Based on an analysis of the causes,the author pointed out that the accident is caused by incorrect switching of breaker auxiliary contacts in the process of operation,then put forward the prevention measure of time-delay switching off the tripping circuit after sending of operation pulse.

This paper analyzes the problems one by one, including why switch auxiliary contact location for a long time does not correspond to the device do not report until 18 M switching operation, if not dealt with what would have serious consequences, and proposes preventive measures that other special units of the dual-bus can cause sufficient attention.