Introduction & Application-

C O R E B A L A N C E C U R R E N T TRANSFORMERS (CBCT) is a sensor to sense earth fault current in conjunction with an EARTH FAULT RELAY to protect the system when an EARTH FAULT occurs on one or more phases. The CBCT is mounted externally & load carrying conductors are passed through CBCT. The CBCT inner diameter is large enough to pass all the three bus-bars or the three phase cables, unlike a metering CT where only one busbar or cable passes through the CT. As the VECTOR SUM of the three currents at any given time is ZERO, the resultant magnetic field through the core of the CT is zero. This is the healthy situation of the system. If required Neutral also can be passed through CBCT in case of 3 phases, 4 wire
system. Ideally Vector Sum will remain zero in case of fault free systems.

In the event of an Earth Fault occurring in any of the phases, the current in that phase rises, inducing a resultant magnetic flux in the CT secondary, energizing the relay & tripping the system.

The current at CBCT secondary will depend on the actual earth fault current level & hence unbalanced loading does not affect the functioning of Earth fault relay.