Electrical Wiring Regs

[dave]

I wish to take a 240v cable from an existing lightswitch through a downstairs internal wall to the other side, chase it in into the wall and feed it through the wall again at ceiling height, to the light fitting in the room where the switch is situated. The cable will run approximately 30 to 40 cm away from a corner and vertically upwards. As an aside I want to do this so I can replace the present 1920s surface mount lightswitch with something modern and get rid of old rubber cable without spoiling the decoration and moldings in the room.

I was under the impression that the wiring regs stipulate an earthed metal sheath or protection should be over cables buried in walls that are not directly above or below a visible fitting and that are some distance from room corners. If this is the case, what do the proffessionals use for the job and how and by what is the metal protection earthed? I asked these questions at my local electrical suppliers and got the answer "we don't bother mate, we use plastic the leakage trip protects people from shocks". I didn't think all domestic properties have leakage trips. Mine doesn't! I could use standard electrical conduit and fittings but that would need a deep chase.

Hoping you experts out there can help.

Regards Dave

[cj]

Hi Dave at the risk of being shot down by others in the group and only having 25 yrs in the trade . The capping you talk of is for mechanical protection of the cable (even though a well placed nail will go through either the steel or plastic types) The norm is to use plastic in most domestic cases in our area and only metal when specified by the design architect. As for earthing the metal capping This opens the can of worms surrounding bonding and earth potential difference. I personally have never seen or been asked to bond metal capping as it would be almost impractical. Conduit on the other hand must be bonded or connected to the main earth as the conductors it carries are not earthed. You should be looking at the cabling you are connecting to to ensure a good earth is available and use twin & earth cable. HTH CJ (for english majors please excuse the spelling)

[John McLean]

Where a cable is concealed in a wall or partition at a depth of less than

50mm from any surface, it must be enclosed in earthed metal conduit, trunking or ducting; or installed either horizontally within 150mm of the top of the wall; or vertically within 150mm of the angle formed by two walls, or horizontally or vertically to an accessory or consumer unit. Regulation 522-06-06 of BS7671 refers. Jaymack

[dave]

It sounds like I need to enclose in earthed metal because I don't fit the straightforward case. What do the professionals use for the ducting. As I said what are the practical considerations??? What do the pros use, and how does one earth the ducting. I can think up ways of doing this but I would like to know the accepted solution.

Dave

[the_constructor]

Houses near to me on a council estate have recently been rewired, by contractors. I went to a friends house who had had his house done and no earthed metal trunking, conduit or ducting was used. Wires were covered with plastic capping at plaster depth and then plastered over. Job was inspected by the BCO's and passed. In my house, I have chiselled the walls out and fitted 20mm plasatic conduit for all sockets, light switches, TV aerials and telephone cables

[dave]

This is worrying. Maybe some have missed the point. The wiring regs plainly say that if you fit buried cables in none "approved" positions i.e. away from corners or not near ceilings of rooms or not running vertically to an electrical device, then you must run your cable in eathed metal protection. This will ensure that a cable buried in the middle of a room say with no outward sign of being there will be protected from nail or drill. The idea is that a drill or nail will contact the earthed metal first and pull the fuse or breaker taking the fault current away from the person holding the metal object and avoiding electric shock. The same protection is NOT afforded by leakge trip without the metal covering.

I think its a good idea but can anyone tell me how to carry out the job practically.

Without any further input, I'm going to run the cable in a 15 mm copper tube and bond it to the mains earth at a local junction box under the floor by means of a pipe bonding clamp. This may not be the proffessional way but at least it seems fairly safe to me and will save me from being sued if I sell the house.

Regards

Dave

I'm sure

[Andrew Gabriel]

Practically, it rarely arises in the first place, but when it does, I've never even once seen an electrician take any notice of this regulation.

[dave]

This is probably OK for registered electricians, but when the DIY man tries to do the job and get it through planning inspection, or someone does kill themselves knocking a nail through a buried cable, the authorities and newspapers will be there like a shot. "These amateurs who don't know what they are doing"!!! Perhaps I'm too paranoid nowadays with Big Brother watching us on everything.

Dave

[John McLean]

The regulations (BS7671), are not binding in law, but the Health and Safety at Work Act is, incidents resulting from contraventions of the regulations, would be dealt with under this act.

Jaymack