Discussion - Earthing a ring main 13A socket metal back-box

[Spike]

It seems that the accepted method of earthing the metal back-box when wiring a ring main socket is to terminate the two T&E cable earth cores (suitably sleeved Gn/Y) into the 13A socket earth terminal, and then to run a separate earth wire from the 13A socket earth terminal back to the earth terminal on the metal back-box.

I don't know if it is mandatory to specifically wire the earths in this way, or not. I have been unable to find anything conclusive on the Internet, or in the IEE 17th Edition Wiring Regulations.

In the past I have used this alternative procedure:

Sleeve the first section of the two T&E cable earth cores (Gn/Y) and take them directly to the earth terminal on the metal back-box. Do NOT cut the earth cores - instead fold them back on themselves, so they enter the back-box earth terminal doubled-over. Now sleeve the trailing ends of the two T&E cable earth cores (Gn/Y) and connect them into the 13A socket earth terminal. The T&E cable earth cores remain continuous and uncut from where they exit the stripped T&E cable, all the way to the 13A socket earth terminal, but they pick up the back-box earth terminal on the way.

My thoughts on this method are:

1. The metal back-box would always be earthed, even if someone disconnected and removed the 13A socket.
2. If the earth wires became loose and fell out of the 13A socket earth terminal, the continuity of the earth would remain whilst the 13A socket was screwed into place on the metal back-box.
3. If the earth wires became loose and fell out of the metal back-box earth terminal, the13A socket would still be earthed, as the earth cores are continuous (uncut).
4. This wiring method does make the earth core wiring tidy and more compact. It also avoids the problem of terminating three earth cores into one earth terminal on the 13A socket (although I accept that many 13A sockets now have two earth terminals). IMO, three cores into one terminal is a recipe for a loose core.

The only downside I can see would be if the earth terminal on the metal back-box was a bit crude and behaved more like a guillotine, cutting through the earth cores.

Is there any reason not to wire the earth in this manner?

Any thoughts, comments, criticisms anyone?

[ARWadsworth]

Hi

The earthing of back boxes is not mandatory if there is at least one fixed lug.

However, if you are going to do it, then either of your methods are fine. When I have to to earth the back box I use method 1 as it is a) faster and b) easier to split the earth if required for testing/ fault finding etc.

I have also frequently encountered a 3rd method. Which is to use just one of the cpcs folded over in the manner you suggested.

Cheers

Adam

[dom]

I also use method 1, simply because the backbox terminal is rather crude.

[Spike]

The back-boxes I usually buy have a nice, brass tunnel terminal.

So there's no code of practice that specifies exactly how the back-box earth should be connected?

[BruceB]

Correct, because as was stated in the first reply there is normally no requirement to earth the back box.

Regards Bruce

[The Natural Philosopher]

which is how you get a nice shock off the screws used to connect the front plate to the back box.

Are you serious?

[Tim Watts]

The Natural Philosopher wibbled on Thursday 15 July 2010 18:18

With the caveat of there being at least one fixed lug, he's right. Accessories usually have earthed metal rings in the screwholes.

Personally I do run a fly to the backbox even though it isn't strictly necessary in my case, because it takes about 30 seconds extra effort.

[BruceB]

I would not criticise anyone who does normally run an extra wire to the back box, many were trained that way. I normally do not run one because I take the view an extra wire is just one more thing to go wrong, come loose and cause trouble. Regards Bruce

[ARWadsworth]

the best link I can find.

Cheers

Adam

[dom]

messagenews:i1nftj$h12$ [e-mail]...

Thanks, very useful.

[Spike]

Terminologically incorrect!

Back-boxes (flush or surface) are classified as 'exposed conductive parts' and are therefore required to be earthed. IMO, earthing of the back-box is mandatory.

You (and Adam in the first reply) obviously meant that there is often no requirement to fit an earthing tail to achieve the earthing of the back-box, not that the back-box itself does not need earthing. Where the cpc is a cable or core connected into the socket outlet earth terminal, you can (allegedly) 'rely' on the back-box fixed lug and the socket outlet earthing strap and eyelet.

Personally, I would always fit an earthing tail, rather than rely on a dubious combination of fixed lugs and eyelets.

