Functional safety is the achievement safety through application of control systems and requires identifying what has to be done and how well it should be done. Such systems are referred to as safety-rated systems (or safety instrumented systems in the process sector). The key purpose of a safety-rated system is the achievement of a tolerable risk.

There is an increasing dependence on safety-related systems to achieve tolerable risk levels across industrial sectors. In addition there is an increasing need to justify that the risk levels that have been achieved are tolerable. Following incidents such as Texas City (USA) and Buncefield (UK), there has been a greater emphasis on the achievement of functional with respect to safety-related systems.

Safety regulators require that such safety-systems are specified, designed and maintained in accordance with accepted good practice. The international standard that underpins both process sector standards is IEC 61508 which is the international standard that deals with functional safety and can be used on its own for the specific application or as a basis for sector standards (such as IEC 61511 the process sector). Such standards are regarded as important in the achievement of accepted good practice. Such standards are regarded as important in the achievement of accepted good practice.

The achievement of functional safety is a multi-disciplinary approach involving; plant managers, plant engineers, mechanical engineers, control and instrumentation engineers and electrical engineers. End users are increasingly involved in ensuring that a tolerable risk target is met by the adoption of various layers of protective systems. Safety-related systems for an essential part of the overall framework for the achievement of a tolerable risk.