Safety instrumentation systems control objectives

A complete lEC 61508 assessment includes a FMEDA, a study of Prior Use and adds an assessment of all fault avoidance and fault control measures during hardware and software development as well as detail study of the testing, modification, user documentation and manufacturing processes. The objective of all this effort is to provide a high level of assurance that an instrument has sufficient quality and integrity for a safety instrumented system application. This is clearly more important for products containing software as many end users have the strong opinion that software is "bad... 

An SIS s correct operation requires a series of equipment to operate properly. The specific control functions carried out by a safety instrument system are known as safety instrumented functions (SIF). The overall objective of design, implementation, and follow-up of an SIS is ensuring that the SIS is able to carry out effectively its intended safety functions, when specific process demands occur. 

For automation-based systems, Chapter 8 presents examples of installations in the oil industry. The lEC 61508 standard allows for a definihon and control of the safety objectives (SIL). Chapter 8 presents an opportunity to revisit this standard and its use. This chapter is supplemented by Chapter 10, which is a snimnaty of the implementation of safety instrumented systems (SIS) in industry. 

The first steps of control system design consist essentially in finding the hardware architecture. The designer has to determine the places where sensors or actuators can be implemented, then for each of them, to determine the number and the type of instruments among those available. The choice is made in order to obtain hardware organisations that can properly perform the mission according to the objectives in terms of financial cost, reliability and safety criteria. 

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