Functional safety is the part of the overall safety of a system or piece of equipment that depends on the system or equipment operating correctly in response to its inputs, including the safe management of likely operator errors, hardware failures and environmental changes.
Objective of functional safety
The objective of functional safety is freedom from unacceptable risk of physical injury or of damage to the health of people either directly or indirectly (through damage to property or to the environment).
Functional safety is intrinsically end-to-end in scope in that it has to treat the function of a component or subsystem as part of the function of the whole system. This means that whilst functional safety standards focus on electrical, electronic, and programmable systems (E/E/PS), the end-to-end scope means that in practice functional safety methods have to extend to the non-E/E/PS parts of the system that the E/E/PS actuates, controls or monitors.
Achieving functional safety
Functional safety is achieved when every specified safety function is carried out and the level of performance required of each safety function is met. This is normally achieved by a process that includes the following steps as a minimum:
- Identifying what the required safety functions are. This means the hazards and safety functions have to be known. A process of function reviews, formal HAZIDs, HAZOPs and accident reviews are applied to identify these.
- Assessment of the risk-reduction required by the safety function. This will involve a safety integrity level (SIL) or performance level or other quantification assessment. A SIL (or PL, AgPL, ASIL) applies to an end-to-end safety function of the safety-related system, not just to a component or part of the system.
- Ensuring the safety function performs to the design intent, including under conditions of incorrect operator input and failure modes. This will involve having the design and lifecycle managed by qualified and competent engineers carrying out processes to a recognised functional safety standard. In Europe, that standard is IEC EN 61508, or one of the industry specific standards derived from IEC EN 61508, or some other standard like ISO 13849.
- Verification that the system meets the assigned SIL, ASIL, PL or agPL by determining the mean time between failures and the safe failure fraction (SFF), along with appropriate tests. The SFF is the probability of the system failing in a safe state: the dangerous (or critical) state states are identified from a failure mode and effects analysis or (failure mode, effects, and criticality analysis) of the system (FMEA or FMECA).
- Conduct functional safety audits to examine and assess the evidence that the appropriate safety lifecycle management techniques were applied consistently and thoroughly in the relevant lifecycle stages of product.
Neither safety nor functional safety can be determined without considering the system as a whole and the environment with which it interacts. Functional safety is inherently end-to-end in scope.
Certifying functional safety
Any claim of functional safety for a component, subsystem or system should be independently certified to one of the recognized functional safety standards. A certified product can then be claimed to be Functionally Safe to a particular Safety Integrity Level or a Performance Level in a specific range of applications: the certificate is provided to the customers with a test report describing the scope and limits of performance.
An important element of functional safety certification is on-going surveillance by the certification agency. This follow-up surveillance ensures that that product, sub-system, or system is still being manufactured in accordance with what was originally certified for functional safety. Follow-up surveillance may occur as various frequencies depending on the certification agency, but will typically look at the product's hardware, software, as well as the manufacturer's ongoing compliance of functional safety management systems.
The principles underpinning functional safety were developed in the military, nuclear and aerospace industries, and then taken up by rail transport, process and control industries developing sector specific standards. Functional safety standards are applied across all industry sectors dealing with safety critical requirements. Thousands of products and processes meet the standards based on IEC 61508: from bathroom showers, automotive safety products, medical devices, sensors, actuators, diving equipment, Process Controllers and their integration to ships, aircraft and major plant.
In Europe, functional safety certification is supported by a well-developed infrastructure. The CASS Scheme is the primary method by which products are certified to IEC EN 61508 and related standards, through accredited quality auditors. It is possible to certify both products and processes that manage the life-cycle of the product, (in which case, the company certified would then issue a certificate of conformity to that certification in respect of its relevant products).
In the USA, NASA developed an infrastructure for safety critical systems adopted widely by industry, both in North America and elsewhere, with a standard, supported by guidelines. The NASA standard and guidelines are built on ISO 12207, which is a software practice standard rather than a safety critical standard, hence the extensive nature of the documentation NASA has been obliged to add, compared to using a purpose designed standard such as EN 61508 with the CASS Templates. A certification process for systems developed in accord with the NASA guidelines exists.
Modern E/E/PS medical devices are being certified to 510(k) on the basis of the industry sector specific IEC EN 62304 standard, based on IEC EN 61508 concepts.
The automotive industry, has developed the ISO 26262 Road Vehicles Functional Safety Standard based on IEC 61508. The certification of those systems ensures the compliance with the relevant regulations and helps to protect the public. The ATEX Directive has also adopted a functional safety standard, it is BS EN 50495:2010 'Safety devices required for the safe functioning of equipment with respect to explosion risks' covers safety related devices such as purge controllers and Ex e motor circuit breakers. It is applied by Notified Bodies under the ATEX Directive. The standard ISO 26262 particularly addresses the automotive development cycle. It is a multi-part standard defining requirements and providing guidelines for achieving functional safety in E/E systems installed in series production passenger cars. The standard ISO 26262 is considered a best practice framework for achieving automotive functional safety. (See also main article: ISO 26262). The compliance process usually takes time as employees need to be trained in order to develop the expected competences.
Contemporary functional safety standards
The primary functional safety standards in current use are listed below:
- IEC EN 61508 Parts 1 to 3 is a core functional safety standard, applied widely to all types of safety critical E/E/PS and to systems with a safety function incorporating E/E/PS. (Safety Integrity Level - SIL)
- UK Defence Standard 00-56 Issue 2
- US RTCA DO-178B North American Avionics Software
- US RTCA DO-254 North American Avionics Hardware
- EUROCAE ED-12B European Airborne Flight Safety Systems
- IEC 62304 - Medical Device Software
- IEC 61513, Nuclear power plants – Instrumentation and control for systems important to safety – General requirements for systems, based on EN 61508
- IEC 61511-1, Functional safety – Safety instrumented systems for the process industry sector – Part 1: Framework, definitions, system, hardware and software requirements, based on EN 61508
- IEC 61511-2, Functional safety – Safety instrumented systems for the process industry sector – Part 2: Guidelines for the application of IEC 61511-1, based on EN 61508
- IEC 61511-3, Functional safety – Safety instrumented systems for the process industry sector – Part 3: Guidance for the determination of the required safety integrity levels, based on EN 61508
- IEC 62061, Safety of machinery - Functional safety of safety-related electrical, electronic and programmable electronic control systems, based on EN 61508
- ISO 13849-1, -2 Safety of machinery - Safety-related parts of control systems. Non-technology dependent standard for control system safety of machinery. (Performance Levels - PL)
- EN 50126, Railway Industry Specific - RAMS review of Operations, System and Maintenance conditions for project equipment
- EN 50128, Railway Industry Specific - Software (Communications, Signaling & Processing systems) safety review
- EN 50129, Railway Industry Specific - System Safety in Electronic Systems
- EN 50495, Safety devices required for the safe functioning of equipment with respect to explosion risks
- NASA Safety Critical Guidelines
- ISO 25119 - Tractors and Machinery for Agriculture and Forestry -- Safety-Related Parts of Control Systems
- ISO 26262 - Road Vehicles Functional Safety
The standard ISO 26262 particularly addresses the automotive development cycle. It is a multi-part standard defining requirements and providing guidelines for achieving functional safety in E/E systems installed in series production passenger cars. The standard ISO 26262 is considered a best practice framework for achieving automotive functional safety.
- IEC 61508
- Hazard and Operability Study
- Safety Integrity Level
- Spurious Trip Level
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