Page 38 - Pocket Guides

POCKET GUIDE 21

CABLES PASSING THROUGH OR OVER JOISTS
Cables that pass through or over timber joists can be particularly vulnerable to damage
from impact, which can lead to a risk of electric shock or fire unless suitable measures
are taken to provide protection.
In order to protect against impact, Regulation 522.6.201 of BS 7671 requires a cable
installed under a floor or above a ceiling to be installed in such a position where it
is not liable to be damaged by contact with the floor or ceiling or their fixings (nails,
screws or the like).
This Regulation also requires that where a cable passes through a joist of a floor or
ceiling construction or through a ceiling support (e.g. under floorboards) the cable
should be:
i Installed at least 50 mm measured vertically from the top, or bottom as
appropriate, of the joist or batten, or
ii meet one (or more) of the requirements given in Regulation 522.6.204; described
below.
Regulation 522.6.204 requires a cable to:
i incorporate an earthed metallic covering which meets with the requirements of
BS 7671 for a protective conductor of the circuit concerned, or
ii be enclosed in earthed metallic conduit meeting the requirements of BS 7671 for
a protective conductor, or
iii be enclosed in earthed metallic trunking or ducting meeting the requirements of 2. Earthed metallic sheath
BS 7671 for a protective conductor, or
iv be provided with mechanical protection having sufficient strength to prevent 3. Earthed containment
penetration of the cable by fixings, or
v form part of a SELV or PELV circuit complying with Regulation 414.4.
An example of the methods to minimise impact to cables of Regulations 522.6.201
Non-metallic
and 522.6.204 (except indent v, which is self-explanatory) are illustrated below:
sheathed cable

2. Earthed metallic sheath
3. Earthed containment
Non-metallic
Ceiling board sheathed cable

Ceiling board

1. Minimum 50 mm
1. Minimum 50 mm 4. Suitable mechanical protection
to top or bottom,
as approriate, of joist
to top or bottom, Notes for methods 1-4 overleaf
as approriate, of joist 4. Suitable mechanical protection
updated to
TH
EDITION
BS 7671:2018
© CERTSURE LLP 2019
updated to
TH
EDITION
BS 7671:2018