multiple T&E cables into back box. ?

[Sargan]

I have 2 new ring final circuits; a simple question has arisen ? when you have a number of cables entering the galvanised back box (all boxes sunk into walls).

If you take a spur point, the back box has a cable in from the ring and cable out to the ring, and a third cable off to spur. The question is should I connect all earths to socket front plate, and run an extra separate earth to connector at rear of socket box ? (all suitably sleeved) Or should I make all connections to back of box and run an earth from that to back of socket ?

Or some mix of the above. ?

Noticed that it is quite difficult to get a good connection when there are a number of earth wires going into single terminal, often when you think it's done .. a test pull on each wire will show one of them has not been caught by screw, I gues this is a problem with screw capture rather than terminals that have compression plates.

Sargan.

[Dave Plowman]

There shouldn't be any problem into any normal box or accessory terminal. You're probably not being brutal enough with the screwdriver. ;-)

[Lurch]

On 31 May 2004 15:08:39 -0700, [e-mail] (Sargan) strung together this:

A twisting together of all the earths will sort that out, or double them all over if the terminals big enough.

[BigWallop]

It's always a good idea to take all cabling to the back box terminal and run a fly lead from this to the face plate. This allows you to remove the face and leave all the earthing in place if you are making changes.

With the solid earth wires it's also a good idea to twist all the bare ends together to make them like one solid piece and they will hold better in the box terminal for you.

[Dave Plowman]

Isn't this frowned on under regs?

[Lurch]

On Tue, 01 Jun 2004 09:28:51 +0100, Dave Plowman strung together this:

It is. But sometimes, as the OP has proved, it's better to twist them and have them all stay in than not and have them fall out!

[Dave Liquorice]

I thought the regs now stated that the frount plate fixing screws did not provide a proper earth circuits so the back now had to be bonded to the frontplate with a bit of wire.

Similary poke the straight wires in and tighten the screw, well. But if you are too brutal you'll pinch the wires to much and weaken them, this is more of a problem if they are twisted together IMHO as the wires cut into each other.

Many sockets now have two earth terminals, high resiliance (reliabilty?) earthing. Put two wires into each terminal.

[Rick Hughes]

(and in your other post you wrote 'optional')

Is there a requirement for the 'link' from front plate to terminal in backbox or not ? I have always put one in, is this not a requisite ? Perhaps I am being over zealous.

Rick

[Dave Plowman]

Just re-read this. It's bad practice. The earth is there for safety, and should be run direct to the important part, ie the socket, not for some possible later convenience of changing the socket.

FWIW, it will make little difference to the ease of changing the socket anyway, given that the 'stiff' conductors are the line and neutral.

[Dave Plowman]

Given that the minimum in a socket will be two 'ring' earth conductors plus the one tail to the box earth, I can't see those falling out. And the OP was asking about a spur, so that makes four conductors. If one(or more) of four conductors is loose in a standard 13 amp socket, then they either haven't been inserted far enough or the screw not tightened fully.

There might well be a problem where the tail goes to the box though, depending on the earth terminal design, so doubling it over is usually a good idea here.

[Lurch]

On Tue, 01 Jun 2004 09:59:04 +0100 (BST), "Dave Liquorice" strung together this:

Depends if the day of the week has a W in it.

Problem with that is sometimes the single wire will end up between the screw and the side of the barrel. It's a case of just knowing what is best for the situation.

You could do that. You'd still need to double them over though.

[Dave Plowman]

Without it you're relying on the connections between the screws and box to earth that box if it's metal. And it's conceivable that a damaged line conductor could touch that box and make it live once the screws had corroded slightly. If this happened (I know it's a long shot) I'd prefer the breaker to go instantly than things to fry away unseen.

Of course, some will save time and money by leaving out the tail in the same way as they don't use grommets. But we'll leave that for the 'pros' and do things properly here. ;-)

[Lurch]

On Tue, 01 Jun 2004 10:16:53 +0100, Dave Plowman strung together this:

You obviously misunderstand, it doesn't matter how many earths you've got, if one of them isn't under the screw but by the side of it then it's probably going to fall out, no matter how tight it is because it isn't under it in the first place.

[Rick Hughes]

I have noticed this ... with 1 or 2 conductors they crimp down under screw OK, but when there are a number ... (4 in case above) the screw crmps down on most but usually one of them goes to side of screw .. and unless you carry out a test 'pull' of the wires you could miss the fact it was not fully connected.

Rick

[romic]

One of the problems I have found when using a screwdriver, is that the normal "electrical" screwdriver often doesn't give enough purchase to fully tighten the screws - the longer the blade, the less satisfactory this is. Also the narrower blade that these screwdrivers have tends to mean that the grub screw isn't gripped properly and the slot gets damaged as pressure is put on the screw. My solution to this was to get a wider bladed screwdriver - a "B&Q 5 x 100mm blade Micro finish slotted" screwdriver. This is ideal for the MK sockets as the blade fits in the hole with space to turn and the tip fits snugly along the length of the grub scre. The handle gives a good purchase and allows the screw to be tightened as much as required without damage to the screw or screwdriver.

Roger (my reader sometimes loses mail/newsgroup messages

- if you think you should have had a reply/comment, please e-mail me again. Ta!)

[usenet]

I've found the best size electrical screwdriver is a 4mm one, 5mm doesn't fit everything but 4mm does fit just about all the electrical accessories I've used. As you say 3mm (the standard small electrical screwdriver) is too small.

4mm bladed electrical screwdrivers are not that easy to find, you certainly can get them though and I have two quite nice ones now.

[Dave Plowman]

Absolutely. You want an (IIRC No 6) 5mm wide screwdriver with a decent chunky handle to give enough torque. The shaft should be about 100mm.

FWIW, most of the faults I've sorted on others' installations are due to inadequate tightening of terminals. IMHO, they should be tightened 'till they groan. I've never found a conductor to shear through this.

[Dave Plowman]

Hmm. I've just had a play with an ordinary new 13 amp socket - a BG Accessories which is a clone of the old MK Logic design and as about as cheap as you'll get, and with a single 1mm bare earth wire I've found it

*impossible* to get it to go down the side of the screw - if you think about it, it would be stupid design if this could happen.

IMHO, a loose earth wire out of several means it wasn't inserted fully - easy enough to do with several when the sleeve can slip so easily.

Careful twisting, of course, prevents one wire slipping relative to the bunch, but may just bring about other problems.

[Lurch]

On Tue, 01 Jun 2004 13:53:19 +0100, Dave Plowman strung together this:

I never said it could happen, I said it does. And yes, it is a design fault.

When you've wired up a fair few of every brand of socket there is and seen it happen on numerous occasions then you'll agree with me.

Which is pretty much what I said originally.

[N. Thornton]

Nothing would fry, there would be no electrocution... the backbox is only accessible via the faceplate fixing screws.

The screws normally make a good connection, not that they actually need to. The issue as I understand it is the one per billion possibility of a live to backbox short, and a very low impedance one at that, plus the fixing screws offering poor connection due to being loose, and corroded, AND the fault current being hot enough to cause damage and low enough not to trip the fuse/breaker. 6 criteria to be met in order for things to go pearshaped.

Now you can decide for yourself whether to add those lil tails.

Regards, NT