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Topic Title: Socket box earthing
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Created On: 26 September 2017 11:12 AM
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 26 September 2017 11:12 AM
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Baldyhugh

Posts: 366
Joined: 01 June 2007

Morning all.

This old chestnut again. Guidance note 1 states that if a flush metal box has at least one fixed lug then a lead from the socket to the back box is not required as it is adequately earthed by the fixing screws. I have gone by this in the past but have decided for my own peace of mind to now fit an earth lead to the back box regardless.
I personally don't like putting more than two cables in a termination as I find that they aren't caught equally by the screw. As most modern socket outlets have two earth terminals, I plan to put the two earths of the ring into one terminal and the earth lead to the back box in the other. I know that this might seem quite an obvious question to some but have seen others where the three earths were in the one terminal. Is there a requirement that they must be in the one terminal or is my plan of the ring earths in one and the fly lead In the other acceptable?

Peter
 26 September 2017 11:44 AM
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KFH

Posts: 567
Joined: 06 November 2010

If the earth terminal can take 3 wires I am happy putting them in one. However I am also happy putting the fly lead into the second earth connector. It could be argued that it is safer to put the ring connections into the separate connectors so that if one comes loose the other should still provide a connection. I have also seen one of the ring cpcs to the backbox with a flyllead to the accessory completing the ring, so only two wires in each connector, I am not so happy about this as the backbox connectors are often, to put it politely, poor and usually difficult enough to get one wire into never mind 2.

Despite GN1 I also prefer to earth/bond the backboxes.
 26 September 2017 12:29 PM
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jonny705

Posts: 157
Joined: 26 September 2015

I know when I did a re-wire the assessment guy said don't bother with the back-box fly lead no more as rcd protected.

I have also seen a lot, the ring cpc terminated into the box, and a single flying lead to the actual fitting, normally when someone has broke the cpc (probably by stripping it) and its now too short , so a way round it i guess.

I had this on a lot of sockets only the other day, so I extended the cpc with through crimps and stuck them in the actual socket.

If the socket has two eath points, the cheaper ones tend not too, I always think its bettter to use indvidual ones as if one breaks ,you at least have the other one on -well thats my theory .
 26 September 2017 03:19 PM
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AJJewsbury

Posts: 16102
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If it's a circuit with high protective conductor currents (i.e. the situation where dual earth terminal accessories were designed for) then you have to have the to ring c.p.c.s in separate terminals - so your back box flylead doubles up with one of them. Probably not a bad approach in general either.

In the distant past I've looped one c.p.c. unbroken into the back box terminal and then that and the other ring c.p.c. into the socket's PE terminal - the idea being earthing remains intact (if via the fixing screws) if one earth terminal screw comes loose (a kind of poor man's high integrity before the advent of dual terminals).

- Andy..
 26 September 2017 05:04 PM
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geoffsd

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Typical guidance - whatever it says,

is the back box an exposed-conductive-part?
 26 September 2017 05:26 PM
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ArduinoXR

Posts: 58
Joined: 16 August 2017

Originally posted by: geoffsd

Typical guidance - whatever it says,



is the back box an exposed-conductive-part?


No, it is not readily accessible.
 26 September 2017 07:18 PM
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AJJewsbury

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is the back box an exposed-conductive-part?

I'm starting to feel very uncomfortable about that definition ... to my mind unless the plasterwork/wallpaper/paint around it reliably provides the equivalent of one of the layers of double insulation, I don't think the need to earth it really goes away.
- Andy.
 26 September 2017 07:32 PM
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KFH

Posts: 567
Joined: 06 November 2010

As an old electrician said to me "if there is no bonding lead and there is a fault that makes the back box live when you pull the accessory off who is going to be the one that finds the fault?"
 26 September 2017 07:40 PM
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dustydazzler

Posts: 1508
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It's metal. It's near a live wire. Earth it.
Many old sparks that I have worked with strip one normal length earth and strip one extra long earth and put a kink in the long one and and tuck the kink into the box lug before placing the cpc s together in the socket as you do
No need for a separate earth tail
 26 September 2017 07:47 PM
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geoffsd

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Originally posted by: KFH

As an old electrician said to me "if there is no bonding lead and there is a fault that makes the back box live when you pull the accessory off who is going to be the one that finds the fault?"

It would be an earth (CPC); not a bond, but -

wouldn't that be working on a live circuit?
Is that the real problem?


IF the box is not an exposed-conductive-part, then why does it need earthing?
 26 September 2017 08:15 PM
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AJJewsbury

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IF the box is not an exposed-conductive-part, then why does it need earthing?

As a thought experiment, let's say a flush box isn't exposed, so therefore not an exposed-conductive-part. But it is screwed (sideways) into a steel C-stud in a partition wall for support. The C-studs themselves aren't in the proximity of live conductors so aren't exposed-conductive-parts, so they don't need earthing either, according to the definitions. Likewise the steel shelf brackets screwed into the same C-stud system further along the wall. A simple single fault within the box would make the whole lot live, with no earthing to initiate ADS.

