The paper describes the accident of a set of 10 kV assembling capacitor at a certain 220 kV substation

which exploded after 9 seconds of operation due to short circuit.Through dissection of the capacitor

careful analyses and demonstration of the record of the faults recorder

it is concluded that the defect of the insulating layer between the inner housing and the ground caused insulation breakdown at the 10 kV rated operating voltage

therefore resulting in short-circuit discharge between phases.The high temperature of arc overheats and vaporizes the insulating oil

causing shell bulges and burst.At the same time

air entered the body and resulted in burning of the insulating oil

so the No.2 capacitor was totally burned down.As for the problem revealed by the fault

this paper propose that some such tests as moisture measurement in insulating oil and chromatographic detection etc not specified by the DL/T596-1996 standard should be carried out.This detection is the necessary and effective way in detecting the main insulating defect inside the assembling capacitor.