PURPOSE: To surely lock trip signal even if a position detector goes out of order by providing a circuit which nullifies the moving stop signal from a position detector detecting the position of a generator.

CONSTITUTION: When a reset motor drive signal is emitted on a line 20 in place of the A side motor drive signal on a line 18, the output of NOT circut 32 becomes 1. Then, the output of an AND circuit 28 becomes 1 and a timer 33 counts the time from the point when a pressure switch 9 or 10 went OFF until the rod returns to the normal position. During this time, a limit switch 1 or 2 goes OFF and the output of a NOT circuit 31 becomes 1 and awaits that the output of the timer 33 becomes 1. The output of the NOT circuit 30 becomes 1 and permits the output of an OR circuit 25 being a pressure switch actuating signal to be ready for turbine trip by thrust abrasion.