(If you don't like the C-stud scenario, try foil backed plasterboard, stainless steel splashbacks or those aluminium partition framing systems that seem so popular in offices)

- Andy.
 26 September 2017 08:22 PM
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aligarjon

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Originally posted by: geoffsd

Typical guidance - whatever it says,



is the back box an exposed-conductive-part?


no, but the screw heads are.

Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 26 September 2017 08:24 PM
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geoffsd

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Then they will be earthed by their contact with the socket.
 26 September 2017 10:03 PM
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leckie

Posts: 4439
Joined: 21 November 2008

Blimey, look if you want to attach a fly lead then crack on, but if the box has a fixed lug you do not have to. Do not judge people that comply with the regulations, if you want to add additional back up protection and comply with the the requirements for 40 years + ago that is also fine.

And if you are going to connect to two different earth terminals, put the CPC's into separate terminals, not the same one, this will be in line with the requirements for high protective conductors currents, and that might actually be beneficial.
 26 September 2017 10:09 PM
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ebee

Posts: 6347
Joined: 02 December 2004

Originally posted by: geoffsd
.

Then they will be earthed by their contact with the socket.


Geoff No No No and No.

I would never rely on the socket screws being of sufficient tightness to form an effective electrical connection between accessory and back box.

Decorators etc leaving slack not to mention damp corrosion.

This at least "one fixed lug" concept is baldylollocks.

On a ring one cpc to earth terminal then tother cpc doubled then connected to back box then on to earth terminal whether dual or single earth terminal.

I have (as previously mentioned) seen nylon screws replacing socket screws (mind boggles)

I`m with AJJ and Dusty on this one

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 26 September 2017 10:16 PM
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geoffsd

Posts: 1783
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Originally posted by: ebee

I would never rely on the socket screws being of sufficient tightness to form an effective electrical connection between accessory and back box.

No, not saying that, Aligarjon said the scews were exposed-c-ps - so they will be earthed by contact with the socket (screw holes).

I`m with AJJ and Dusty on this one

That's fair enough - and I do the same.

However, I am trying to determine IF (people think) the back box IS an exposed-c-p.
 26 September 2017 11:04 PM
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ebee

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Joined: 02 December 2004

no it`s not exposed.

but bearing in mind it may well be surrounded by damp plaster then it would be prudent to treat it as such

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 27 September 2017 12:09 AM
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mapj1

Posts: 9692
Joined: 22 July 2004

The real unknown is to ask "is there a credible failure that livens the backbox ? "
I guess it depends who does it, if the T and E comes in through a non grommet hole, already stripped as singles, and the socket is a deep one squashed tight into a box that is too shallow, then yes.
If it comes in via a grommet, and there is plenty of room, and the length that is stripped to singles is kept suitably short, then probably pretty incredible, or at least no more risky than other things we accept, thinking of T and E in unearthed metal capping, or meter tails on TT into a metal CU.
The problem is that the one without the grommet is also probably also going to be the one with the missing link and loose screws as well
To be fair, you do have to pretty much lose the screw completely to get a non-contact with the eyelets in most sockets, a bit loose to stuff the new wall paper behind is not enough to break the connection.
I'm happy without the link wire in most cases.

-------------------------
regards Mike
 27 September 2017 05:04 AM
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leckie

Posts: 4439
Joined: 21 November 2008

Apart from people taking off faceplates while the power is still on, I don't think there is a credible risk of anyone getting a shock if a socket is screwed back onto a fixed lug. I have never heard or experienced that and so I don't bother fixing a fly lead. BS7671 has to my mind agreed with that for many, many years. I'm not going to account for what if scenarios regarding people loosening sockets to wall paper etc., if you start taking account of every possibility you can go on forever. If some think that this does represent a risk and then they should of course fit a fly lead. I suspect we all have habits that we think are an improvement on complying with the minimum standard for our own reasons. I like to install supplementary bonding in bathrooms regardless of RCD protection being installed to all the circuit on the basis of touch voltages being reduced in the event of a fault, but you don't see that being done very often now. I have been clamping meter tails entering consumer unit for at least twenty five years before anyone had ever mentioned a need for it to me and insisted that my employees did the same as well as rechecking the connection of tails and any larger conductors with their torque calibrated screwdriver hand before fitting the lid on. That always was a risk of a loose connection from the moment manufacturers started using cage clamps. I have always glued in grommet strip because of the amount of times I have seen it not fitted correctl, or coming off at the slightest touch. These are all little things that in my opinion make sense based on witnessed evidence and thing that I think reduce the risk to extremely low of a tangible hazard occurring. I am not of the opinion that fitting an earth lead does that. Dog rough Leckie.

Edited: 27 September 2017 at 05:11 AM by leckie
 27 September 2017 05:37 AM
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ebee

Posts: 6347
Joined: 02 December 2004

Leckie.

Ditto Supp bond bathrooms.

Ditto Tails secured.

Map & Leckie,

Reliance on what often becomes a slack connection does not sit well with me. 2 seconds extra work and tuppence extra materials to ensure continuity seems worthwhile

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
IET » Wiring and the regulations » Socket box earthing

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