The CT series toroidal transformers allow to sense the leaking currents to earth . These transformers have been designed to be coupled to the earth leakage current relays which can be used to generate the tripping signal . All active wires (phase and neutral) of the three phase or single phase lines should pass through the internal core of the transformers .
The net resultant flux being zero does not induce any current in the secondary of the transformer. Thus the secondary current of the core balance current transformer when all the three phases are healthy is zero . When an earth fault occurs in one of the phases, the zero-sequence fault current which flows is not cancelled by the flux of the other two phases and hence induces a current in the secondary.
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