Correct method of wiring a double socket

[fuzzytrifle]

I want to replace some double sockets but I'm not sure which is the correct wiring method.

I've had three electricians work on our barn conversion and workshops over the past eight years, and each one has wired the ring main sockets differently.

The first one twisted the pairs of wires together - 3 twisted cables for the earth (one connected to the back box)

The second folded all of the cable ends then stacked them - one socket has 3 stacked wires where it supplies a spur, and also has four cables in the earth terminal, but I haven't checked to see whether they are folded too.

The third electrician fitted all of the sockets with the wires going straight in, with three wires going straight in to the earth terminals. For the sockets where there are spurs to fused connection units there are three wires straight in and four straight wires in the earth - the cable used connecting the back boxes is a twisted earth cable whereas the T&E earth is a thin single strand, all three (or four cables where there is a spur) are in a single earth terminal.

Please could someone tell me which of these is correct - or if there's another standard wiring method.

 

[securespark]

Twisting is a big no-no.

Doubling over the ends is fine. Obviously there needs to be enough room in the terminal to do this! Some are not big enough to take 3 x doubled over 2.5 conductors.

You don't have to double over, but where there is just one conductor, it is a good idea to give the terminal screw more surface area to grip onto.

Sounds like the twisted cable you speak of is an off cut of flexible cable.

It's best to use solid strand like the others. Sometimes mismatched conductors get pushed out of the terminal as it is being tightened up, or else the wire ends up not being gripped properly.

There does not need to be a link from the earth terminal to the box if the box has one fixed lug, but it is considered good practice to do this.

 

[JohnD]

the idea is to have enough copper in the tunnel to more or less fill it, and pack against the brass, so that when you tighten the screw it is pushing down on a mass, and the copper core can't slip to the side of the screw.

I was thinking that the G&Y links to the backbox were probably cut from 6mm stranded that the electrician may have had to hand, which is fine. These strands are much thicker than you find in flex.

The twisting method was once common. The people who did it may also have poked all the earth cores from the T&E cables into a single green sheath. It can be a bit of a nuisance untangling these, and they occasionally break.

 

[ericmark]

There are pros and cons with twisting wires. It does mean it is less likely for any wire to have a bad connection but is a real pain when doing inspection and testing because one risks work hardening the cable as it is untwisted to allow testing.

To lightly twist the earth wires so a single sleeve can be used is common but only lightly just enough to keep them together to slide on the earth sleeve.

With stranded flexible cable one would always twist but with just 7 strands then it can be debated and one could not really say any of the methods are wrong or right. It's a personal thing electrician to electrician.

It also depends on the terminal type. Where there is a plate which is tightened onto the wires the screw is unlikely to damage the wires. But where a screw acts directly on the wire it can bight into the wire to such a degree to cause it to be necked off. If not twisted pulling on the wire can confirm it's held solid but when twisted it can seem a wire is being held when in fact it is really loose.

The old screwit connectors clearly twisted wires together when being used and it would be hard to say twisting is wrong even though I now don't twist wires myself.

I think the big change with inspection and testing where today people do inspect and test every 10 years or change of occupant means wires are removed and refitted much more than in days gone by. The slow change from direct screw contact to using a sliding plate between screw and wire means with flex cables there is less chance of a stray wire getting in the thread giving a false indication of tightness.

But with some terminals using wires of different cross sectional area it is possible to grip only one wire if not twisted so to say something like twisting is a no no is clearly wrong as there are times when it makes sense.

Many sockets are designed to take up to 2 x 6 mm² wires and fitting a single 1.5 mm² wire may result in lack of grip. Folding them can help get the extra thickness. But it can also result in one of three wires not having the pressure required. Three round wires will naturally arrange themselves to give grip to all if in a round hole. But this may not be the case with a rectangular hole. So likely the twisting there can be no hard and fast rule.

Near every electrician can tell a story of why one method should be used in preference to another. But with different shapes of terminal and different clamping methods there can't be a hard and fast rule.

I was taught in industry and was told one terminal = one wire if you want two wires then you need two terminals with the exception of when eyes were fitted then as long as the crimped parts did not fowl each other not problem. But with most sockets that would be impossible with domestic. When I first moved to domestic it really went against the grain.

Today with maintenance free terminals domestic is also moving towards one wire into one hole idea. Even then we see arguments some feel the pressure should be removed with a terminal driver before wire is inserted other use the stiffness of the wire to open the terminal. Because relatively new often instructions are included and some makes do recommend using a terminal driver others don't.

