PURPOSE:To maintain a lock out valve in its lock out condition even if a lock out electromagnetic valve is demagnetized by connecting a self holding circuit to the lock out valve at the time of a normal governor oil trip test. CONSTITUTION:When a lock out electromagnetic valve 25 is magnetized, a valve body 25a is moved to the left side opposing to a spring 25b, emergency oil supplied from an emergency oil supply port b3 is supplied into the first oil pressure chamber 31a of an oil pressure relay 31 through ports f1, f2. The spool valve 31c of the abovementioned oil pressure relay 31 is thus moved to the left side opposing to a spring 31b. Also, the spool valve 31c is maintained in its condition moved to the left side opposing to the spring 31b by emergency oil supplied from a port g1. Therefore, even if magnetization of the lock out electromagnetic valve 25 is released in this condition, the spool valve 31c is maintained at its operating location.
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