I have this wiring diagram included with a UK 3 pin double socket with a USB port.

What does "functional earth" mean? On the physical socket it appears to be bonded directly to the regular earth terminal at the top.

This is the specific product I'm fitting, if it makes any difference: GoodHome Brushed Steel Double 13A Switched Socket with USB x2 & White inserts

What (if anything) should I do with it when fitting a replacement socket?

Wiring diagram

Photo of the back of the socket

I've found some descriptions online, such as this one on av-info.eu, but I'm not sure I really understand it in the context of this socket, so I'm not sure if that's the same thing. An answer from someone with a bit more knowledge in this area would be helpful.

Update: I found this little snippet buried further in the instructions:


This question was brought up by the answer to Is it OK to use the two separate earth cables in a mains socket, one for the socket and one for the mounting box?

  • I've found some descriptions online, such as here: av-info.eu/index.html?https&&&av-info.eu/power/… but I'm not sure I really understand it in the context of this socket, so I'm not sure if that's the same thing. An answer from someone with a bit more knowledge in this area would be helpful. Commented Feb 3, 2022 at 12:15
  • 2
    Comments disappear. Put that link & text into the question itself to make sure people see it.
    – FreeMan
    Commented Feb 3, 2022 at 13:12
  • 2
    Can you link (or at least provide the make and model) the device you're trying to install?
    – Machavity
    Commented Feb 3, 2022 at 14:28
  • 1
    @jay613 I found some more details in the instructions. See my update. Commented Feb 4, 2022 at 9:33
  • 1
    Simon - that new bit of the instructions makes it much clearer… in other words, for a domestic install with a known-good earth already on the ring… don't bother ;))
    – Tetsujin
    Commented Feb 6, 2022 at 17:28

2 Answers 2


Functional earth refers to earthing for a particular function, not for safety. The function is usually noise reduction. I would have thought the purpose would be to decouple the DC-side shielding from protective earth as part of a systemic design to eliminate ground loops in the shielding of all signal wiring.


  1. This device (now that we have the model and link) is not built with that level of thought, engineering, or documentation. It appears to be a cheap no-name brand from a hardware superstore.
  2. The functional earth is permanently bonded to the protective earth. It's basically another screw terminal. The documentation already encourages up to three wires in the main terminal (two for the ring, one for the box). I cannot think of why you'd want to connect another wire to a separate terminal.

Functional ground usually means you want to connect directly to the wire leading to a grounding rod. It's an odd requirement that you have both here, because your neutral is typically bonded to the ground at your service entry, and then probably tied to a grounding rod on top of that (depending how old your service connection is).

It could be they are to ensure that a ground fault can occur. UK codes require a Residual Current Device breaker (we call them Ground Fault Circuit Interruptors, or GFCI in the US). They detect an imbalance in the hot and neutral and cut the power before any lethal shocks can occur. As mentioned here

A new, revised version of Regulation 411.3.3, relating to RCD protection of socket-outlets, forms part of BS 7671:2008+A3:2015 (IET Wiring Regulations Seventeenth Edition), which was published in January 2015 and comes into effect on 1 July. The current and new versions of the regulation are shown side by side below. By comparing the two versions, it can be seen that the new version

  • requires RCD protection to be provided for all socket-outlets rated at not more than 20 A, not just those that are for general use by ordinary persons

BS 1363 (the UK standard for wall sockets) is a 13A outlet, so this applies. The other moving part here we need to be concerned with is the step-down transformer that's dropping the voltage from 230V to 5V for the USB portion. Apparently BS 1363 changed in 2016 to include in-plug chargers like this, but within certain size restrictions. This teardown of a UK USB socket notes

The underside of the circuits show a heavy reliance on surface mount components. This is most likely due to the strict standards of the British Standards Institute which define the size and shape of socket outlets (in other words, not much room to play with!). Any device that does not meet the BSI standards is considered a deviation and therefore must be checked by a competent person (which individuals may not want to do).

The circuitry to step the voltage down has to be small to fit the mandated form factor. It could mean this device uses cheaper parts to fit in, which . I should note that it seems that BS 1363-2:2016 also precludes relying on the RCD to prevent shorts, but I could easily see cheaper devices not paying attention to that and doing that anyways. It's also possible they're hedging their bets because a great many parts of the world use 230V standards set by the UK

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.