Cable protection

[Graham Jones]

Hello,

In what circumstances does electrical cable need to be protected when buried in plaster? I am guessing that wires leading up from under the floor directly to the socket do not need any kind of protection (steel conduit for example). Is that right? I mean it would take a complete idiot to start drilling under a socket! Or maybe we have gone so far down the health and safety route that all buired cable needs protectetion?

Anyone help?

Thanks,

Graham

[Owain]

All buried cable does need protection now, unless it's RCD'd

Or to put it the other way, RCD protection is now required for all buried cables unless they are in conduit or have an continuous earthed 'screen'

This may change with an amendment to the Regs, especially for minor alterations to existing installations.

Owain

[ARWadsworth]

That would have to be earthed steel conduit.

Or be at least 50mm below the surface.

Of course plenty of idiots still drill holes above and below sockets and switches:-)

[Roger Mills]

Even if it doesn't *need* it for protection - and that's moot - it's much easier to plaster over galvanised top-hat section that it is over un-covered cables.

[Graham Jones]

Thanks for the replies, what about a stud wall, how should cable be protected in that?

Graham

[John Rumm]

Same answer - Bury deeper than 50mm (from both sides!), RCD, or earthed metallic enclosure (i.e. conduit, earthshield (or other similar foil screen cable), SWA or MICC)

[Andy Wade]

There's nothing particularly moot about this; the regs are quite clear.

No mechanical protection [1] is needed for cables that are in the 'safe zones' [2] AND are 30 mA RCD protected.

If you want to stray outside the safe zones you need mechanical protection, or one of the prescribed protected types of cable. The RCD is than optional from the point of view of cable protection, but may be required for other reasons - circuits feeding general-purpose socket-outlets, high earth fault loop impedance, etc.

The use of top-hat capping (metal or plastic) is good practice and protects the cables during construction and plastering, but it does not constitute mechanical protection in the sense of this thread.

[1] Prescribed 'self-protected' cable types, or earthed conduit or trunking, or measures to prevent penetration by nails or screws (generally interpreted as min. 3 mm thick mild steel). [2] Horizontally or vertically to a visible electrical fitting or wiring accessory (which can be on the 'other' side of the wall if this is accessible), or within 150 mm of a corner, or within 150 mm of a ceiling.

[ARWadsworth]

Have a read of

[Graham Jones]

OK, it is all making more sense now, but there is one thing I just don't get.

My situation is that I am planning to add a socket in a stud wall, my idea is to add an extra noggin with a cut out in the noggin to fit a metal back box. The cut out will be exactly the right depth so the mounting box is flush with the wall surface.

As the cable needs to be in an earthed metal conduit, I could use something like:

how to "connect" this to the mounting box and satisfy the earthing requirement?

Although my main problem is that with this conduit in place there is no longer any room for the plasterboard! The conduit needs to be at least 12.5 mm from the front of the mounting box, and obviously this isn't so. So how to do it?

Incidentally are there any guidelines about how to prevent someone drilling from the other side of the stud wall straight through the mounting box and into the socket itself. If a plastic drywall box had been used then this is easily do-able with a SDS drill. It seems daft to go to all this trouble to protect the cable when the actual fitting is quite defenceless. This is why I have chosen a metal mounting box.

Thanks,

Graham

[ARWadsworth]

Are your sockets RCD protected?

[Graham Jones]

No.

[ARWadsworth]

Okay. Have you considered just running the cable in a "safe zone" without any additional protection? It sound as though your installation does not meet the 17th edition so one extra socket should not make any difference.

[Graham Jones]

Thanks Adam, this is a relief. The house was built in the 1960's and when we moved in 11 years ago we had the electrics upgraded by an electrician. He replaced the fuse box with a comsumer unit containing

7 MCBs (no RCDs), replaced the lighting circuit and added a new ring circuit for some additional sockets. Since then two more rings have been added, one for the kitchen and one for garage and outside lighting. The only original cabling left is for the original sockets in the house and this is T&E but of the stranded core type. The new socket I require will be added to the new sockets ring circuit, so solid T&E.

So can I add the socket with no additional protection then?

Thanks,

Graham

[Graham Jones]

Thanks Adam, this is a relief. The house was built in the 1960's and when we moved in 11 years ago we had the electrics upgraded by an electrician. He replaced the fuse box with a comsumer unit containing

7 MCBs (no RCDs), replaced the lighting circuit and added a new ring circuit for some additional sockets. Since then two more rings have been added, one for the kitchen and one for garage and outside lighting. The only original cabling left is for the original sockets in the house and this is T&E but of the stranded core type. The new socket I require will be added to the new sockets ring circuit, so solid T&E.

So can I add the socket with no additional protection then?

Thanks,

Graham

[ARWadsworth]

I am a little surprised that no-one installed an RCD at some point during the works. Split load CUs were a standard installation 11 years ago.

Technically you would be breaking the 17th edition rules by adding this extra socket without additional protection. Had you done this work before

1st July 2008 then it would have been legal. But don't worry, the socket police will not come and arrest you. It is also worth noting that a future ammendment to the 17th edition may make what you are proposing to do legal again!

I am suggesting (and I am sure others will agree) that your best course of action is to just install the socket without additional protection (it will be as safe as all your other sockets)and at some time in the future look towards adding some RCD protection (especially for your sockets that power any outside appliances).

IMHO the money saved on not buying a length of steel conduit would be better of spent on RCDing some of your installation.

[Graham Jones]

Thanks for all your help Adam, I will do as you suggest. The wife is on about building an extension, so I reckon we will probably get a more modern CU then.

[John Rumm]

You don't, basically!

If you were going to do it "by the book", then this is the case where one of the foil protected cables would get you out of the predicament you find yourself in.

However, swapping the circuit MCB for a RCBO if possible would solve the problem anyway and enhance the safety of the whole circuit.

As Adam said, even if you just extend it as it currently is, it will be no less safe than currently - even if the current setup is not ideal.

If you were using metal conduit, then you would also need to use a metal clad mounting box to terminate it to as well - problem solved ;-)

(chances are a future amendment to the regs will allow extensions to non RCD protected circuits without this problem where it can be argued that the extension makes it no worse than current).

[John Rumm]

Depending on the make and model of CU you currently have, you may be able to retrofit a RCBO (i.e. combined MCB and RCD protecting a single circuit) on those circuits that would benefit. This would particularly apply to circuits that may be used to feed appliances used outside for example.

[John Rumm]

Had to tease a mate of mine once for nailing a picture rail back on a wall with masonry nails - right into the new plaster that was filling a cable chase drop to a light switch!

(by fluke it seemed he missed the cable!)

[ARWadsworth]

A Fluke 2042 would help him miss the cable:-)