" NO DIG POLICY "

ACTIVITY ON 14/4/10


DUCTBANK @ PUTRAJAYA

TNBD Putrajaya has a unique underground cable installation which is using ductbank and manhole system. This is because the relevant authority had imposed a ' no dig policy ' in Putrajaya area. For the cable installed in ductbank and manhole, the maximum current carrying capacity or ampacity is the main concern. It is crucial for the planner or engineer to know the ampacity of each cable in order to avoid overloading. Overloading the cable will cause the cables to operate at temperature greater than 90 degree celcius which is the maximum operating temperature of XLPE cable. this should be avoided since it will cause premature failures and decreases the insulation strength.

In the project the pipe used in the ductbank was High Density polyethylene (HDPE). In early 2009, due to 400km ductbank collapse, UNITEN had give recommendation to use Polyvinylchloride (PVC) pipe instead using HDPE pipe. this recommendation given by UNITEN was solely based on civil and mechanical reasons. If different material of pipe are used, it might effect the ampacity since the ampacity is a function of the heat generated in a cable (I^2R) and the amount of heat conducted away from the cable. This heat conduction is very dependent to the thernal resistivity of the material used. It should be noted that the thermal resistivity of PVC is higher than HDPE.

samples of cable installation in ductbank:

Few of cable laying terms used by TNB has different meaning in the IEC 60287 and CymCAP software. The cable lying terms used in the IEC 60287 and CymCAP are :

Direct Buried : The cable are buried directly in its original or native soils

Ducts : The cables are inside the PVC, PE, GI or other pipes, which are buried directly in its original or native soils.

Ductbanks : Ductbank consists of series of ducts (e.g PE pipes) which is surrounded with concrete. The cables are inserted through the ducts from one end to another end.

Trough : Long shallow rectangular-shaped excavation, where the walls, bottom and cover are made of concrete. The trough can be filled with a thermally good material or left unfilled (air filled)

R&D Project
Prepared by : Zairul Aida (Researcher@TNBR)