Earthing Steel Conduit/Trunking??

[thankyousam]

I'm replacing an old section of cable to a light switch. The position of the existing cable does not comply with the 150mm vertical channel above the light switch, as stated in my BS7671 on-site guide. It's not an option to move the cable because of some period architectual features, so I've been forced to buy a length of stainless-steel trunking to protect the cable.

The trunking will eventually be buried in the plaster, but I'm not sure how to earth this section. Can I just take a earth feed from the light switch and bolt it to the stainless-steel? Or do I need to take a separate line from the earth on the CU, in which case does this work become notifiable?

Thanks in advance :)

[Lobster]

What do you mean by this 150mm vertical channel?

David

[thankyousam]

The regulations state that, from my on-site guide:

"Where a cable is concealed in a wall or partition ata depth of less than 50mm from any surface it must be enclosed in earthed metal conduit (trunking or ducting) or installed either horizontally within 100mm of the top of the wall or partition or vertically within 150mm of the angle formed by two walls, or run horizontally or vertically to an accessory or consumer unit".

There's a PDF Pocket Guide at the NICEIC site.

[Lobster]

confused me!

I'm interested in the answer to your query too actually. If I may tag on a supplementary query: in the above PDF file, the diagram shows a pale green zone where it's OK to bury the cable for (say) a lightswitch. But what is supposed to happen at the top of the run, where the cable passes up into the ceiling space? In the diagram, it is shown as having to pass through a pale blue 'no-go' zone at the top of the wall. How should that be dealt with according to the regs?

David

[ARWadsworth]

The pale blue part is not a no go zone, it is an allowed zone. You could run a cable vertically from the light switch upto the blue zone and then all the way around the room in the blue zone before entering the ceiling.

As for metal capping etc I would consider it an exposed conductive part so a

4mm earth that runs from the capping to the fitting or switch earth should be fine.

Adam

[chris French]

In message , thankyousam writes

vertical channel above the light switch' is confusing. Because the 150mm bit has got nothing to do with being above the light swicth

[thankyousam]

I think it's just the way I have tried to convey it - in hindsight I used the wrong phrase :).

I understand the restrictions on the cable placement, it was only because I have been forced to run cable outside the 150mm wide area that the metal protection is necessary.

[jim_in_sussex]

SFAIUI you can earth of the trunking by connecting it to the metal backing box of your fitting/switch/socket: A 'proper' conduit would be fastened with a nut into one of the knockout holes in the backing box. Thereby being earthed via the connection from box to the earthing conductor in the cable. But a sheathed yellow/green 4mm^2 link should be OK.

An alternative method of solving your problem is to run your cable hozizontally then verticaly & somewhere in the vertical line place a backing box with a plain cover.

BTW you are talking proper conduit or trunking (ie the cable is completely surrounded with metal? Capping - sometimes c alled channelling - is different [and is NOT conduit or trunking] and requires no earthing SFAIUI, it is simply additional protection for the cable. Running capping on a diagonal is not a satifactory alternative to conduit or trunking. Conduit is a closed tube: trunking is usually rectangular cross section and has an access lid running its length.

[Andy Wade]

Are you sure about that?

Unless the switch is within a 150 mm wide "corner zone" an unprotected cable should go straight up vertically (or across horizontally). There's no definition provided for the permitted deviation from "straight" in this case, but I think the usual interpretation would be that the cable must not stray outside a zone equal to the width (or height)of the back-box. So, assuming a single-gang box, the "zone" is only 75 mm wide in this case.

As to earthing the conduit/trunking, a connection to the CPC (i.e. the circuit's own earth wire) is fine but remember that the connection of a wire to conduit must be accessible unless soldered, brazed, welded or crimped [526-04-01]. The simplest way, as has been said, is to use 20 mm round conduit, thread the end and mount it directly to a knock-out in the switch box in the usual way (using a female coupler and male bush). Alternatively, if the top of the conduit is accessible, take the switch cable through a junction box and break out a 1 or 1.5 mm^2 earth wire and connect that to the conduit with an ordinary earth clamp.

FAOD, thin metal capping is not regarded as giving mechanical protection for the purposes of Reg. 522-06-06. Its only purposes, really, are to hold cables in place until the plasterer arrives and to protect them during plastering.

[Lobster]

So is there actually any application where metal capping should be used in preference to the plastic stuff? Are the two types in effect interchangeable?

David

[Andy Wade]

IME the metal type is easier to use as it tends to stay put better and requires fewer fixings. It also (probably) gives better protection prior to plastering on a busy building site where all manner of big heavy things are getting bashed around.