Underground Cable System Configurations

Cable system configurations
Trefoil and flat formation
The three cables in a 3-phase circuit can be placed in different formation. Typical formations include trefoil (triangular) and flat formations. The choice depends on several factors like screen bonding method, conductor area and available space for installation.

Bonding of the metallic screens
The electric power losses in cable in cable circuit are dependent on the currents flowing in the metallic sheaths of the cables. Therefore, by reducing or eliminating the metallic sheath currents through different methods of bonding, it is possible to increase the load current carrying capacity (ampacity) of the cable circuit. The usual bonding methods are described below:

Earthing methods, induced voltage
High voltage cables have a metallic sheath, along which a voltage is induced as a function of the operating current. In order to handle this induced voltage, both cable ends have to be bonded sufficiently to the earthing system. The following table gives an overview of the possible methods and their characteristics: