Safety integrity level , concept

This International Standard has two concepts, which are fundamental to its application safety lifecycle and safety integrity levels. The safety lifecycle forms the central framework which links together most of the concepts in this International Standard. 

The determination of Safety Integrity Level (SIL) for safety instrumented functions (SIF) is a basic concept of performance based safety standards. The safety integrity or performance of a SIF must increase with higher SIL levels. There are a number of factors that influence the performance of any safety instrumented function. Some of the key factors are ... 

During the 1990s the concept of Safety Integrity Levels (SIL) was developed [1]. It serves to assess safety-related systems and concerns aU components and subsystems required to realize safety functions from the sensor to the final element. Apart from that it applies to application software, which was developed for systems with limited variability language (no branching) or programmable logic controllers (PLC). 

A basic concept of functional safety analysis related to a process of determining required safety integrity level (SIL) is described as below ... 

The concept of safety integrity level (SIL) has two aspects, the first relating to functions and the second relating to systems. 

Phases 1 to 5 deal with concept and analysis—risk assessments to discover the Safety Instrumented Functions (SIF) and assign an appropriate Safety Integrity Level (SIL) rating. 

For safety-critical systems and software, the concept of Safety Integrity levels (SIL) is a source of great debate. The table below sets out limit claims that can be made for a given SIL level. 

The concept of safety integrity levels (SELs) was introduced during the development of BS EN 61508 (BSI 2002) as a measure of the quality or depen bility of a system which has a safety iimction - a measure of the confidence with which the system can be expected to perform that fimctioa It is also used in BS lEC 61511(BSI 2003), the process sector specific application of BSEN 61508. 

Defining criticality as a requirement on evidence for individual services may be more intelligible and acceptable to OS suppliers than the concept of a Safety Integrity Level (SIL) applied to the OS as a whole. 

A Service Assurance Level (SAL) concept may be useful when all or part of the safety mitigation required to produce a sufficiently safe service is placed within the service operation itself, rather than within the software or systems (as is usual with a Development Assurance Level or Safety Integrity Level (SIL)). 

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. 

During the 1990s this concept of safety-integrity levels (known as SILs) evolved and is used in the majority of documents in this area. The concept is to divide the spectrum of integrity into four discrete levels and then to lay down requirements for each level. Clearly, the higher the SIL then the more stringent the requirements become. In lEC 61508 (and in most other documents) the four levels are defined as shown in Table 1.1. 

Now that we have introduced the concept of safety integrity levels and described the life-cycle approach it is now appropriate to describe the structure of the lEC 61508 Standard. Parts 1—3 are the main parts (Figure 1.3) and Parts 4—7 provide supplementary material. 

The acronyms for the overall design safety concepts of the two standards are PL (Performance Level, ISO 13849-1) and SIL (Safety Integrity Level, lEC 62061). 

When the concept confirmation phase has been completed, all software requirements specifications and design documents need to be approved in order to enable commitment from all relevant parties to the implementation All these documents have to ensure that an adequate safety integrity level has been achieved. 

Since the 1970s, TOTAL has developed and implemented a very poweifiil range of methods and tools based on traditional approaches. They ate nsed on a daily basis to perform onr lehability analysis. However, in recent years international standards such as the lEC 61508 [lEC 00] and lEC 61511 [lEC 03] have proposed a new concept of safety integrity level (SIL) required to achieve a level of acceptable risk. They are progressively imposed on the design of SlSs and as their scope naturally concern HIPS, their implementation carmot be ignored. Much work was necessary to establish hnks between these two approaches and identify their similarities and differences. 

The ISO 26262 [ISO 08] standard is the variation of the lEC 61508 [lEC 98] standard of the automotive industry. Levels SIL 1 to SIL 3 are adjusted on a scale known as ASIL (automotive safety integrity level) described below with the impacting key points for the safety concepts. 

Finally, in Chapter 9 we present examples in the automotive field. The automotive field is currently evolving. This development will result in the establishment of a variation of the lEC 61508 standard for the automotive industry called ISO 26262. This standard takes the safety level concept (called here the automotive safety integrity level, or ASIL) and identifies recommendations for activities and methodologies for implementation in order to achieve a given safety... 

With an understanding of the role of safety instrumented systems in risk reduction we are able to introduce the concept of safety integrity and safety integrity level , (SIL). 

In order to get a scale of performance safety practitioners have adopted the concept of safety integrity levels or (SILs). The SILs are derived from earlier concepts of grading or classification of safety systems. The principle is illustrated in the next figure where the layer of protection provided by an SIS is seen to be quantified as a risk reduction factor from which it can be converted to a PFDavg and referenced to an SIL classification table. 

One advantage of the SIL concept is that it provides a 10 1 performance band for risk reduction and for SIS in each safety integrity level. Hence the classification of the safety system can be matched to a broad classification of the risk and the whole scheme is able to accept a reasonable tolerance band for the estimates of risks and risk reduction targets. 

Chapter 2 Hazards and risk reduction. An examination of basic hazards, the chemical process, hazards studies, the lEC model, protection layers, risk reduction and classification and the important concept of the safety integrity level (SIL). 

This section describes the multicore partitioning based safety concept for the failsafe wind turbine control and protection , which aims to reach the same integrity level while providing a suitable solution that overcomes limitations of the current federated approach. The safety concept is based on the strategy described in [32] and updates the safety concept described in [33], which summarizes the complete safety concept document positively assessed by a certification body [34], It is structured as follows ... 

This concept is further supplemented in die TIDE proposal by a speed cut-off directly at the prime movers if the zero velocity control command fails to act within a certain time span. Safety integrity is further enhanced by the use of safety monitoring, both of the central control functions and at the sub-system power module levels. The safety monitors would remove power from different areas, depending on the fault, and report back to the central display unit. In addition to the continuous action of closing a DMS, there is also need for a large red button, prominently displayed, as a final emergency off switdi. 

Facility System Safety (FSS), which is the application of system safety concepts to the facility acquisition process, has recently gained acceptance throughout the Department of Defense and most recently within the Department of Army with the conception of SAFEARMY 1990. The Army s goal is to fully integrate the total system safety, human factors, and health hazard assessments into continuous comprehensive evaluation of selected systems and facilities. The Chemical Research Development and Engineering Center (CRDEC) has mandated appropriate levels of system safety throughout the lifecycle of facility development for many reasons. These include ... 

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