The purpose of this paper is to illustrate the new phenomenon in a full-voltage switching test of a 126 kV vacuum circuit breaker（VCB）

i.e. pre-brokendown vacuum arcs self-quench

the gap between contacts is broken several times

and the current generating time fluctuates significantly. Synthetic test method and direct test method were adopted in the full-voltage switching test for the 126 kV single-break VCB. In the synthetic test circuit

two kinds of switching devices were adopted. Test results show that in the synthetic test circuit

the time delay of two switching devices is much longer than 300 μs

which can not meet the requirement in the standard IEC 62271-100—2008 for full-voltage switching test. And in the direct test circuit

the current generating time fluctuates from 200 μs to 800 μs. The delay period is from the first prestrike to the start of power frequency high-current rising

which depends on the characteristics of the high-voltage VCB.