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Spring 10

What is SIL? What is IEC 61508?

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IEC 61508 and SIL (Safety Integrity Levels) are fast becoming ‘buzz’ words in the gas detection industry. As a result, it’s no surprise that Crowcon are making sure that they don’t miss out on the most up to date information on this standard and are assessing its products against it.



Related Standards

IEC 61508 is a safety standard adopted for safety systems (such as gas detection systems) that:

  • Demands specific quality requirements on the development and manufacture of equipment
  • Statistically demonstrates that the system will perform its key safety function (eg. trigger a shut-down function) in the event of a specified failure event (for example, a gas being detected).


A ‘system’ will usually incorporate many components, all of which may contribute to the overall safety function and ultimately the SIL rating it achieves. However, only the whole ‘system’ can be validated to the standard not individual components. As such, individual components (eg. a gas detector, a control panel, a valve) can only be validated for use within a SIL 2 system but not themselves classified as SIL 2. Individual components can not perform the complete safety function in isolation of the rest of the ‘system’.

The SIL level defines the level of safety performance of a critical control system. SIL levels are ranked 1 to 4, with the highest level (SIL 4) indicating greater safety performance.

To increase the SIL rating of a system, a technique called ‘system redundancy’ may be used. System redundancy provides a safeguard so that if a component fails, services are retained or can be restored in the shortest time possible. Some systems can be dual and triple redundant (e.g. dual redundancy could be achieved by using two gas detectors instead of one).


Validation to the IEC 61508 standard demonstrates the probability that a system will perform a stipulated safety function. If for example, a system is designed to close a gas valve upon detecting a gas leak, a SIL 1 ranked ‘system’ has a 90% probability that the executive action chosen (i.e. the closing of the gas valve) would be taken. If a ‘system’ is validated to SIL 2 it has a 99.00% probability that the chosen executive action would be taken. In turn, a SIL 3 and SIL 4 ’system’ would have a 99.90% and 99.99% probability, respectively.


In most applications, gas detection systems are typically required to be validated to SIL 1, 2 or 3 though SIL 1 and 2 are generally adequate.


As well as “SIL” and “61508” some of the other ‘buzz’ words you may hear being used are listed below. If you hear any of these terms being used it is very likely that the application requires an IEC 61508 compliant system.

SIL: Safety Integrity Level: SIL 1-SIL 4. The higher the SIL of a safety related system, the lower the probability that it will not perform the required safety functions.

SIS: Safety Instrumented System – provides safe control functions for processes.

SFF: Safe Failure Fraction – A measure of the effectiveness of the fail safe design and/or the built-in diagnostic tests. In other words, a system with a high SFF will indicate a low probability of a dangerous failure.

PFD: Probability of Failure on Demand – Average probability of failure of a safety function working in low demand mode of operation.

Parameters such a SFF and PFD will usually be stated on the IEC 61508 Declaration for each product. (This information would be used by the engineer compiling the system to assess its overall SIL rating.)

Crowcon Validated Products

Crowcon products validated to IEC 61508 are:

Product SIL 1 SIL 2 SIL 3 SIL 4
Xgard Type 1 (toxic)
Xgard Type 1 (O2)
Xgard Type 1 (toxic; biased-NO, HCl, ETO, VOCs)
Xgard Type 2 (toxic)
Xgard Type 2 (O2)
Xgard Type 3
Xgard Type 4
Xgard Type 5
Xgard Type 6

Crowcon products that are validated to IEC 61508 can be issued with a Declaration of Conformity like the one below. This declaration states that the product has been tested by Crowcon and has been found to conform to IEC 61508. This proves that the product is SIL compliant and can be used in a SIL system.


Related Standards

EN 50402 – “Electrical apparatus for the detection and measurement of combustible or toxic gases or vapours or of oxygen. Requirements on the functional safety of fixed gas detection systems”.

This standard considers the safe and reliable integration of gas detection systems. It defines frameworks to ensure that the SIL levels of the different combination of parts of the gas detection system comply with the reliability of other systems already installed in the plant. The standard illustrates the functional parts as being the outputs and inputs from/to the control units, including the detectors, signal transmission and gas sampling.

IEC 60079-29-3 – “Explosive atmospheres – Part 29-3: Gas detectors – Requirements of the functional safety of the fixed gas detection systems”.

This standard is forecast for formal adoption in 2011, and is currently under development.

To find out more about IEC 61508 or SIL ratings contact Crowcon at or visit the website at

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