The insulation defects in Opening/Closing coil of circuit breaker were studied in this paper by analysing the structure of coil, the current of coil when operating, causes and characteristics of internal defects. The results show that the insulation defects caused as coil manufacturing will produce local overheating or inter-turn short circuit under the operating current. Faulty coil does not affect performances of circuit breaker. However the DC resistance decreases, coil current increases and waveform is abnormal, and the correctness of the analysis was verified by return test and anatomy. Finally, effective protection measures to the Opening/Closing coil of circuit breaker were proposed and a pulse detection method which can effectively detect defects in coils was introduced. The practice shows that the pulse detection method significantly improves the qualified rate of the coil.