The process sector allows devices to be selected for SIFs on the basis of prior use: documented evidence that the device has performed satisfactorily in a similar environment and application. It is an alternative to IEC 61508 certification for many field devices.
What the evidence must show
- The device version and its manufacturer's quality and change management.
- Operating hours accumulated in similar service and environment.
- Failures recorded, with modes, and the resulting failure rates.
- That the device's use in the SIF matches how it was used before (same function, configuration, diagnostics).
How to build it
Use maintenance records, proof test results and failure investigations. Define a minimum population and operating time before claiming prior use, and keep tracking failures afterwards.
Limits
Prior use evidence supports systematic suitability for lower SILs in particular; at higher SILs more is expected. A new firmware version or a different service may invalidate the evidence.
In First Out AI
Record "prior use" and its evidence reference in the source field of the group and set SC accordingly; the SRS draft lists every source.
Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.