Frequently Asked Questions - Exida

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3.2 SCOPE OF THE STANDARDThe IEC 61508 standard covers safety-related systems when one or more of such systemsincorporates mechanical/electrical/electronic/programmable electronic devices. These devicescan include anything from ball valves, solenoid valves, electrical relays and switches through tocomplex Programmable Logic Controllers (PLCs). The standard specifically covers possiblehazards created when failures of the safety functions performed by E/E/PE safety-relatedsystems occur. The overall program to insure that the safety-related E/E/PE system bringsabout a safe state when called upon to do so is defined as “functional safety.”IEC 61508 does not cover safety issues like electric shock, hazardous falls, long-term exposureto a toxic substance, etc.; these issues are covered by other standards. IEC 61508 also doesnot cover low safety E/E/PE systems where a single E/E/PE system is capable of providing thenecessary risk reduction and the required safety integrity of the E/E/PE system is less thansafety integrity level 1, i.e., the E/E/PE system is only available 90 percent of the time or less.3.3 FUNDAMENTAL CONCEPTSThe standard is based on two fundamental concepts: the safety life cycle and safety integritylevels. The safety life cycle is defined as an engineering process that includes all of the stepsnecessary to achieve required functional safety. The safety life cycle from IEC 61508 is shownin Figure 1.1Concept“ANALYSIS”(End User / Consultant)234Overall ScopeDefinitionHazard & RiskAnalysisOverall SafetyRequirements5Safety RequirementsAllocationOverall PlanningOperation &Installation &Validation6 Maintenance 78 CommissioningPlanningPlanningPlanning1213149Safety-relatedsystems :E/E/PESRealisationOverall Installation& CommissioningOverall SafetyValidationOverall Operation &Maintenance16 Decommissioning10Safety-relatedsystems : otherTechnologyRealisation11Overall Modification15& RetrofitExternal RiskReductionFacilitiesRealisationFigure 1: Safety life cycle from IEC 61508.“REALISATION”(Vendor / Contractor /End User)“OPERATION”(End User / Contractor)The basic philosophy behind the safety life cycle is to develop and document a safety plan,execute that plan, document its execution (to show that the plan has been met) and continue tofollow that safety plan through to decommissioning with further appropriate documentationthroughout the life of the system. Changes along the way must similarly follow the pattern ofplanning, execution, validation, and documentation. Although the standard is written in thecontext of a bespoke system, the requirements are applicable to general product design anddevelopment.© exida, 2007 IEC 61508 Certification Program – Frequently Asked QuestionsPage 6 of 13
Safety integrity levels (SILs) are order of magnitude levels of risk reduction. There are four SILsdefined in IEC 61508. SIL1 has the lowest level of risk reduction. SIL4 has the highest level ofrisk reduction. The SIL table for “demand mode” is shown in Figure 2. The SIL table for thecontinuous mode is shown in Figure 3.Safety IntegrityLevelProbability of failureon demand, average(Low Demand mode ofoperation)Risk ReductionFactorSIL 4SIL 3SIL 2SIL 1>=10 -5 to =10 -4 to =10 -3 to =10 -2 to =10 -9 to =10 -8 to =10 -7 to =10 -6 to
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Frequently Asked Questions - Exida

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