Blog Functional Safety

Functional Safety

The following is a short, but excellent, introduction to the topic of ‘Functional Safety’ by the United Kingdom Health and Safety Executive (UK HSE). It is equally applicable outside the UK, and the British Standards (‘BS EN’) are versions of international ISO/IEC standards – e.g. the Australian version (‘AS/NZS’) is often identical to the British standard.

My comments and explanations are shown [thus].

[Functional Safety]

“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).

[Functional Safety is popular, in fact almost ubiquitous, in the process industry, where large amounts of flammable liquids and gasses are handled. That said, the systems and techniques developed by and for the process industry have been so successful that they are found in many other industrial, transport and defence applications.]

SIS [Safety Instrumented Systems]

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

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 [Basic Process Control Systems]

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 maximize 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.”

[To be honest, I would have put this the other way around. The BCPS came first, although they were just called ‘control systems’, and some had alarms to get the operators’ attention. As the complexity of these control systems increased, then cascading alarms became a problem and alarms had to be managed as a ‘thing’. Finally, the process industry used additional systems, when the control system/alarm system combo became inadequate, and thus the terms SIS and BCPS were born.]

[It’s worth noting that for very rapid processes where a human either cannot intervene fast enough or lacks the data to do so reliably, the SIS becomes an automatic protection system, as found in rail signaling systems, or ‘autonomous’ vehicles. Also for domains where there is no ‘fail-safe’ state, for example in aircraft flight control systems, the tendency has been to engineer multiple, redundant, high-integrity control systems, rather than use a BCPS/SIS combo.]


The above text is reproduced under Creative Commons Licence from the UK HSE’s webpage. The Safety Artisan complies with such licensing conditions in full.

[Functional Safety – END]

Back to Home Page