Abstract: A system and method for complete on line testing of the emergency trip system of a turbine power plant is disclosed. The mechanical trip valve and a portion of a line to drain, of the emergency trip system, are isolated from the remaining portions thereof. The testing of the valve is permitted without an opening to drain of the line, which would cause a turbine trip. A three-way solenoid valve obstructs the line to isolate an upstream portion from a downstream portion while permitting introduction of fluid into the downstream portion at a pressure distinct from that in the isolated upstream portion. Pressure sensitive switches, which operate suitable indications are provided in fluid communication with both portions of the line to drain to sense the differential pressure therebetween upon operation of the solenoid valve and, subsequently, a rapid diminishing of pressure in the downstream portion indicative of a successful test of the mechanical trip valve.