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I hope to run a spur to a plastic ,wall mounted, twin switch, single pole socket in my conservatory. The donor socket has two twin and earths in it with the earth connection connected again to a lug in the corner of the metal box. As I have a plastic box for the receiving socket is it ok to connect an earth wire from the earth connection on the socket to a screw drilled through the back of the box into the wall?

The plastic doesnt need earthing, only if the box is metal it needs a earth. Just connect your t&e from your socket to your spur, make sure you connect the earth in the socket connections.

Hi, Excuse the pun but there is no EARTHLY reason to connect an earth wire to ANY back box if wiring is in twin/earth! the Earth wires called CPC's should go direct to the earth terminal on the socket!
The unnecessary and in my view undesirable practice of crowding an unrequired wire in a cramped box is a piece of folklaw perpetrated by numpties at the NICEIC on their members.
This originates from a mis reading of reg.543.2.7 which says :
where using the earthed metal work of a wiring system such as conduit,trunking, steelwire armouring as the CPC then a wire shall be connected to bring the earth FROM the box TO the accessory, not vice-versa.
So in answer to your question connect CPC from donor socket to new sockets earth terminal, you can't earth a plastic box!

regards SPARX

Thank you for your helpful information. I liked the pun!

Davyess

[quote="chriscba"]The plastic doesnt need earthing, only if the box is metal it needs a earth. Just connect your t&e from your socket to your spur, make sure you connect the earth in the socket connections.[/quote]

Thanks for your help it clears up my query.
Davyess