Handbook of Electrical Installation Practice - BeKnowledge

Selection of Wiring Systems 539
Trunking
Trunking is available manufactured out of folded sheet steel, extruded plastic or
extruded aluminium. By definition, trunking has a detachable lid and is intended to
have the cables laid in; it is not intended that cables should be drawn in.
Standard cable trunking is installed fixed to the structure of a building; it may
be installed flush with a wall, but in all cases its removable cover makes for great
flexibility in wiring. In long vertical lengths, steps must be taken to provide
arrangements at suitable intervals to relieve the cables of stress arising from
their own weight.
If it is suitably electrically connected, it may be used as a protective conductor;
the cross-sectional area of the interconnections is subject to the requirements of
Chapter 54 of BS 7671.
Trunking is available divided into compartments to allow for the segregation of
various categories of circuit. The same restrictions which apply to the use of PVC
conduit also apply to PVC trunking. There are a number of variants on the trunking
theme.
Skirting trunking
Skirting or dado trunking, commonly manufactured out of steel, aluminium or PVC,
provides a ready means of both distributing cables around an office and providing
ready access to them. Accessories can be mounted on, or flush with, the surface of
such trunking. This type of trunking often has separate compartments for wiring,
communications and data cables.
Floor ducting and trunking
Where it is necessary to provide supplies in open plan offices, floor ducting is available;
it can be provided with compartments for segregation purposes. Floor ducting
and trunking are manufactured in sheet steel, plastic or mixed fibre and are cast
into the floor finish screed, with interconnecting outlet boxes at intervals as
required. Once cast in, alterations cannot be made without difficulty.
Some floor trunking systems have a removable cover allowing access to the whole
of the wiring system.
Service post and poles
Alternatively, electrical services can be brought down to user positions from above
the working area by use of service poles or posts. These are essentially vertical
self-supporting trunking, divided into compartments into which the various
categories of circuits can be placed. They are commonly used in association with
lighting trunking.
Lighting trunking
Lighting trunking is in many ways similar to standard trunking but incorporates
flanges and other features enabling it to be used either in association with a false
ceiling structure or as a structural part of the false ceiling. It is intended to provide
accommodation for cable and to support luminaires. It is sometimes, but not commonly,
available in compartmented form for segregation purposes.