Which devices are considered suitable for use in a SIF compliant with IEC 61511

There are 3 different ways that devices can be shown to be suitable for use in a 61511 Safety Instrumented Function. If any SIF device isn’t demonstrably compliant by one of these 3 ways, then the entire SIF is non-compliant. The 3 ways (or routes) are:

61508 Route 1 compliant (”by design”)

Route 1 “By Design” is the only way a product manufacturer can launch a new product on the market. By definition “new” means untried and untested in the real world. This Route requires product manufacturers to follow all the requirements of 61508 parts 1 to 4. This is a considerable undertaking – time consuming and often requires additional circuitry being added to the design to add diagnostics.

61508 Route 2 compliant (“proven in use”)

Route 2 “Proven in Use” allows a product manufacturer to claim that an existing design has turned out to be “good enough for SIL” mainly by reviewing product faults reported by customers. If the returns show that the typical rate of dangerous undetected failures is low enough, then this claim can be made. (There is a statistical significance test included, to ensure confidence in the numbers claimed). This route is defined in 61508.

61511 Prior Use compliant

“Prior Use” is defined in 61511 and follows similar thinking to Route 2 “Proven in Use”. Again the “good enough for SIL?” test is based on a review of actual failures – but this time done by the end user (plant operator) themselves. And again, there is a statistical confidence test for the values obtained from the test data of the device population. (Of course, you must have been keeping good proof test records to be able to do this. A typical operator may not have kept good records and may not have enough devices, that have been installed for long enough for prior use to be a viable option to demonstrate device compliance.)