Extra light switch cable question


Postby lopie » Wed Aug 17, 2011 8:42 am

I'm adding an extra light switch on a long corridor (so that the lights can be turned off and on at both ends).

The cable specified is 3 core and earth - BUT there's no earth on the switch I have and I'm using a dry lining box again with no earth connection (this is the same at the other end of the corridor), so I'm presuming that it doesn't need to be earthed? Also if the switch doesn't need to be earthed can I use twin and earth - 1.5mm and use the redundant earth for say the common, although the earth has no protective sheath?

I ask because I currently have no 3 core and earth but will buy a box if needed - although I only need 5 meters!

Great site - use it A LOT
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Postby BLAKEY1963 » Wed Aug 17, 2011 8:05 pm

LOPIE
You need to use 3 core and earth and terminate earth into a terminal block and earth sleeve , If you two way or intermeadiate switch your lights
deatails recorded on a minor works sheet with your schedule of test results.

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Last edited by BLAKEY1963 on Tue Aug 23, 2011 8:22 pm, edited 1 time in total.
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Postby ericmark » Mon Aug 22, 2011 3:10 am

It is forbidden to use an earth cable for any other use.
514.4.2 Protective conductor
The bi-colour combination green-and-yellow shall be used exclusively for identification of a protective conductor and this combination shall not be used for any other purpose.
Single-core cables that are Coloured green-and-yellow throughout their length shall only be used as a protective conductor and shall not be over-marked at their terminations, except as permitted by Regulation 514.4.3.
In this combination one of the colours shall cover at least 30 % and at most 70 % of the surface being coloured, while the other colour shall cover the remainder of the surface.
A bare conductor or busbar used as a protective conductor shall be identified. where necessary, by equal green-and yellow stripes, each not less than 15 mm and not more than 100 mm wide, close together, either throughout the length of the conductor or in each compartment and unit and at each accessible position. If adhesive tape is used, it shall be bi-coloured.

It is normal to fold the earth wire and sleeve so it can be used in the future or where there is already an earth present as with the original switch to connect it to that earth.

In theory every minor works should have a minor works certificate [url=http://electrical.theiet.org/wiring-regulations/forms/index.cfm]Down load here from IET[/url] BS 7671:2008 forms (431 k). You will note these forms ask for the loop impedance and without the earth wire being connected you would be unable to measure this value.

I know in practice most DIY people will not have the meters to measure loop impedance direct you have to do a risk assessment and consider if cutting corners is what you want to do? Also any wire buried must also be RCD protected.

522.6.6 A cable concealed in a wall or partition at a depth of less than 50 mm from a surface of the wall or partition shall:
(i) incorporate an earthed metallic covering which complies with the requirements of these Regulations for a protective conductor of the circuit concerned. the cable complying with BS 5467, BS 6346. BS 6724, BS 7846, BS EN 60702-1 or BS 8436, or
(ii) be enclosed in earthed conduit complying with BS EN 61386 and satisfying the requirements of these Regulations for a protective conductor, or
(iii) be enclosed in earthed trunking or ducting complying with BS EN 50085 and satisfying the requirements of these Regulations for a protective conductor, or
(iv) be mechanically protected against damage sufficient to prevent penetration of the cable by nails, screws and the like, or
(v) be installed in a zone within 150 mm from the top of the wall or partition or within 150 mm of an angle formed by two adjoining walls or partitions. Where the cable is connected to a point, accessory or switchgear on any surface of the wall or partition, the cable may be installed in a zone either horizontally or vertically, to the point, accessory or switchgear. Where the location of the accessory, point or switchgear can be determined from the reverse side, a zone formed on one side of a wall of 100 mm thickness or less or partition of 100 mm thickness or less extends to the reverse side.

Using Ali-tube cable BS 8436 one could get around this problem however it is expensive cable and in may area only available by the role. In order to use this cable the earth to comply [b]MUST[/b] be connected. Otherwise it would not work. Only new houses since 2008 when BS7671:2008 came out are likely to have lighting on RCD protection.

Again if you intend to break the rules then you must realise you are doing that and accept the risks. I know you may now know where the wires run and you think it is safe. But in 10 years time will you remember and will you never sell the house?
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