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An Introduction to Functional Safety and Safety Integrity Levels (SIL)
Introduction
Whenever something is built (Of IT equipment or plant) which are liable to introduce people, or even safety legislation is likely to be involved!
If that is reasonably expect action or inaction leads to risk an unacceptable risk arising from the equipment or plant, then the safety features needed in order to achieve or maintain a safe operation. These safety functions are performed by one or more safety-related systems.
Initially, significant attention to the elimination of hazards. This could apply the inherent safety of the principles or the application of the rules of good craftsmanship. The is likely however that in many cases will not be possible (or cost will be priceless) and some residual risk will remain.
It is at this stage that we analyze risks and take appropriate action. One of the most common forms of appropriate measures (and a defined and published methodology) is the application of functional safety and integrity of the security levels.
Functional safety
There are two different aspects of functional security, safety function requirements (what the function is the safety) and "Safety Integrity 'requirement (the probability of the safety function properly when called upon to do so).
The safety function requirements are derived from the 'hazard analysis' (ie Process Hazard Analysis, Failure Mode Analysis) and safety integrity requirements are derived from a 'risk assessment' (ie impact analysis). To ensure that safety is achieved, both hazard and risk assessment is necessary.
The risk analysis identifies the hazards associated with the process or operation, risk assessment determines the performance requirements safety function. The aim is to ensure that the safety integrity of the security function is adequate to the risks associated with this dangerous event guaranteed is reduced to an acceptable level.
Example of functional safety
To strengthen the definitions, we will consider a machine tool with a guillotine blade that is protected by a sliding guard access to the shearing element of the blade to avoid. The guillotine blade is accessible for routine maintenance to pass the guard opened. The guard is attached, so whenever it is opened by an electrical circuit to power the machine tool. Therefore, the operation of the guillotine blade is stopped before the operator can access and the possibility of shearing injury is prevented. To ensure that safety is achieved, both hazard and risk evaluation.
a) the risk, the risks associated with the maintenance of the guillotine blade. The machine tool may demonstrate that it is not possible for the guard to open the machine tool without the energy supply. That article describes the security function.
b) Risk assessment determines the performance requirements of the safety function. The aim is to ensure that the safety integrity of the safety function is sufficient to ensure that nobody is exposed to an unacceptable risk from this dangerous event.
The damage resulting from a failure of the safety function can be shear amputation parts of the operator's limbs. The risk also depends on how often the guard to open. The level of safety integrity required increases with the severity of the injury and the frequency of exposure (how often the guard is opened) to the hazard.
Safety Integrity Levels
Process and Machinery is often achieved through the use of Safety Instrumented Systems (SIS) to provide safe control functions for processes, functions like this would STOP, gas or fire detection, constructive and even dangerous level / pressure control.
Safety Instrumented Systems are typically composed of a certain type of sensors (eg motion, pressure, temperature, etc.), analyzers / processors (eg relay logic, PLC) and control elements (eg drive, alarm). The integrity of each of these elements are combined to a "system" to produce Safety Integrity Level, so individual potential failure rate and manner of each component must be known to the integrity of the system to measure.
Safety Integrity Level related to probability of failure on demand
SIL PFD Risk Reduction Factor (1/PFD)
April 10-5 10-4 100000-10000
March 10-4 to 10-3 for 10.000 1000
10 to 3 February at 10-2000 to 100
January 10 to 2 at 10 to 1 100-10
A Safety Integrity Level (SIL) is a simple numerical representation of the reliability of Safety Instrumented Systems (SIS) was correlated with the probability of failure on demand (PFD), which is expressed as the unavailability of a system when a certain undesirable event, such a process could not injure people.
Integrity Level related to probable damage levels
SIL qualitative terms
4 Potential number fatalities in the community or major facilities on site
3 Potential for multiple fatalities
2 Potential for major injuries on site or single fatality
1 Potential for minor injuries on site
The level of SIL necessary for a system will be determined by analyzing the frequency of occurrence, the probability of detection or avoidance of the event and the result of the event. SIL is therefore determine the degree of protection needed to reduce the risk of an undesirable event to an acceptable level.
