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Understanding Proof Test Intervals for Simatic Safety Systems

Dear Ghassan,

You've raised a common and important point regarding Proof Test Intervals (PTI) and the associated PfD claims for Simatic Safety Systems. You are correct to identify the relationship between Mission Time and Proof Test Interval – Siemens designs these systems with Working Life = Proof Test Interval.

However, the situation can appear complex due to different values presented. Here's clarification:

• Mission Time: Represents the maximum intended operational life of a component, as per IEC 61508.
• Proof Test Interval (PTI): The frequency at which a proof test must be executed to maintain the safety function's target Safety Integrity Level (SIL).
• Component vs. Function: A component (like 6ES7136-6RA00-0BF0) may have a long Mission Time (e.g., 20 years). However, the function it supports within a safety system requires proof tests at intervals determined by the required SIL (e.g., 1 year or 1 month).

The shorter PTI for a function doesn’t contradict the component’s long Mission Time. It reflects the overall system's need to demonstrate continued compliance with the target SIL throughout its operational life.

To determine the correct PTI for your specific safety functions, please refer to the following resources:

• Simatic Safety System Manuals: Specifically, the sections detailing safety function diagnostics and testing.
• Safety Integrity Level (SIL) documentation: Siemens provides documentation outlining PTI requirements based on SIL level. See this FAQ for more details.
• SiePortal: Use SiePortal to search for specific component data sheets and safety manuals. Link to SiePortal

Essentially, the PfD is *claimed* based on the assumption that these proof tests are performed at the specified intervals.

Sincerely,
Siemens Self Support