Safety-integrity level targets

Table 1. Safety integrity levels target failure measures for a safety function operating in low demand mode.

Safety Integrity Level Target failure measure (Average probability of failure to perform Its design function on demand)... 

To establish a Safety-Integrity Level target, vis-d-vis lEC 61508, for a Boiler Control System which is regarded as safety-related. 

Safety integrity levels (SILs) provide targets for risk reduction (Security EAL-Evaluation Assurance Level Correlation to SIL )... 

The required safety integrity level of the instrumented function shall be derived by taking into account the required risk reduction that is to be provided by that function. For those SILs, the target PFDj,yg on demand and the target frequencies of dangerous failure are hsted in Table 3.8 [ANSI/ISA-84.00.01(2004) Part 3] for each SIF. Several risk analysis methods ranging from qualitative to fully quantitative can be deployed based on the severity and complexity of the scope, as listed in Table 3.9. 

Table 3.8 Safety integrity levels probability of failure on demand, target risk reduction factor and target frequency of dangerous failure to perform the SIF.

Once the technology, architecture, and periodic test intervals are defined, the designers do a reliability and safety evaluation (Ref. 14 and 15) to verify that the design has met the target safety integrity level and reliability requirements. In the past, this probabilistic evaluation has not been part of a conventional design process. The effort requires gathering failure rate data as a function of failure modes for each piece of equipment in the safety instrumented function. 

Within the framework of [1] the standards [2]-[4] refer to the process industry. In these standards the continuous spectrum of failure frequencies and unavailabilities is divided into four discrete bands, the safety integrity levels, as shown in Tables 11.1 and 11.2. The bands apply to safety-related systems. These are systems which play a role for safety and can therefore in addition to safety systems also comprise elements from the operating level. The bands in Tables 11.1 and 11.2 are targets whose selection and fulfilment are presented below. 

Safety integrity level (SIL) Target bands for the frequency of dangerous failures of the safety-related function in h ... 

Suimners, Angela 1998. Techniques for Assigning a Target Safety Integrity Level, ISA Transactions 37 (1998). pages 95-104. 

In order to determine the required SIL level, a detailed hazard analysis is performed for the equipment under control (EUC). From the hazard analysis, all safety functions are identified (example—Detect failure of braking). A target safety integrity level is assigned to each of the safety functions (example—detect failure of braking—SIL 3) in order to ensure the residual risk is lower than the acceptable risk (in other words, the risk is sufficiently reduced). The outcome will be an EUC safety function specification detailing the function and target SIL level (between 1 to %) required for each safety function identified in the hazard analysis. 

Target Safety Integrity Level (SIL) ind it probability of dcingerous failure per hour value... 

The safety core-process itself starts with the Preliminary Hazard Identification (PHI) and the Preliminary Hazard Assessment (PHA). During diat phase a preliminary hazard list with severities is created via brainstorming and the use of historical data and checklists. Outputs are the preliminary hazard list, including severities and hazard target rates, and initial development process integrity level allocations as detailed in various standards, e.g. Safety Integrity Level (SIL) in lEC 61508 or CENELEC EN 50128. 

This chapter details all standards which have to be addressed, failure rate targets, allocated safety integrity levels, a description of requirements handed over from other safety cases and emerging from the hazard analysis and a reference forward to where each requirement is addressed. It also includes operational safety requirements. 

NOTE 1 The target failure measures for the safety integrity levels are specified in Tables 3 and 4. 

NOTE 2 The safety integrity level is defined numerically so as to provide an objective target to compare alternative designs and solutions. However, it is recognized that, given the current state of knowledge, many systematic causes of failure can only be assessed qualitatively. 

NOTE 1 In the case of safety instrumented functions operating in the demand mode of operation, the target failure measure should be expressed in terms of the average probability of failure to perform its design function on demand, as determined by the safety integrity level of the safety instrumented function (see Table 3). 

It uses safety integrity levels (SILs) for speeifying the target level of safety integrity for the safety functions to be implemented by the E/E/PE safety-related systems. 

In the first chapter we wiU introduce the concept of functional safety and the need to express targets hy means of safety integrity levels. Functional safety will be placed in context, along with risk assessment, likelihood of fatality, and the cost of conformance. 

Part 3 gives more detailed guidance on targeting the Safety Integrity Levels and has a number of Appendixes covering both quantitative and qualitative methods. 

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