Building Services # 1970 Southampton
Since the early days of telecommunications the British Post Office has used ducts to provide a flexible cabling system which allows prior provision for development and facilitates the renewal and recovery of faulty lengths of cable.
For the past half century the majority of ducts laid have been manufactured from earthenware, which has well-known characteristics of long life and stability.
Other materials have been used, notably cast iron, asbestos cement, pitch fibre and precast concrete, but none has proved as satisfactory as earthenware.
The introduction of light-weight, polythene-sheathed cable with a low coefficient of friction, together with light-weight improved rodding systems, hay made possible the use of longer cable lengths between jointing points. These new techniques can only be used to maximum advantage if improvements in the air and water tightness, frictional properties, and concentricity of the sockets of duct lines can be achieved. Recent improvements in the jointing of singleway earthenware ducts have met these requirements but it has not been possible to extend the same improvements to multiway earthenware ducts. Ducts made from unplasticized PVC have shown promise of meeting the requirements of a satisfactory all-purpose duct during extended trials.
Polythene ducts of gu bore are also used on new housing estates for protecting lead-in cables.