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Dear,
Under actuator group, under consideration of safety integrity, I need to answer following questions which are not very clear to me:
* MTTFd of testing channel? >50% / <50% of the functional channel.
* Safe state via Testing Channel? Yes/No.
My application:
A safety actuator (contactor) is switched according to level 2 (single channel with feedbackloop). The contactor is switched by a safety PLC output channel. The connector has a feedbackloop to a normal PLC input. This input is used as evaluation in the safety program.
How can I evaluate the MTTFd of the feedback channel?
What do they mean with safety state via testing channel?
Thanks in advance,
Tom Bauwens
Dear Tom Bauwens,
Regarding your questions concerning safety integrity for your actuator group (contactor switched by a safety PLC with feedback), here’s a clarification based on your Level 2 (single channel with feedback loop) application:
The MTTFd (Mean Time To Fault Detected) of the testing channel should ideally be >50% of the functional channel. This ensures adequate diagnostic coverage. The testing channel's diagnostic coverage directly impacts the overall safety level achievable. Calculating this requires considering the failure rates of all components within the testing channel (feedback loop in your case, including the PLC input module).
Yes, a safe state *can* be achieved via the testing channel. In your setup, the feedback loop to the normal PLC input, evaluated within the safety program, *is* your testing channel. A failure in the contactor (stuck closed) or feedback loop (wire break) should be detected by the safety program, driving the system to a safe state (e.g., de-energizing the contactor through the safety PLC).
To evaluate the MTTFd of your feedback channel:
Consider using Siemens' safety evaluation tools and consulting relevant safety standards (e.g., IEC 61508, IEC 61511) for detailed guidance.
For further assistance, please refer to SiePortal's documentation and knowledge base: SiePortal.
Sincerely,
Siemens Self Support
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