PE from metal back box to socket

[robd]

I don't do it on plastic I started in 2006 so guess we were on 16th and no longer a requirement then. I know from doing periodics thinking back the one fixed lug making contact with the brass ring round the screw hole was deemed sufficient, was brought up on site work so wouldn't often do more than required in that situation and old habits die hard whichever side you fall.

 

[MJPD29]

Not required. Back box is earthed by face plate screws. Could be argued you shouldn't be working live hence you shouldnt be opening it without killing power anyway

 

[johnduffell]

It’s niether extraneous or exposed

May sound bizarre on first reading, but the metal backboxes is indeed an exposed conductive part.
This is because it is not insulating and may become live under fault conditions.
The touchable criteria could be indirectly, for example installed into a metal framed wall with a shelf fixed on the other side. Regardless, There is no requirement for any specific standard of insulation to the wall that encloses the back box and the back box is the only thing under the control of the electrical installation.

Having said all that, the screw is deemed enough to earth it!

 

[Pete999]

Agreed on both point. But A) why have the termination screw o the box if it's not mandatory and B) why was I told to do it and have always done it since I was a lad?

Somewhere to hang your cpcs if the wiring is Twin and cpc, on a lighting switch maybe, where there is no earth connection on the switch?

 

[pirate]

Seems just like a nice finishing touch, takes less than a minute, and on surface-mounted metal-clad stuff in a workshop with multiple users of varying skills it makes sense as an extra safety element, imho

 

[Rpa07]

Also helps to use up that roll of 1.5mm green/yellow 6491 that will not ever end.
As with @Murdoch - any upgrades to metal switch faceplates is a must.

 

[Pete999]

Also helps to use up that roll of 1.5mm green/yellow 6491 that will not ever end.
As with @Murdoch - any upgrades to metal switch faceplates is a must.

The termination block is there for when a switch is to be fitted, stops a floating earth

You bin watching Sy Fy movies Mate floating earth WTFs that all about.

 

[JOURNEYMAN]

Appols for my spelling, I have dyslexia, so rely on auto correct.

I knew a dyslexic pimp once who bought a warehouse! ..........sorry, I'll get my coat!

 

[chris gray]

A floating termination is how a none fixed terminated connection is described.

 

[Flanders]

Made me laugh when I came across a earth lead from the earth terminal in a plastic back box to the earth termial in the spur.

 

[Upton Sparks]

Apologies for the late arrival to the conversation,

So, does anyone know where in the regulations (BS7671) it says, you can use the lug hole of the metal box as an earthing point?

I think the regulation 411.3.1.1 applies, As does 132.1

The only question is. Are the metal back boxes classed as an exposed conductive part when buried in the wall?????.... I would agree when it’s fitted into the wall it may not be an exposed conductive part, but again if it’s not stated within the regulations, it’s all about interpretation.

Then again, it’s all about the idiots who live in the house, if they take of the socket of the wall then the back box is an exposed conductive part. Agree they would deserve an electric shock for being stupid in doing it live, (but these are the same people who put 30 amp fuse wiring into a 5 amp fuse carrier because the 5 amp kept blowing) … therefore its incumbent on the designer to take into account the layman and woman who will be living in the installation afterwards. hence 132.1

I don’t know if I agree its not good practice to earth the metal box. I think it is, and makes a statement about the standards the electrician is aiming for.

but I am on the fence now about it being a bad practice in not doing it… possibly it’s just lazy….

OK Tin Hat on …..

And

Merry Christmas and happy new Year to everyone.

 

[ipf]

NOBODY is using one of my lug holes as an earth connection point....

 

[richy3333]

Just as an addition/clarification GN8 Amd3 states at 9.7
'For flush metal back boxes with one fixed lug and one adjustable accessory fixing lug, as shown in figure 9.8, it is always desirable to provide and earthing tail.'

 

[Flanders]

The only question is. Are the metal back boxes classed as an exposed conductive part when buried in the wall?????.... I would agree when it’s fitted into the wall it may not be an exposed conductive part, but again if it’s not stated within the regulations, it’s all about interpretation.

The metal screws holding the switch to the back box would be an exposed conductive part though , so I would say that a metal box would have to be earthed even if the accessory did not need an earth, unless of course they where nylon screws.

 

[chris gray]

Over 50mm2 to be classed as exposed conductive part

 

[Silly Sausage]

The metal screws holding the switch to the back box would be an exposed conductive part though , so I would say that a metal box would have to be earthed even if the accessory did not need an earth, unless of course they where nylon screws.

Indeed, but if you look at the construction of a BS1363 socket outlet, the screws become Earthed when fully tightened up, and thereby, Earthing the backbox via 1 or 2 of the fixed lugs.

 

[Flanders]

Indeed, but if you look at the construction of a BS1363 socket outlet, the screws become Earthed when fully tightened up, and thereby, Earthing the backbox via 1 or 2 of the fixed lugs.

Yes I know That's why I said an accessories that does not need an earth for example a plastic lights switch

 

[SJD]

I have found on sockets (on several occasions) on an RFC, where the two incoming earths are terminated in the metal back box. And the installer has relied upon the screws to earth the socket, with no strap fitted!

Measuring a somewhat higher than expected Zs at the socket, and giving the screws a quick tighten (when Zs drops) usually gives the game away.

 

[chris gray]

I have found on sockets (on several occasions) on an RFC, where the two incoming earths are terminated in the metal back box. And the installer has relied upon the screws to earth the socket, with no strap fitted!

Measuring a somewhat higher than expected Zs at the socket, and giving the screws a quick tighten (when Zs drops) usually gives the game away.

I’ve honestly never seen this done?

 

[johnduffell]

Over 50mm2 to be classed as exposed conductive part

Maybe truet, but in terms of IP ratings that's not good. To be effectively class II it would need double or reinforced insulation, neither of which a socket has.
Wagobox or even surface mount back box yes, but metal socket box even though it's not an appliance i would still treat the back box as the enclosure as the wall is not part of the electrical installation.