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core balance current transformers (CBCT) 3 phase protection

hello friends in this viode i a going to describe about the CBCT . Core Balance Current Transformer or CBCT is a ring type current transformer through center of which a three core cable or three single core cables of three phase system passes. This type of current transformer is normally used for earth fault protection for low and medium voltage system. Secondary of CBCT is connected to Earth Fault Relay. During normal operating condition as the vector sum of three phase current i.e. (Īa + Īb + Īc =0) is zero therefore no residual current in the primary will be present. Here residual current means zero sequence current. Therefore there will not be any flux developed in the CBCT core and hence no current in the secondary circuit of CBCT. The advantage of using CBCT for earth fault protectionis that only one CT core is used instead of three core as in conventional system where the secondary winding of three cores are connected residually. Thus the magnetizing current required for the production of a particular secondary current is reduced by one third which is a great advantage as the sensitivity of protection is increased.