Testable voted logic power circuit and method of testing the same

Abstract

In a protection system for a complex process control system which generates three independent actuation signals, the power circuit includes three pairs of switches arranged in three groups connected in series with each other and with a load device and a voltage source. Each group of switches includes two switches selected from different pairs of switches and connected in parallel. The same circuit topography is used for normally energized and normally deenergized loads with normally closed siwtches used with the former and normally open switches used with the latter. For testing the circuit, each group of switches is shunted by a resistor to form a leakage path through the configuration of switches. The impedance of each resistor is several times that of a closed switch such that the leakage current is insufficient to energize the load, but sufficient to be detected by a current detector which measures the current drawn from the voltage source. Actuation of each switch has an observable effect on the detected current, and thus the circuit may be tested by selective actuation of the switches without changing the circuit topography and while maintaining the protection function, all without susceptibility to any single failure. In addition, by monitoring the current continuously, the failure of any switch can be detected at the time it occurs.