Wiring not in safe zones - need mechanical protection or no?

[adamr7747]

Hi All,

I'm in the process of re-wiring a room. Most of the cable runs will be in safe zones, but in a couple of places this won't be practical. Everything is protected by a 30mA RCD at the CU.

I've been tearing my hair out trying to establish whether or not I need additional protection for the non-safe-zone runs. Is RCD protection sufficient? I seem to find a different answer every time I try and research it

If I do need earthed conduit, how does the conduit fit to the back box & how is the earth connection made? Similarly, do the connections between the conduit and conduit accessories (e.g. bends) make robust earth connections?

Thanks a lot,
Adam

 

[PrenticeBoyofDerry]

Firstly,why can't you route cable safe zones?
For cable outside permitted safe zones, RCD protection is not sufficient against mechanical damage.
You must either use containment that is mechanically protected, use cable that incorporates mechanical protection within it sheathing or have cable buried 50mm from wall surface. often difficult when complying to the building regulations, concern the depths of chases, if brick/block wall.

//www.diynot.com/wiki/electrics:installation_techniques:walls

 

[plugwash]

There are a variety of acceptable methods for running cable outside of safe zones but your main choices are either to use a cable with an acceptable built-in earthed metal layer (e.g. MICC or flexishield) or use steel conduit.

Steel conduit is earthed by the joins in the system. To join it to a backbox the end of the conduit is threaded, a coupler is screwed on and then a bush is screwed through the backbox into the other side of the coupler joining them mechanically and electrically.

 

[bernardgreen]

Similarly, do the connections between the conduit and conduit accessories (e.g. bends) make robust earth connections

Initially at time of installation a robust connection is possible. But over time the robustness can be reduced, corrosion in damp walls being one reason.

So best to not also rely on the conduit for safety earthing of the sockets. I have seen installs where the conduit was the only CPC ( earth connection ) between sockets. Not good.

 

[adamr7747]

Firstly,why can't you route cable safe zones?

In one case because the socket will be hidden inside a built-in cupboard, so no clue from outside that there's a potential cable run. For another because I'd like to run a lighting cable down one side of a wall and then pass it through to the other side where the light fitting will be. And for the other(!) because I want to hide an existing surface cable which currently goes through a doorframe to provide power to a small extension.

Steel conduit is earthed by the joins in the system. To join it to a backbox the end of the conduit is threaded, a coupler is screwed on and then a bush is screwed through the backbox into the other side of the coupler joining them mechanically and electrically.

Thanks, this makes sense, but looking at my options I think I'm stumped with rigid conduit because (even if I can actually find some locally to buy!) I'll have to cut it, then it won't have a thread.

Are the flexible steel conduit options (e.g. flexicon, adaptaflex) suitable as these look easier to work with?

Bernard, yes, I'll be running T&E and earthing the back boxes as normal.

Thanks,
Adam

 

[RF Lighting]

Unfortunately adaptaflex is not permitted for use for this purpose.

It must be rigid steel conduit of one of the other previously mentioned wiring types.

 

[plugwash]

Steel conduit is earthed by the joins in the system. To join it to a backbox the end of the conduit is threaded, a coupler is screwed on and then a bush is screwed through the backbox into the other side of the coupler joining them mechanically and electrically.

Thanks, this makes sense, but looking at my options I think I'm stumped with rigid conduit because (even if I can actually find some locally to buy!) I'll have to cut it, then it won't have a thread.

Indeed, you have to cut the threads after cutting the conduit to length.

http://www.screwfix.com/p/conduit-die-set/68284

 

[Adam_151]

So best to not also rely on the conduit for safety earthing of the sockets. I have seen installs where the conduit was the only CPC ( earth connection ) between sockets. Not good.

Its quite common on installations older than about 30 years or so, and is usually fine providing its been properly installed. I did recently come across some surface metal clad sockets with no earth continuity... it transpired that although being correctly specified with flanged coupler and lead washer, that no one had actually bothered to remove any paint from the metal clad enclosure... which is more or less why its fallen out of favor, no one specifies it anymore because they can't be sure it'll be installed properly... safer to insist on a copper CPC being pulled in

 

[ban-all-sheds]

I seem to find a different answer every time I try and research it

I find that hard to believe, because all you need to do is to look at the appropriate regulation in BS 7671, which is perfectly clear.

If your reply is "I don't have a copy of that", can you predict what mine will be?

 

[adamr7747]

Steel conduit is earthed by the joins in the system. To join it to a backbox the end of the conduit is threaded, a coupler is screwed on and then a bush is screwed through the backbox into the other side of the coupler joining them mechanically and electrically.

Thanks, this makes sense, but looking at my options I think I'm stumped with rigid conduit because (even if I can actually find some locally to buy!) I'll have to cut it, then it won't have a thread.

Indeed, you have to cut the threads after cutting the conduit to length.

http://www.screwfix.com/p/conduit-die-set/68284[/QUOTE]Thanks for the constructive comments. In the end I found a helpful seller on eBay who cut & threaded the conduit for me

My last problem is how to protect the cable from the sharp-ish ends of the conduit. Is there a standard solution for this? What do other people do?

Adam

 

[Adam_151]

Burs should be removed after cutting the conduit and once it has been made off into a box there won't be anywhere for the cable to catch. If you mean the 'top' end in the roof void, then the normal way is a female bush after putting just a few threads on that end