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Cables passing through or over joists bs 7671 2008 requires a cable installed under a floor or above a

POCKET GUIDE Cables passing through or over joists Cables that pass through or over timber joists can be particularly vulnerable to damage which can lead to the dangers of electric shock or ?re unless suitable measures are taken In order to protect against these dangers Regulation of BS requires a cable installed under a oor or above a ceiling to be run in such a position so that it is not liable to be damaged by contact with the oor the ceiling or any of their ?xings The Regulation also requires that where a cable has to pass through a timber joist within a oor or ceiling construction or through a ceiling support e g under oorboards the cable shall i be at least mm measured vertically from the top or bottom as appropriate of the joist or batten or ii incorporate an earthed metallic covering which complies with the requirements of BS for a protective conductor of the circuit concerned the cable complying with BS BS BS BS BS EN - or BS or iii be enclosed in earthed conduit complying with BS EN and satisfying the requirements of BS for a protective conductor or iv be enclosed in earthed trunking or ducting complying with BS EN and satisfying the requirements of BS for a protective conductor or v be mechanically protected against damage su ?cient to prevent penetration of the cable by nails screws and the like The ?ve conditions are illustrated below ii iii iv Ceiling board i Minimum mm to top or bottom v as approriate of joist NOTES The titles of the British Standards listed in Condition ii are given in Appendix of BS Explanatory notes on the ?ve conditions are provided on the reverse of this guide www niceic com Based on information as at September ? Pocket Guide CPOCKET GUIDE Cables passing through or over joists Explanatory notes Condition i The speci ?ed minimum depth of mm from the top or bottom as appropriate of a joist or batten is intended to avoid the cable from being penetrated by a nail screw or the like that is used to ?x the oorboards or ceiling boards to the joist However if the electrical installer believes that a mm depth will not be su ?cient for this purpose due to the length of the nails etc then either the cable must be placed at a su ?ciently increased depth to avoid penetration or one of the other conditions ii ?? v must be complied with Condition ii The use of a cable incorporating an earthed metallic covering such as mineral insulated copper sheathed MICS or steel wire armoured SWA cable is intended to ensure that a nail screw or the like cannot make contact with a line conductor of the cable without ?rst coming into good electrical contact with the earthed metallic covering by piercing thereby providing a direct path for earth fault current to ow to cause automatic disconnection of the circuit concerned Regulation