer to as "An Old Chestnut" because it keeps coming up

 

[SUNRAY]

In the old days it was quite normal to fit all metal conduit and back boxes without any earth wiring, accessories would simply be mounted on the backbox with the fixing screws and that was deemed to be enough. I have done loads like it. Would I do that now?

NFW.

Our school stage lighting for example was all round 5A BS546 sockets screwed into Besaboxes, something like 50 of them. Installed in 1966 all conduit was coupled to a metal trunking, as was the main isolator, as was the Strand Junior8. The feed was a MICC into the isolator. There was not a single earthwire in the system. In fact the whole school was wired like that.

 

[ebee]

In the old days it was quite normal to fit all metal conduit and back boxes without any earth wiring, accessories would simply be mounted on the backbox with the fixing screws and that was deemed to be enough. I have done loads like it. Would I do that now?

NFW.

and to think you'd probably be more than likely to have an RCD in circuit than back in those days so , in a few circumstances, not altogether as crucial perhaps.

Those of us that years ago did rewireable fuses and no RCDs didn`t flinch as often as we did back then

 

[SUNRAY]

and to think you'd probably be more than likely to have an RCD in circuit than back in those days so , in a few circumstances, not altogether as crucial perhaps.

Those of us that years ago did rewireable fuses and no RCDs didn`t flinch as often as we did back then

RCD's what were they and yes rewireable fuses throughout the brand new school building, complete with asbestos insert in 1966.

 

[blup]

The back box is quite simply not exposed.

Just concentrating on this for the time being, it is (the argument goes) an ECP because the combination of the accessory box and the screws (which attach via the back box lugs) from a single exposed conductive part

 

[JohnW2]

Just concentrating on this for the time being, it is (the argument goes) an ECP because the combination of the accessory box and the screws (which attach via the back box lugs) from a single exposed conductive part

The back box, per see, cannot be an "exposed-c-p", because it is not 'exposed' ('during normal use'), but the heads of the faceplate screws (if metal) will be exposed-c-ps.

In the case or metal accessories,the heads of the screws will be earthed by their contact with the (required to be earthed) faceplace. Also, as has been said, in the case of plastic sockets (or FCUs), the heads of the screws will also be earthed. by contract with the rivets into the socket's 'earth bar'.

That leaves only plastic switches as presenting a potential 'issue'. Plasric caps/covers over the screws might be considered adequate to address this theoretical issue but, for those who are really concerned, nylon screws are available.

Kind Regards, John

 

[blup]

The back box, per see, cannot be an "exposed-c-p", because it is not 'exposed' ('during normal use'), but the heads of the faceplate screws (if metal) will be exposed-c-ps.

Why not, the fact that they are mechanically connected to the back box creates a standalone single exposed conductive part, so need to rely on IET guidance, just earth it,

 

[JohnW2]

Why not, the fact that they are mechanically connected to the back box creates a standalone single exposed conductive part, so need to rely on IET guidance, just earth it,

As I've often said, and for what it's worth, I personally always do earth metal back boxes - but that's 'just me', and doesn't necessarily mean that it is 'required'.

Both in terms of common sense and the spirit of BS7671, your argument above is essentially academic/semantic. The concept ('spirit') obviously is that anything which could become live under fault conditions and which is "exposed" (i.e. touchable) should be earthed. Whether or not something else which is not exposed (i.e. not touchable) is in electrical continuity with the exposed/touchable part is not really relevant.

As I said, if one is really concerned, one should probably use nylon screws, in which case I can'ts see how anyone could suggest that the back box was an 'exposed-c-p'..

Kind Regards, John