I was recently approached with a question regarding why it would be necessary to certify mechanical devices, such as valves, per IEC 61508 when it is not clearly stated in the standard that the standard is applicable to such devices.  After looking through IEC 61511 and IEC 61508 regarding the application of the standards to mechanical components, I came to the conclusion that mechanical components are not specifically mentioned but are also not specifically excluded from the requirements of the standards. 

The IEC has addressed this very issue in a FAQ for Edition 2 of 61508.  The question posed was “How does IEC 61508 apply where E/E/PE technology makes up only a small part of the safety-related system?”  The response was as follows:

“The IEC 61508 website states that IEC 61508 is applicable to any safety-related system that contains an E/E/PE device.”

Also from the IEC 61508 website:

“This applicability is appropriate because many requirements, particularly in IEC 61508-1, are not technology specific. Indeed, early development phases (such as initial concept, overall scope definition, hazard and risk analysis and specifying the overall safety requirements) may take place before the implementation technology has been decided.”

“Even during later phases such as realization, specific functional safety requirements apply directly to non-E/E/PE devices, such as mechanical components, as well as E/E/PE devices. For example, the requirements for hardware reliability and fault tolerance in IEC 61508-2 directly relate to the properties of all components in the E/E/PE safety-related system, whether or not they include E/E/PE technology.”

The greater issue here is one of safety.  If a device is not qualified for the Safety Integrity Level (SIL) required by the SIF by either a third party certification process or a proven in use analysis by the end user, there is significant risk that that device may not meet the SIL requirements of the SIF.  Third party certification such as the one exida performs assures that the device meets the requirements of IEC 61508 and that the manufacturer’s quality management procedures also meet the requirements of IEC 61508.

The exida certification process places particular emphasis on the requirements for the avoidance of systematic faults.  (See Clause 7.4.6 of IEC 61508 part 2)

exida has certified approximately 100 devices and it is rare that a manufacturer will pass the initial audit without strengthening of procedures that directly relate to the avoidance of systematic faults.

The bottom line here is that an IEC 61508 certification assures that the manufacturer of the device has a quality management system in place that meets the requirements for avoidance of systematic faults for the SIL level required.  The certification process also assures that any detected systematic faults on the device have been properly addressed through an approved modification procedure.