The “primary integrity parameters” — Design parameters for safety systems

This paper discusses the Primary Integrity Parameters (PIPs) - design attributes that, as a group, determine the level of safety integrity achieved by a Programmable Electronic System (PES). These parameters include redundancy level, failure rates and modes, diagnostic coverage factor, common cause failure rates, on-line manual test interval/duration, maintainability and security. The paper demonstrates that the level of safety provided by a PES is not simply a factor of any one of these attributes, but is determined by the total combination of the PIPs. Examples are given to show the dependency of the system safety on each of the parameters.