But the screw terminal has been going for years so unlikely to find a manufacturers instruction on how to use.

 

[Cabbyfan]

Not sure if this has been covered already but I thought current guidelines were for a ring socket that an earth from each cable went to each of the earth connectors on the socket ?

That's what the diagram shows in the current on site guide anyways.

 

[EFLImpudence]

Not sure if this has been covered already but I thought current guidelines were for a ring socket that an earth from each cable went to each of the earth connectors on the socket ?

That's what the diagram shows in the current on site guide anyways.

That refers to circuits with high CPC currents. 7.5.3

Normally it makes no difference.

 

[RF Lighting]

That's only a requirement for circuits with high integrity earthing.

I keep wires single and straight, unless there's only one conductor then I double the end over.

 

[Cabbyfan]

Not sure if this has been covered already but I thought current guidelines were for a ring socket that an earth from each cable went to each of the earth connectors on the socket ?

That's what the diagram shows in the current on site guide anyways.

That refers to circuits with high CPC currents. 7.5.3

Normally it makes no difference.

Fair point. I still do it as I think it's neater

 

[Jackrae]

There is a potential problem with doubling a wire end. If the wire is fitted into a pocket with a compression pad rather than a bite-down screw and the bent-over wire happens to lie in the pocket in the "up-down" position rather than the "side-by-side" mode and an extra cable is added without a bent-over end then the latter wire may not be gripped correctly. OK, one can argue that a "mix-and match" of ends shouldn't be done, but it can happen and when it does there is a potential problem.

Part of today's termination problems come from the rubbish cable design of solid cores as against when we had decent flexible multi-stranded twin and earth; the day before the bean counters and our continental colleagues decided the new "improved" recipe was better.

 

[JohnW2]

There is a potential problem with doubling a wire end. If the wire is fitted into a pocket with a compression pad rather than a bite-down screw and the bent-over wire happens to lie in the pocket in the "up-down" position rather than the "side-by-side" mode and an extra cable is added without a bent-over end then the latter wire may not be gripped correctly.

True, and the same can happen if conductors (particularly solid ones) of different sizes are used with such a terminal. I don't think it's too bad with just two conductors, since the pads/plates can usually tilt enough to grip both satisfactory - but if there are three or more (of differing sizes) then there is potential for big problems.

Kind Regards, John

 

[fuzzytrifle]

Thank you for all of the replies - I think I've got a pretty good idea of what to do now.

The flexible earth connecting some of the back boxes is identical to the 2.5mm earth that runs in the conduit in the workshop - and was fitted by the same electrician. I'll replace it with single strand.

Do metal back boxes have to be earthed? I'm only asking as the plumber told me that I didn't have to fit earthing strips to taps etc. because everything is RCD protected, and that the 10mm earth connection at the incoming water supply and 6mm wires connecting incoming and outgoing heating pipes is all that is required.

 

[fuzzytrifle]

Not sure if this has been covered already but I thought current guidelines were for a ring socket that an earth from each cable went to each of the earth connectors on the socket ?

That's what the diagram shows in the current on site guide anyways.

The replacement sockets are MK DP switched double sockets, I was a bit surprised to find that they only have one earth terminal.

 

[JohnW2]

Do metal back boxes have to be earthed?

Yes, definitely.

I'm only asking as the plumber told me that I didn't have to fit earthing strips to taps etc. because everything is RCD protected, and that the 10mm earth connection at the incoming water supply and 6mm wires connecting incoming and outgoing heating pipes is all that is required.

That's different - he's presumably talking about relaxations of bonding requirements in bathrooms when you have RCD protection. In any event, unlike back boxes, taps are not enclosing live electrical conductors (which, in the case of a box, could touch the box if they fell out) - and, if you have metal pipework, taps will be earthed, anyway.

Kind Regards, John

 

[DaftPunk]

A great example of how a little knowledge is dangerous.

 

[fuzzytrifle]

Do metal back boxes have to be earthed?

Yes, definitely.

I thought it would be unsafe not to, there was a previous comment in the thread:

'There does not need to be a link from the earth terminal to the box if the box has one fixed lug, but it is considered good practice to do this.'

All of the back boxes in my LAN, TV/audio and telephone sockets are earthed too, is that standard practice?