Reliability model of a relay output card with diagnostic circuitry for safety instrumented system

High reliability Safety Instrumented Systems (SIS) in critical applications employ external or built-in testing to detect dangerous failures. In a nuclear power plant, typically, electromagnetic relays are used to communicate shutdown signals between various parts of safety instrumentation. Relay contact welding is a failure mode of concern. An online technique to diagnose this failure mode has already been published by this author and her team. The aim of the paper is to show that the implementation of this technique is practical using a Relay Output Card (ROC) and to highlight reliability improvement. Printed circuit board (PCB) schematic of ROC is described. Markov model is established to determine unsafe state probability of the system. Reliability analysis proves that the diagnostic circuitry in ROC significantly reduces unsafe state probability. It is also demonstrated that failures in diagnostic circuitry have very less significance on the unsafe state (steady state) probability of the system. Sensitivity analysis has been carried out by varying the test interval and proof test interval.