Current transformer（CT） is a key component that links high voltage primary circuit and control protection circuit.This paper ascertains the structural dimension of CT on the basis of electromagnetic conversion principle.It is found that the leakage flux and exciting current are main sources to introduce the error of CT

and the exciting current depends on electrical parameters

core material and structural dimension of CT.It is demonstrated that uniformly distributed secondary windings of CT can minimize the leakage flux

and indirect test and direct test of composite error are equivalent only for the CT with uniform distribution of tapped windings.On the premise that performance requirements of CT are met

the error and manufacturing cost of CT can be reduced by appropriately changing CT’s electrical parameters

such as grading load of the CT with multi-tapped windings

and changing duty cycle of the CT with transient protection and without reclosing requirement from double-cycle to single-cycle.In addition

the selection of insulation materials for internal CT and the moisture resistance for external CT are described.Especially

the reason why closed circuit is formed in spot installation of external CT

the effect of the closed circuit on CT’s error

and the methods for detecting the closed circuits are discussed.