SHEATH

ACTIVITY ON 13/4/10


The sheath - to provide continuous earthing connections and return path for fault current.

In TNBD system, they applied concentric copper wires and cooper tape as sheaths for the cables. (single core and ABC cables)

Where was sheath located??

Internal cable compartment

cable cross section area

Cable sheath bonding arrangement :


- single point bonding
- both end bodings



Arrangement 1 : Single Point Bonding

- metallic sheath of 3 single core cables are connected and grounded at one point-usually at the cables' end.

- since it is only grounded at one end >> current flow in the conductor produced magnetic flux in the metallic sheath and hence, induces a voltage in the sheath.

- in conductor >> as the current increase, voltage will increase as well (V proportional to I)

- voltage increases as the distance from the ground bond increase

- induced voltage in the sheath = standing voltage


* figure below illustrates the phenomena of induced voltage

Arrangement 2 : Both Ends Bonding

- metallic sheath of the 3 single core cables are connected and grounded at both cable ends.

- the magnetic flux induces a current to flow in the sheath of the completed circuit.

- sheath current inversely proportional to the resistance of the sheath. V=0

- induced current in the sheath = circulating current


* figure below illustrate the phenomena of induced current

what happen if standing voltage in the metallic sheath is high ??

- it's unsafe and causes degradation of the jacket and insullation material

what happen if circulating current in the metallic sheath is high ??

- temperature rise by the contributed heat energy and overheating of the cables.



to overcome thus two problems, it is a must to know the value of it's induced volatge and current of different bonding arrangements. so that, the further action will fits the needss of particular installation without seriously affecting the ampacity of the cable.

*ampacity = current carrying capacity

the CymcCAP software was used to calculate the cables induced voltage for single point bonding and induced current for both ends bonding with respect to cable loading as well as at maximum ampacity on the IEC 602887.




** R&D Project : Sheath Induced Voltage and Current of Different Bonding Arrangements with Respect to Cable Loading for MV Single Core Cables and Aerial Bundle Cables
Prepared by : Tashia Marie (Reseacher@TNBR)