Determining the SIL level according to IEC61508
While the table for determining SIL IEC 61508, relatively simple, the task of Target should Sils involved people with relevant expertise and experience in the systems, processes and risks into account. IEC 61508 specifically states: "All persons involved in an overall safety lifecycle activity, including management activities, the right training, technical knowledge, experience and qualifications relevant to the specific duties they have to perform. "
Appropriate hazard analysis tools should be used (eg HAZOP, Fault Tree Analysis) and, as with any hazard analysis, all considerations and decisions must be documented. Once SIL is a set (through the risk assessment and analysis process) design of the SIS operation and maintenance choices must then be implemented and tested against the target SIL standards require.
One of the primary international standards IEC 61508 Safety Integrity is, "Functional safety of electrical / electronic / programmable electronic safety-related systems. "IEC 61508 has defined four categories of safety as SIL 1,2,3 and 4. (In North America under ANSI/ISA-S84.01 there are only three categories defined SIL 1, 2 and 3). In simple terms, the higher the SIL, the more reliable or more efficient the system is (and how it will be difficult to achieve)
Application of IEC 61508 series of standards.
IEC 61508 series of standards is under the common title "Functional safety of electrical / electronic / programmable electronic safety related systems" and consists of the following components;
Part 0: Functional safety and IEC 61508
Part 1: General requirements;
Part 2: Requirements for E / E / PES related systems safety;
Part 3: Software requirements;
Part 4: definitions and abbreviations;
Part 5: Examples of methods for determining the safety integrity levels;
Part 6: Guidelines on the application of IEC 61508;
Part 7: Overview of measures and techniques.
There is a large amount of information within these standards (approaching 1,000 pages) and it can be a daunting task to approach and understand through the SIL standards seem It is recommended that if you are new to the standards (and / or subject of SIL) that you start reading the following sections.
Annex A of IEC 61508-5, relating to the integrity of risk and safety concepts in a simplified form
Figure 2 and Table 1 of IEC 61508-1, the total lifetime of a machine and a list of the objectives of each phase of the life cycle to illustrate. The lifecycle phases and objectives are a key to understanding the requirements of Article 7 of IEC 61508-1.
Appendix A of IEC 61508-6 who 61508-3 outlines the requirements of IEC and IEC 61508-2.
Clauses 6 and 8 of IEC 61508-1, which require regarding the management of functional safety and functional safety contain.
Figure 2 and Table 1 of IEC 61508-2 and Figure 3 and Table 1 of IEC 61508-3, which is a key to understanding the requirements of IEC 61508-2 and IEC 61508-3 provide
If you drive to perform your first functional safety assessment or SIL, It is highly desirable to undertake training in this area and / or partner with a company that has experience in this field if you have no experience in this field.
SIL and the Machinery Directive
BS EN 62061 standard is an industry with the seven-part standard IEC 61508, "Functional safety of electrical / electronics / programmable electronic safety related systems', written especially for the machinery sector. It takes a quantitative risk-based approach similar to that found in EN 61508, which is rather more work than the qualitative 'risk graph' That was in the former required standard EN 954-1.
Besides managing and documenting requirements, Specific technical measures are needed and described. There are requirements for the specification of safety-related functions Control (SRCFs), and the standard explains how the functional requirements specification and safety integrity requirements for each SRCF should be drawn to the safety requirements (SRS) to create. Three safety (SIL 1, SIL 2 and SIL 3), specified and they require that the probability of dangerous failure per hour (PFHd) must fall between certain targets as follows:
SIL P robability of dangerous failures per hour
SIL 1 ≥ 10-6 to <10-5 (or 1 error in 100,000 h)
SIL 10-7 to ≥ 2 <10-6 (or 1 in 1,000,000 non-h)
SIL 3 ≥ 10-8 to <10-7 (or an error of 10,000,000 h)
There are also specific requirements for the selection or Design of safety related electronic control (SRECS) to meet the functional security and integrity requirements specified safety requirements (SRS). This is also identify the likelihood of harmful interference (PFHd), estimation of Safe Failure Fractions (SSF), Common Cause failures (CCF) and diagnostic functions.
SIL and ATEX
It should be noted that design standards ATEX line with the requirements of SIL. They concern the safety related devices (for ignition prevention) and will be implemented by the notified bodies under the ATEX Directive. Under DSEAR (ATEX 137) should be noted that if safety devices or systems are added to the formation of an explosive atmosphere (air to avoid dilution, gas monitoring) or reduction of an explosion likely a functional safety.
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