In this paper

two sphere-sphere gaps are in series connection to simulate double breaks circuit breakermodel. The two sphere gaps in series withstand standard lightning impulse voltage（1.2/50 μs）and the photos ofsphere gap breakdown is taken by camera. The breakdown phenomena in the experimental results can be divided intothree types:The breakdown of only the upper or lower sphere gap is defined as breakdown type I;the non-synchro-nous breakdown of series double series sphere gaps is defined as breakdown type II

while the synchronous break-down of the series double sphere gaps is defined as breakdown type III.It is found by further study that the causes ofthe three types of breakdown are related to the distribution of the electric field strength of the upper and lower gaps.The ratio of the maximum field strength of the upper gap to the lower gap between 6.9 ～ 1.52 is breakdown type I andbetween 0.8 ～ 0.2 is breakdown type II. The ratio between 1.5 ～ 1.52 and between 0.7 ～ 0.8 is breakdown type II andbetween 1.5 ～ 0.7 is breakdown type III.The study result has a definite significance for the breakdown mechanism ofmultiple break circuit breaker.