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| Cross bonding for high voltage underground cable sheath earthing |
How Bond Your Cable?
Widely used bonding system for High Voltage or Extra High Voltage power cables are as follows:- Both End Bonding Systems
- Single Point Bonding System
- Cross Bonding System
- Both End Bonding systems
The simplest arrangement to eliminate sheath induced voltage is Both End Bonding system. It is very easy and simple to ground both end of cable sheath. Minimum materials are required which lead to economical.
But circulating current flows through cable sheath and heat produces which increase loss and reduce cable life. Circulating current is proportional to the length of the cable and the magnitude of the load current. This system normally applies to short cable section of tens of meters long.
Single point bonded system
A simple system is single point bonding which arrangement does not allow any path to flow circulating currents or external fault currents. In this system the sheaths of cables are grounded at one end and other end are open or connected with SVL and grounded through earth continuity cable, consequently induced voltage is present at open end.
The magnitude of sheath induced voltage depends on the length of the cable, longer the cable length higher the induced voltage. Using of this bonding system is limited only for short distance of cable length to keep induced sheath voltage within a safe limit. Generally suitable for less than 500 m cable length.
Advantages of Single Point Bonding:
No circulating current.No heating in the cable screen.
Economical
Disadvantages of Single Point Bonding:
Induced voltage at the un–earthed end
SVL required if induced sheath voltage during fault is extreme.
Earth Continuity Conductor is required for undesirable fault current.
Cross Bonding system
This arrangement is such that the circuit provides electrically continuous sheath runs from earthed termination to earthed termination. Voltage will be induced between screen and earth, but sectionalized cross bonding system will reduce the sheath circulating currents.
Screen separation and additional link boxes are require for this system, arrangement carries 3 minor sections in each major section. When number of minor sections is not exactly divisible by 3, then system may consist of mixture of single point bonding with cross bonding. This system is technically complicated and more expensive, but for long route length cross bonding system is suitable because of following advantages-
-Earth continuity conductor no need.
-Virtually zero circulating current in the screen.
-Standing induced voltage in the sheath is controlled.
-Technically superior than other methods.
-Suitable for long route length.
