Permitted cable routes / zones BS7671:2018
I’ve recently been asked a question by one of our 18th Edition students and think it would be useful to see it here.
” My inquiry is quite simple really, but I am a little confused as to what is acceptable practice for cables that are installed in walls outside of the permitted zones and especially if that wall is plastered brick….
Any time in the that I have carried out any work for myself or friends/family that involves installing new sockets, switches etc – where the walls are brick, I have chased out the wall to a decent depth, installed a back-box and put a metal 20mm conduit in place – secured by a brass bush and canvil clamps along its length in the brickwork and taking it above the ceiling or below the floor level.
A 50mm depth to me for an unprotected cable in a wall – almost half a brick’s worth – seems to me, to be more than would be required to dig out for a conduit!
What is best practice here? I’ve seen some work where the wall has been chased and capping placed over the cables but it is far less than 50mm in the wall and when I saw it, I thought it was a cowboy install! Maybe I’m being overly picky, but is the method of install with conduit that I have mentioned acceptable practice? Is there a more widely used method that I am not aware of?
With regard to your question it might be better first to define what is a permitted zone for the cables (See 522.6.202 ‘i’ BS7671:2018). Basically where a cable is installed at a depth of less than 50 mm ‘i.e at plaster depth’ from the surface of the wall then the permitted zones / cable routes are 150 mm from the top of the wall and 150 mm at each side of a corner and also horizontally and vertically to any point / accessory mounted on the wall.
So any cable buried in a wall less than 50 mm deep and running vertically up or down or even horizontally to a switch, socket, spur etc is within a permitted zone. Note – additional protection using a 30 mA RCD must be used for any cable installed using this method that is not protected by one of the methods in regulation 522.6.204 which would be the case if capping or clips were used below the plaster.
Only if the cable is not in a permitted zone would it need to comply with one of the methods ‘i to v’ in regulation 522.6.204 which your method of using an earthed steel conduit would comply with (see point ii).
To sum up. Using your method if the cables are in a permitted zone is not necessary and would prove too time consuming and expensive and may even cause damage to the wall if it’s chased too deep into the brickwork (See Approved Document A – Building Regulations – Vertically not deeper than one third of the thickness, horizontally not more than one sixth the thickness of the wall).
The standard method is to use capping with the cables routed within permitted zones and to RCD protect them using a 30 mA RCD.
The thought behind this is that even if someone puts a nail through a cable the RCD should trip before any serious injury can occur ‘i.e less than 40 ms’.
Note that due to various regulations in the 18th Edition all circuits within a domestic installation will have additional 30 mA RCD protection.