"Lobster" wrote in message
...
On 15/02/2013 22:29, John Rumm wrote:
On 15/02/2013 20:15, mike wrote:
Just came to fit some socket outlets from Toolstation and realised the
faceplate screws aren't connected to earth.

They're the top ones in this pic:

http://i46.tinypic.com/9glo1z.jpg

Older MK version with earth connection is below.


I thought all sockets followed the MK pattern with the result that
using either the earth connection on the faceplate or the earth
connection in the back box would result in everything being connected
to earth.

As you have seen, not always...

Don't think I've ever seen them before - I always assumed the earthed
screw holes were mandatory

But if someone made the earth connection to the back box on
the Toolstation version, any connected appliance (double insulation
and plastic earth prongs aside) wouldn't be earthed. Is that right?

No, because the circuit earth wires will always be connected to the
earth terminal on the socket face plate. You would not simply terminate
the circuit earth wires on the backbox and rely on that to make an earth
connection to the backbox.

With a "normal" socket that has earthed screw holes the reverse
situation is allowable - i.e. you can re;y on them to earth the backbox
(assuming it has at least one fixed tag), however good practice would
suggest adding a separate fly lead between circuit terminal and the
earth terminal on the back box.

So, if you have sockets with earthed screw holes does that mean you
actually don't need the fly lead twixt socket and backbox? I thought it
must be a belt-and-braces thing or something, but didn't think you could
just do away with the flylead?

--
David


With the 'main' earth connection in the circuit being the earth pin on the
socket face, I can't imagine that anyone in their right mind would not make
this the primary connection point for the safety earth wire(s). It would be
madness to try to use the (sometimes present) faceplate fixing screw earth
straps 'backwards' - that is to try to make the socket's primary earth
*from* the backbox via the screws. If those screws were the slightest bit
loose, or corroded, the earth could be almost totally ineffective.

In a correct installation, the possibility of the faceplate screws becoming
live is pretty damn small anyway, and almost non-existent if the backbox is
plastic. Never-the-less, if the backbox is metal, I would always run a
secondary flying earth wire to the earth terminal that is always present in
such boxes, and then the faceplate screws would be earthed anyway, courtesy
of the fact that they are screwed into the earthed backbox. In my opinion,
within practical constraints, 'belt and braces' is good when it comes to
electrics ...

Arfa