Methods :

Single Point Bonding : This is applicable for short cable circuit such as interconnection with sub-stations or termination of an overhead line into a sub-station. In this system, the three sheaths are bonded and earthed at one of the cable route. As a result:

• A voltage will appear from sheath to earth which will be maximum at the farthest point from the earth bond at other end.
• No induced current hence, no sheath losses result.

Mid Point Bonding : This is a modification of the single point bonding method whereby the circuit is effectively split up into two elementary sections. By Mid Point Bonding the length of the cable circuit can be increased to almost twice that of Single Point Bonded system. In this system, the three sheaths are bonded and earthed at the centre of the cable route. As a result:

• Voltage will appear from sheath to earth which will be maximum at both ends of the cable route.
• No induced current hence, no sheath losses result.

Both Ends Bonded : This is applicable for longer circuit lengths .In this system. The sheathes are bonded and earthed at both the ends of the cable route. As a result of the flow of sheath currents sheath losses take place which slightly de-rates the current rating. This system has the following distinct advantages:

• There is no sheath voltage to earth hence, ensures safety.
• Installation is simple, and no maintenance or monitoring of the installation is necessary.

Cross-Bonding : This is applicable for longer circuit lengths. In this system, the sheaths of the adjoining cables are connected in a manner so as the sheaths are electrically 120° apart. Cross - bonding essentially consists of the sectionalising the sheaths into minor sections which are cross - bonded. The minor sections together make a major section. For cables in trefoil formation, their sheath voltage under the cross - bonded systems will be equal across a major section and have a phase displacement of 120c (by ensuring the minor sections have identical lengths). The vector sum of the voltage will be zero, hence, eliminating circulating current and sheath losses.

This system however has the following disadvantages:

• Complexity of the system
• Maintenance of link boxes (prevention of water seepage to be ensured)
• Higher capital cost.