Grading capacitor is normally parallel with two interrupters of circuit breaker（CB）respectively in order to reduce the uneven voltage division of interrupter due to stray capacitance

assure the two interrupters to bear the same voltage

limit the recovery voltage gradient and improve the operation reliability of circuit breaker. It is found in the preventative test of the grading capacitor that partial full film insulation and film paper composite insulation grading capacitors have the phenomena of 10 kV dielectric loss exceeding standard or fast increase. Such tests as high voltage dielectric loss

withstand voltage and partial discharge on 5 pieces of defect capacitors and the works such as physical and chemical analysis of insulation oil and disassembly are performed. It is judged that the direct cause for exceeding standard or fast increase of 10 kV dielectric loss of grading capacitor is due to physical and chemical performance deterioration of insulation oil and core breakdown

thinking that high voltage dielectric loss test cannot be regarded as the only standard whether the grading capacitor possesses continuous operation condition. Based on above work

the suggestion for the preventative test work of the grading capacitor is proposed.