Functional safety is the part of the overall safety of plant and equipment that depends on the correct functioning of safety-related systems and other risk reduction measures such as safety instrumented systems (SIS), alarm systems and basic process control systems (BPCS).
SIS are instrumented systems that provide a significant level of risk reduction against accident hazards. They typically consist of sensors and logic functions that detect a dangerous condition and final elements, such as valves, that are manipulated to achieve a safe state.
The general benchmark of good practice is BS EN 61508, Functional safety of electrical/electronic/programmable electronic safety related systems. BS EN 61508 has been used as the basis for application-specific standards such as:
- BS EN 61511: process industry
- BS EN 62061: machinery
- BS EN 61513: nuclear power plants
BS EN 61511, Functional safety - Safety instrumented systems for the process industry sector, is the benchmark standard for the management of functional safety in the process industries. It defines the safety lifecycle and describes how functional safety should be managed throughout that lifecycle. It sets out many engineering and management requirements, however, the key principles of the safety lifecycle are to:
- use hazard and risk assessment to identify requirements for risk reduction
- allocate risk reduction to SIS or to other risk reduction measures (including instrumented systems providing safety functions of low / undefined safety integrity)
- specify the required function, integrity and other requirements of the SIS
- design and implement the SIS to satisfy the safety requirements specification
- install, commission and validate the SIS
- operate, maintain and periodically proof-test the SIS
- manage modifications to the SIS
- decommission the SIS
BS EN 61511 also defines requirements for management processes (plan, assess, verify, monitor and audit) and for the competence of people and organisations engaged in functional safety. An important management process is Functional Safety Assessment (FSA) which is used to make a judgement as to the functional safety and safety integrity achieved by the safety instrumented system.
Alarm systems are instrumented systems designed to notify an operator that a process is moving out of its normal operating envelope to allow them to take corrective action. Where these systems reduce the risk of accidents, they need to be designed to good practice requirements considering both the E,C&I design and human factors issues to ensure they provide the necessary risk reduction.
In certain limited cases, alarm systems may provide significant accident risk reduction, where they also might be considered as a SIS. The general benchmark of good practice for management of alarm systems is BS EN 62682.
BPCS are instrumented systems that provide the normal, everyday control of the process. They typically consist of field instrumentation such as sensors and control elements like valves which are connected to a control system, interfaced and could be operated by a plant operator. A control system may consist of simple electronic devices like relays or complicated programmable systems like DCS (Distributed Control System) or PLCs (Programmable Logic Controllers).
BPCS are normally designed for flexible and complex operation and to maximise production rather than to prevent accidents. However, it is often their failure that can lead to accidents and therefore they should be designed to good practice requirements. The general benchmark of good practice for instrumentation in process control systems is BS 6739.