Commonly, the logic systems for protective systems are electronic, but programmable and other technology systems (magnetic or fluidic/pneumatic) have been used.
The architecture of the logic system will be determined by the hardware fault tolerance requirements, for example dual redundant channels. Where a high level of integrity for the system is required (SIL3 or SIL4) then diverse hardware between channels may be employed. This should not be confused with diversity of independent protective systems.
Logic systems are likely to incorporate provisions for fault alarms and overrides, for which there should be suitable management control arrangements. They may also provide monitoring of input and output signal lines for detection of wiring (open circuit, short circuit) and sensors/actuators (stuck-at, out of range). Such monitoring may initiate an alarm, a trip action or, in a voting arrangement, disable the faulty element.
Software based systems should be adequately protected against systematic failures, for example by an appropriate hardware and software safety lifecycles, and suitable techniques and quality systems. Guidance is available in BS IEC 61508 Part 3, PES Parts 1 & 2, EEMUA 160, Out of Control, and IGasE SR15 - Programmable equipment in safety related applications.