The Application of Two-Out-of-Three Logic for the Control of Electric Motor Operated Valves in Candu safety Systems

Traditionally, the operation of electric motor operated (EMO) valves has been controlled by a position limit switch and a torque limit switch which are part of the actuator. A malfunction or improper maintenance of either of these switches has resulted in numerous actuator failures. The reliability of EMO valves and, in the case of nuclear power plants, the frequency of forced shutdowns could be significantly improved by using two-out-of-three logic in the control circuits. The feasibility of mounting additional switches on the valve and the control circuits was verified in tests and has been assessed as suitable for both new and existing nuclear installations.