The acceptance of the functional safety standards, IEC 61508 [1] and IEC 61511 [2] / ANSI/ISA 84.00.01:2004 [3] has increased drastically over the last year. Many end-users are requesting manufacturers to provide reliability information on specific products to perform the Safety Integrity Level (SIL) verifications that are required by the functional safety standards.

The SIL performance parameter for Safety Instrumented Functions relates to a range of average Probability of Failure on Demand (PFDavg) values for low demand mode (or demand mode) applications and a range of Probability of a Dangerous Failure per Hour (PFH) values for high demand mode (or continuous mode) applications.

The reliability calculations that are performed as part of the SIL verification allow end-users to optimize their Safety Instrumented Function design, through technology, redundancy, and/or proof test interval selections. Many SIL certified devices have been designed to achieve a low dangerous undetected failure rate which allows for longer intervals between proof tests while still achieving a low PFDavg. The advantages are obvious, less process interruption, less maintenance work, lower lifecycle costs.

The extension of proof test intervals could however result in a SIF application being classified as a high / continuous demand mode application. This paper will explain the rules for classifying an application as low demand or high / continuous demand. It will explain how exida uses three different modes of operation and the motivation for this. Why does an application that seems to be a low demand application, all of a sudden need to be considered as a high demand application? Finally, the paper will also explain the differences between low demand and high demand applications and the consequences for the SIL verification.

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