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I'm fitting a replacement electrical socket. (UK 3 pin double socket with a USB port). Just the front plate, the mounting box that is already there is staying.

I have the wiring diagram below. In the diagram it shows the two earth cables from the ring lines both running to the socket, and then a separate wire running to the mounting box's earth terminal.

Wiring diagram

My question is, as both of these cables are earth, can I just use one for the socket and one for the mounting box? Or is it important that both run to the socket first?

In picture form, can I do this:

Wiring diagram
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I can't figure out what the wire labelled "functional earth" is for.
All earths should be 'functional' & all should be bonded together.

TL:DR - No, don't do it like that.
It is not sufficient to use the patress screws as your bond [which is what you appear to be doing in your photograph]. The ring should be physically connected to earth at both ends, but this is potentially removing the continuity, which is unsafe. A ring has redundancy built-in, so don't intentionally remove that redundancy.

Your ring main earths should both go into the Earth terminal[s] on the socket. The diagram shows two earth terminals, bonded to each other & also to the patress screw-holes. This is good. In practise you can use one or both, but don't crowd one just for the sake of it. From either of those terminals, then run a separate 6" of green/yellow sheathed wire to the Earth tab on the back box.

Earth bonding in the socket…

enter image description here

All earths need to connect to this - mains in, mains out, back box [& whatever is considered a "functional earth"] This means the patress screws & back box can never accidentally go live, even with the socket removed from the back box.

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    Thank you very much. Of course, it makes total sense now, the two earth wires coming into the box form the ring, so if I don't connect them together the ring has a break in it, which could impact other sockets on the ring. And relying on the mounting screws to form part of that ring is not as reliable a connection.
    – Simon P Stevens
    Commented Feb 3, 2022 at 12:01
  • Got it in one! :)) British mains is very over-engineered… but it's safe.
    – Tetsujin
    Commented Feb 3, 2022 at 12:02
  • I've added a separate question about the functional earth label, over here: diy.stackexchange.com/questions/244140/… I think I've found the answer on other sites, but it might be nice if someone with better knowledge confirms it.
    – Simon P Stevens
    Commented Feb 3, 2022 at 12:13
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    If you have clues about the "functional earth" you should add them to your other question, either in the question, in a comment, or by answering it yourself if you think you have the answer.
    – jay613
    Commented Feb 3, 2022 at 14:39
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    @Harper-ReinstateMonica - I don't get what you mean. The earths should all be hard-wired to both ends of the ring & also all metal components of box & patress, even if the patress is unscrewed from the box. The whole idea is that nothing is ever left floating. The 'functional' earth seems totally pointless to me, more something that would increase ground-loop potential in any audio device than remove it. The photo appears to show one half of the ring to the socket earth, the other to the back-box earth, which will not do of course.
    – Tetsujin
    Commented Feb 3, 2022 at 18:07

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