On Feb 16, 12:45*pm, John Rumm wrote:
On 16/02/2013 09:21, mike wrote:

If everything is reliably earthed via the socket screw in the
situation you describe, how would it be any less reliable if the earth
were connected to the back box rather than the face plate?

Because in reality its a less reliable connection...

While not desirable, an unearthed backbox is unlikely to kill you, but
an unearthed socket might.

Imagine a situation where some one is decorating and has release socket
screws to get paper etc behind the faceplate. Anything plugged into that
socket *must* still be earthed, but whether the box still is in those
circumstances is less important.

I had thought of that situation and I take your point but, in
fairness, you have introduced three further variables into the
situation. Firstly, someone takes the socket off the wall; secondly,
they plug something into it; thirdly, whatever is plugged in has a
metal earth prong and needs an earth connection.

In the case of a socket screwed to the wall properly with the MK style
earth connection to the mounting screws, if it provides a "reliable"
earth to the mounting box it would necessarily have to work equally
reliably to the earth terminal. I'm surprised the MK style earth
connection isn't mandated but, as Adam has pointed out in another
thread:

'From BS 1363-2 1995 (Specification of 13A switched and unswitched
socket outlets)
10.3 If means are provided for electrically bonding the mounting box
to the earthing circuit of the socket-outlet
by means of the fixing screws the connection between the screw and the
earthing terminal shall be of low resistance.
Note the use of the word "if".'

.... so the Toolstation sockets are just built-to-save-a-penny crap
that sneak in under the wire.

As for saying that, in practice, no-one would use the mounting box
earth, you've surely seen work infinitely worse than that.

I've just replaced a lighting circuit where a "professional"
electrician had managed to cram sixteen cables and three connector
blocks into one 30-amp junction box and, when he ran out of room for
screwed connections, twisted wires together and wrapped them in
insulation tape. It was like a rat's nest up a fish's arsehole. The
earth wires that hadn't simply been cut off were twisted together
unsheathed outside the junction box, the lid of which had been refixed
at a jaunty angle with a woodscrew.

Compared to that kind of workmanship, the mounting box earth seems
like the height of British standards.