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Understanding Functional Safety an Overview of the Iec 61508 Standard and Its Application and Benefits

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Understanding Functional Safety an Overview of the Iec 61508 Standard and Its Application and Benefits UNDERSTANDING FUNCTIONAL SAFETY AN OVERVIEW OF THE IEC 61508 STANDARD AND ITS APPLICATION AND BENEFITS As the integration of automated safety systems expands globally across diverse industries – including process, household and commercial products, medical, nuclear, automotive, railway and avionics – the importance of functional safety evaluation and certification has become recognized internationally. Today functional safety certification is widely considered to be an essential tool to control and mitigate risk, particularly in those cases where a failure could lead to serious injury or death. Those unfamiliar with the concept of functional safety may find the subject difficult to understand and place within the context of traditional safety and reliability assessments. However, designers who understand and embrace the concept of functional safety – and who demonstrate that products or systems conform to the requirements of recognized functional safety standards – are equipped to better manage risk while also capture increased share of market among the growing ranks of customers who also seek to meet the requirements of functional safety standards. This paper provides an overview of functional safety concepts, standards requirements and methods of compliance. It is intended to help readers understand the importance of functional safety and the advantages of obtaining formal evaluation and certification services performed by an independent third-party. Why Functional Safety? Evaluation and certification of systems involved in a safety related system and products to confirm that the functional application, with SIL4 used to protect IEC 61508 is the international standard for safety requirements of IEC 61508 have been against the highest risks. safety related systems associated with met is just one of the methods of dealing Safety function requirements are defined electrical, electronic and software-based with hazards control. An appropriate initial though a hazard analysis while safety technologies. The principles of the standard hazard analysis must be implemented to integrity requirements are derived from an can also be extended to assess mechanical define the level of risk and determine if assessment of acceptable risk. IEC 61508 elements if they are used in the safety function. functional safety is necessary to ensure may cover both determining the SIL level of adequate safety protection. IEC 61508 is an umbrella (generic) standard, a product and verifying the manufacturer intended to form a basis for sector-specific The IEC 61508 standard defines requirements specified safety integrity level (SIL) level. standards, including: for determining the level of risks and describes The higher the SIL assigned to the safety • IEC 61511 process industry the lifecycle process for ensuring that system or component, the lower the systems are designed, validated, verified, likelihood of dangerous failure. Instruments • IEC 61513 nuclear industry operated and maintained to perform a covered by these requirements might • IEC 62061 & ISO 13849 machinery industry specific function or functions to ensure include sensors, detectors, signal risk is kept at an acceptable level. IEC 61508 conditioners, logic controllers, monitors, • EN 50402 gas detector systems defines four SILs according to the risks alarms, actuators, valves and motors. • EN 50126 rail industry North America | Europe | Asia • www.csagroup.org UNDERSTANDING FUNCTIONAL SAFETY Demonstrating Functional Safety The Benefits of Functional Safety Organizations that provide a safety related service, or operation involving safety systems, In a highly complex, safety-related system Evaluation of systems and components, and can be approved for the technical and where functional safety is required, equipment certification that they meet the requirements management processes that govern their suppliers should identify an accredited of the applicable functional safety standards, functional safety activities (e.g., plant approval body (third body) that can evaluate provides designers, owners, operators and operators, systems integrators, contract and certify compliance with the IEC 61508 other stakeholders with increased confidence designers, product suppliers etc). This or applicable industry-specific standard. that processes operate safely, products meet type of approval covers the organization’s Accreditation means that the third-party regulations and industry requirements, risk generic processes as well as the competency agency has an internationally recognized has been appropriately managed, and the of its staff. This approval can be very useful in approval that qualifies it for functional potential for costly litigation has been earning new business, or in satisfying ongoing safety conformity assessment. minimized. contractual or regulatory requirements. The agency should demonstrate its Equipment suppliers can also leverage their experience and expertise in functional functional safety certifications to gain market Example of Functional Safety safety with a highly knowledgeable staff advantage, access new markets and achieve The following example explains the basic capable of carefully performing conformity sales growth among customers who require principles of functional safety. The diagram assessment requirements while providing products that meet a given SIL for use in below shows a relatively simple operation levels of service that help clients optimize their safety related applications or systems. from the process industry: filling a bulk their businesses. Agencies should be This “product-of-choice” status is reinforced storage fuel tank. Questions that must be independent third parties subject to annual by subsequent positive assessment reports answered initially include: What hazards audits by the accrediting body. Qualifying from customers whose products and systems are associated with this application? What agencies must provide annual compliance are also certified, further demonstrating can go wrong in the process? What are the evidence to the accreditation agency as full compliance with functional safety risks? How safe is the application? How proof of full conformance with the requirements. safe does it need to be? Other questions requirements of IEC 61508. may also apply. In this illustration, tank overfill is clearly one of the main hazards that must be addressed. North America | Europe | Asia • www.csagroup.org UNDERSTANDING FUNCTIONAL SAFETY That might sound basic, but consider what supplier and one of the instrument SIS Implementation can happen if an overflow occurs… suppliers. The incident could have been prevented if the hazard and risk had been With the hazard and risks identified, safety correctly identified and an appropriate requirements can be assessed and an target SIL established, resulting in the acceptable target SIL established, resulting implementation of an appropriate overfill in the design of a safety related protection protection system and an operational system – an SIS loop – implemented as functional safety management system. shown below. In a scenario like this, specifying a “safety- The SIS (in red) in this example might be related system,” usually called a “Safety specified as follows: Instrumented System” (SIS) in the process Safety Function: To close the emergency industry, can reduce risk to a target SIL shut off valve and switch off the pump in deemed acceptable based on assessment the event that the high-high level switch Oil storage depot, Buncefield, UK, December 2005 of the hazard. Depending on the target SIL, contacts are opened risk level can be reduced by at least… Miraculously, no-one was killed in this Safety Integrity: To perform the safety incident (it was 6:00 am on a Sunday • SIL1 by ≥10 times function to SIL2 (that’s a probability of the morning), but dozens of surrounding • SIL2 by ≥100 times independent safety function failing to work businesses were devastated. Several of less than 1 in 100 trips) companies were prosecuted and found • SIL3 by ≥1,000 times guilty in criminal and civil courts – including • SIL4 by ≥10,000 times the owner/operator, the control system North America | Europe | Asia • www.csagroup.org UNDERSTANDING FUNCTIONAL SAFETY Basic Steps in Achieving prepared, as it is one of the most important Functional Safety phases in the lifecycle. Functional safety Common Functional Safety standards emphasize the importance of Terms and Concepts 1) SIL Determination capturing functional requirements, deriving Once the hazards and risks have been more detailed design requirements (right • Functional Safety is when safety identified, using “HAZOP” analysis, a SIL down to low level hardware and software) relies on: Determination study can be prepared and tracing these through the design and – Safety function(s) – what the (normally arranged by the plant/machine development stages, integration and testing equipment does, and operator) to establish the Safety Function(s) process, and through to final validation – Safety integrity – how reliably and the amount of risk reduction required (assessment to the product lifecycle). At the the equipment does it end of every stage of the product lifecycle, of the safety system, which then defines • It is aimed at systems, typically a verification process must be followed its SIL. The IEC 61508 standard shows the formed from discrete instruments to capture any details not fully addressed requirements for failure data which are such as: that can affect compliance. This supports
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