Regulatory Analysis for the resolution of Generic Issue 142: Leakage through electrical isolators in instrumentation circuits

Abstract

Generic Issue (GI) 142 deals with staff concerns about the design of isolation devices used to ensure separation between Class 1E and non-Class 1E electrical control and instrumentation circuits. This issue was initiated in June 1987. Staff reviews of the implementation of the Safety Parameter Display System (SPDS) requirement indicated that some isolation devices used to provide an interface between the non-Class 1E SPDS and the Class 1E safety systems would allow signal leakage if electrically challenged. It was unknown if the amount of leakage posed a hazard to safe operation of the Class 1E system. A review of failure records does not reveal any incidents of system damage caused by isolation device challenge. Furthermore, a review of existing PRA data indicates that the safety significance of ID challenge is low, at the expected challenge event frequency. However, based upon the potential design variations in future control systems resulting from application of computer technology, additional design and qualification test requirements for future plants are recommended.