Functional safety
Definition
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.
In North America, Function Safety is considered as a part of "Safety Critical" systems, extending from software to electronics.
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. Whilst Functional Safety standards focus on Electrical, Electronic and Programmable Systems (E/E/PS), the end-to-end scope means that it is often applied to the mechanical systems that operate with that E/E/PS.
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 involves several concepts, including:
1. Identifying what the required safety functions are. This means the safety functions have to be known, and will involve a process of function reviews, HAZIDs [1], HAZOPs[2] and Accident Reviews to determine.
2. Assessment of the risk-reduction required by the safety function. This will involve a Safety Integrity Level (SIL) Assessment [3]. A safety integrity level (SIL) applies to an end-to-end safety function of the safety-related system, not just to a component or part of the system.
3. 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.
Neither safety nor functional safety can be determined without considering the systems as a whole and the environment with which they interact.
Certifying Functional Safety
Any claim of Functional Safety for a component, subsystem or system should be independently certified to one of the recognised functional safety standards. A certified product can then be claimed to be Functionally Safe to a particular Safety Integrity Level in a specific range of applications.
In Europe, functional safety certification is supported by a well developed infrastructure [4][5]. The CASS Scheme is the primary method by products are certified to EN 61508 and related standards, through accredited quality auditors[6][7][8][9]. It is possible to certify both products and processes that manage the lifecycle 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, though with a particular focus on software [10], supported by guidelines [11]. A certification process for systems developed in accord with that standard exists [12]. The US FAA have similar certification processes, based on US RTCA DO-178B. The North American standards are built on ISO 12207, which is a good software practice standard rather than specifically a safety critical software standard, but is augmented by the NASA guidelines and FAA guidelines in its application to safety systems.
Contemporary Functional Safety Standards
The primary functional safety standards in current use are listed below:
- IEC EN 61508 Parts 1 to 3 is a basic safety standard, applied particularly in Europe to E/E/PS and to systems with a safety function incorporating E/E/PS.
- IEC 61513, Nuclear power plants – Instrumentation and control for systems important to safety – General requirements for systems
- IEC 61511-1, Functional safety – Safety instrumented systems for the process industry sector – Part 1: Framework, definitions, system, hardware and software requirements
- IEC 61511-2, Functional safety – Safety instrumented systems for the process industry sector – Part 2: Guidelines for the application of IEC 61511-1
- 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
- IEC 62061, Safety of machinery - Functional safety of safety-related electrical, electronic and programmable electronic control systems
- EN 50128, Railway Industry Specific
- EN 50129, Railway Industry Specific
- UK Defence Standard 00-56 Issue 2
- US RTCA DO-178B North American Safety Critical Systems
- EUROCAE ED-12B European Airborne Flight Safety Systems
References
- ^ Wikipedia Hazard Identification http://en.wikipedia.org/wiki/HAZID
- ^ Wikipedia "Hazard and Operability Study http://en.wikipedia.org/wiki/Hazop
- ^ Wikipedia "Safety Integrity Level" http://en.wikipedia.org/wiki/Safety_Integrity_Level
- ^ The 61508 Association http:www.61508.org
- ^ Institution of Engineering and Technology, Safety Zone http://www.theiet.org/
- ^ CASS Scheme, Conformity Assessment of Safety Systems, http://www.cass.uk.net/
- ^ SIRA Certification http://www.siracertification.com/safety.aspx
- ^ 61508 Association, Conformity Assessment http://www.61508.org/ca.htm
- ^ TUV Anlagentechnik, Dept ASI, http://www.tüvasi.com/downloads/Certification_Information_2003_05_16.pdf
- ^ NASA Software Safety Standard NASA STD 8719.13A
- ^ NASA-GB-1740.13-96, NASA Guidebook for Safety Critical Software.
- ^ S. Nelson, Certification Processes for Safety-Critical and Mission- Critical Aerospace Software, June 2003, NASA/CR–2003-212806 http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040014965_2004000657.pdf
See also
External links
- IEC Functional safety zone
- Functional Safety and IEC 61508: A basic guide
- Safety Users Group - Functional Safety-Information Resources
- Inside Functional Safety - Technical magazine focusing on functional safety
- 61508.org The 61508 Association