Sensitive Earth Fault
The core-balance current transformer (or CBCT) is normally of the ring type, through the centre of which is passed cable that forms the primary winding.
An earth fault relay, connected to the secondary winding, is energised only when there is residual current in the primary system.
The advantage in using this method of earth fault protection lies in the fact that only one CT core is used in place of three phase CT’s whose secondary windings are residually connected. In this way the CT magnetising current at relay operation is reduced by approximately three-to-one, an important consideration in sensitive earth fault relays where a low effective setting is required. The number of secondary turns does not need to be related to the cable rated current because no secondary current would flow under normal balanced conditions.
This allows the number of secondary turns to be chosen such as to optimise the effective primary pick-up current.
Core-balance transformers are normally mounted over a cable at a point close up to the cable gland of switchgear or other apparatus. Physically split cores (‘slip-over’ types) are normally available for applications in which the cables are already made up, as on existing switchgear.
SOURCE: Network Protection & Automation Guide -Current and Voltage Transformers