Abstract: The pressure monitoring circuit of relay protection secondary circuit was introduced. The defect of pressure monitoring circuit of protection devices in new line protection design specification was analyzed, which is that when the first group operating power source is cut off; the second group operating circuit will be broken simultaneously, the breaker will be blocked incorrectly and can not trip after the line fault occurred. A solving scheme was put forward, in which the protection device will only block the reclosing mechanism, and the blocking of breaker open and close will be realized by secondary circuit of the breaker itself.

Key words: pressure monitoring, power supply fail, breaker, incorrect blocking