The TetraSentry is a dual parallel-redundant hydraulic trip block that’s on-line testable and on-line repairable. The TetraSentry is connected to the hydraulic trip header in parallel with the turbine trip valve. During a trip action, the high capacity solenoid valves dump the trip header fluid to the reservoir faster than the supply orifice can refill it. Testing is accomplished using pressure transmitters to monitor the state of each solenoid valve. They provide feedback to the control system to allow confirmation of the success of each step in the test. Isolation valves allow each pair of solenoid valves to be taken out of service and replaced without compromising the system’s ability to trip.