ZCT (Zero-phase-sequence Current Transformer)

is used widely for the detection of the ground fault
current in low or high voltage distribution power line.
Especially, the use of pole air switches equipped with
ZCT is increasing to prevent the extended accident
caused by the ground fault. The accurate detection
of low level ground fault is needed from the view of
early stage discovery.
The internal residual current of ZCT must be small as
possible for the detection of low level groundfault.