Capacitance and dielectric loss measurement of the grading capacitor is a very important task to assure safe operation of circuit breaker.Such problem as critical interference of induction power exists in the site test

and such case as more and more grading capacitors’ dielectric loss appeared over normal value at 10 kV test voltage according to the standard

but become normal when re-measuring back to manufactory.The site test methods are studied and the methods for capacitance and dielectric loss measurement

changing frequency of test power supply and increasing supply voltage

are proposed.Dielectric loss variation of circuit breaker capacitor with test voltage and the shielding effect to electromagnetic interference

when a different frequency power supply was used for test

is firstly analyzed.The results of capacitance and dielectric loss measurements of grading capacitors in a certain 500 kV transformer substation are proved to be good.This research activity has significance to the site capacitance and dielectric loss measurement of grading capacitors.