Automation equipment in control applications is selected primarily based on operational needs, such as functionality, reliability, repeatability, accuracy, communication options and ease of maintenance. Demonstrated in-service performance earns the equipment and its manufacturer a coveted place on the approved equipment list. Equipment in safety applications must also support risk reduction requirements in addition to meeting operational needs. For safety instrumented systems (SIS), the concept of approving the initial selection and continued use of equipment based on in-service performance is referred to as proven in use in IEC 61508 and prior use in IEC 61511. Ultimately, the intent of both standards is to collect data that proves the installed equipment is capable of providing satisfactory in-service performance.
Manufacturers often make safety integrity level (SIL) claims on their products, but SIL is a loop concept and not a device property. An SIL 1 sensor connected to an SIL 1 logic solver with an output to an SIL 1 final element may not achieve an SIL 1 loop. Like links in a chain, the integrity is limited by the weakest link, but the chain is only as strong as the whole. In many cases, it is necessary to design some, if not all, subsystems (links) to achieve a higher SIL claim so the loop (chain) meets the required SIL.
The most troublesome result of the IEC 61508 certification process is how nonconservative the manufacturer claims appear to be. The failure rate data published by various industry data collection programs has revealed dangerous in-service failure rates are significantly higher than what is claimed in the majority of third-party approval reports. For field devices, the reported values are generally 3-10 times lower than what is seen in actual installations. For programmable logic controllers (PLCs), the manufacturerâs claims can be nonconservatively lower than in-service performance by a factor of 10 or more. This means a lot of certified equipment is sold based on an SIL claim at least one level higher than achievable in the installation.
IEC 61511 only requires the use of compliant equipment when applying PLCs in SIL 3 applications. For all other technologies, clause 11.5.2.1 states, âDevices selected for use as part of a SIS with a specified SIL shall be in accordance with IEC 61508-2:2010 and IEC 61508- 3:2010, and/or 11.5.3 to 11.5.6, as appropriate.â This clause lists two forms of evidence that can be used to select devices for SIS applications:
⢠Evaluation for compliance with IEC 61508 Part 2 (hardware) and Part 3 (software). This typically involves third-party approval of a specific configuration of a product.
⢠Prior use or historical data. This data is typically derived from the deviceâs performance in similar operating environments.
When weighing these two forms of evidence, recognize the more relevant the information is to the in-service environment, the higher the certainty the actual failure rate will align with the assumed reliability parameters. In-service data is essential to understanding the real potential for human (or systematic) errors. In contrast to IEC 61508, prior use identifies not only hardware failures and their root causes but also systematic failures, which is essential for achieving industry benchmarked performance.
IEC 61511 acknowledges the importance of in-service records for justifying the continued use of existing equipment. The newly required functional safety assessment involves a periodic examination of site operating and maintenance records to determine whether the installed SIS is being managed as planned and complies with the safety requirementsâ specifications.
IEC 61511âs quality metrics are also appropriate for proving the fitness for purpose of equipment in any safety control, alarm and interlock application. Fundamentally, this approval process involves making an engineering judgment on the equipmentâs design quality, functional capabilities, use factors, in-service history, failure rate in the operating environment, and ability to fulfill the safety requirementsâ specifications for the particular application.
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