[Spike]

Interesting document, thanks Adam - but all this convolution as to whether the back-box has no fixed lugs, or one fixed lug, or two fixed lugs; and whether the socket outlet has no earthing strap, or an earthing strap and one eyelet, or an earthing strap and two eyelets; and whether the back-box has been mounted the right way up, so that the fixed lug corresponds to the eyelet on a single-eyelet socket outlet, or whether the customer will accept their 13A socket mounted upside-down so that it does. Good grief, what sort of committee wrote this ridiculous, labyrinthine precept?

What if someone (e.g. the householder) replaces a double-eyelet socket outlet at some future time with a single eyelet socket outlet and ends up with the eyelet screwed to the adjustable lug? What if the socket outlet mounting screw(s) become loose?

As back-boxes (flush or surface) are classified as 'exposed conductive parts' and are therefore required to be earthed, surely the short answer is to ALWAYS install an earthing tail between the socket earth terminal and the back-box?

I think I shall continue to pick-up the back-box earth terminal with the (uncut) earth core in the T&E ring main cable, as I described in my op.

[ARWadsworth]

Not sure I have seen any sockets with just one eyelet so that should not be a problem.

What if the socket outlet

Then they should be tightened up. It is easy enough to notice and rectify. Also I have never known of anyone getting shocks from these screws with or without a flylead.

Not according to the NICEIC.

And there is nothing wrong with you doing that. I gave my reasons why I prefered the method of going from the socket to the back box in preference to your method.

Your method is safe and that is all that matters.

Cheers

Adam

[BruceB]

Yes OK I was incorrect in the pedantic way you describe. That is the risk of opting for brevity in a reply!

Introducing an earth tail when not required does introduce additional failure modes.

On another pedantic point, note the pdf link in another message dates from the 16th edition so the reg references are incorrect

Regards Bruce

[Spike]

Well, the NICEIC doesn't preclude always fitting an earth tail - they just seem to have seriously overcomplicated an otherwise simple earthing procedure with too many options. Give people a choice, and some will get it wrong!

Always fit an earth tail, and the back-box/socket outlet earthing requirements will always be met. Why make it any more complicated than that?

Inspection Council, existence, justify?

[Spike]

Such as?

By always fitting an earth tail, I think the benefits to the integrity of the earthing scheme will outweigh any envisaged adverse failure modes.

At worst, fitting an earth tail between the back-box and the socket outlet earth terminal will provide a parallel earth path in some instances. The converse could result in no earth path.

Making the earth tail mandatory would certainly remove any uncertainty about how to interpret the socket outlet/back-box earthing requirements.

[ARWadsworth]

Of course they will. And one of the reasons people post here is to make sure that they get it right by getting views from people who have been there before or are qualified in such things.

Because it is not needed. Two adustable lugs on back boxes have long gone and so have single eyelet sockets. The rules cover your arse in older installations.

It means that the person doing work at your house should be competent. It could be any one of the electrical domestic installer schemes, GasSafe, Fensa etc depending upon what work you have done at home. Not everyone here DIYs and not everyone DIYs everything.

Notice the words "should be competent":-) There are often posts about bad workmanship done by people who are qualified. Some posts are justified and some are not.

Cheers

Adam

[js.b1]

There is an issue about any earth terminal must be used.

This created hilarious situations with Masterseal plastic boxes having a brass terminal which were intended for loop-in earth use, rather than someone adding an earth to earth the terminal.

Some councils still insist on it, can not recall the reg in 17th, but it prompted MK to specifically state they need not be connected and later on to remove them for a time. Someone nit-picking. Another nit- picking is the various IPx4 fan heaters for kitchens/bathrooms/toilets

- they are actually German to IPx3 but get re-written to IPx4. Dimplex deceitly boast about how their new fan heater is IPx4 when the old one was labelled IPx4 with an open fan cover on the lid (the new one is somewhat shielded).

[BruceB]

Such as one end coming out, touching a neutral or phase terminal and tripping the rcd or mcb.

[js.b1]

You can find them with MK Grid frame on a conventional backbox. A grid frame has an adjustable lug which depending on orientation may combine with the conventional box adjustable lug to result in two adjustable lugs. The grid frame has its own earth terminal. MK grid backboxes (flush n surface) have two fixed lugs since the grid frame provides an adjustable one.