An accident of an insulated tension pole of 550 kV porcelain column circuit breaker is introduced. After the accident

the primary and secondary equipments were checked

and tests both on site and in the manufacturing company were conducted for the circuit breaker to analyze the accident. It was found that the insulation defect of B-phase insulated pull rod resulted in partial discharge to gradually degrade the insulation of the circuit breaker

eventually leading to this accident. Therefore

it is necessary to draw up standard and put in practice for detection and judgment of number and size of air gap in insulated pull rod of a circuit breaker.