INTRODUCTION

This article provides an overview of the ISA Standard on Safety Systems (S84), with an emphasison the layers of protection and the estimates of the availability of different types of instrumentation and controls. The definition of a safety instrumented system is a system designed to respond to conditions of a plant, which may be hazardous in themselves, or if no action were taken could eventually give rise to a hazard. It must generate the correct outputs to prevent the hazard or mitigate the consequences.

The ISA (International Society for Measurement and Control) S84 and IEC (International Electrotechnical Commission) 61508/61511 standards, along with the AIChE CCPS (American Institute of Chemical Engineers, Center for Chemical Process Safety) Guidelines on safety instrumented (interlock) systems, as well as process safety management legislation [1] [4] are performance oriented, not prescriptive. They do not tell people what technology logic system to use (relay, solid state, or software based), what logic and field device configuration to use (single, dual, or triplicated), or how often to test a system (monthly, quarterly, or yearly). They merely list the performance requirements for the overall system. In other words, the greater the level of risk of the process, the greater the performance needed of the safety instrumented system.