Siemens safety or fail safe technology

Hi guys.Can somebody explain me or give me a practical example what is the fail-safe technology/signals . For what i read it is force plant into safe state after shut-down. I work in a paper industry, and for the press section of the paper machine there are some fail safe signals. Practically i know (correct me if i am wrong), that unlike standard signals, for fail-safe we double every signal (the master or first signal and the same compared signal) and if one of them fail we have a fault. How this help to increase safety for the machine and the people , compare to standard (non fail-safe) signals. Thank you in advance

TheoPap said:

Hi guys.Can somebody explain me or give me a practical example what is the fail-safe technology/signals . For what i read it is force plant into safe state after shut-down. I work in a paper industry, and for the press section of the paper machine there are some fail safe signals. Practically i know (correct me if i am wrong), that unlike standard signals, for fail-safe we double every signal (the master or first signal and the same compared signal) and if one of them fail we have a fault. How this help to increase safety for the machine and the people , compare to standard (non fail-safe) signals. Thank you in advance

Click to expand...

Simple version:

You have a light curtain around an automatic press, 20 feet away. If anyone crosses the line, the machine is supposed to freeze in place. The safety engineer has calculated that 20 ft is far enough away that the machine will stop before the person could reach it.

If that light curtain had only one signal back, then a short on that signal, or a failure in a contract or an input would prevent anything from detecting that the light curtain tripped. Two signals makes that much less likely. The two signals are supposed to be routed differently, or via different cables, or with other perfection methods. The modules also do fancy things with electricity to prove that the signal is not short circuited.

Thank you for your respond,I appreciate. But I would like to ask if we use for the same process standard technology,for what I know the result be the same..because if in normal operation we send 24V signal back to input module and we have open circuit,cable problem, the machine will stop..again if we have short circuit,the fuse will blow,interrupt 24V back to place, tripped condition and again the machine will stop..in case of module fault I think again machine will stop for the same reason.so for me is the same situation. Correct me if I am wrong.Thank you again

Fail-safe is a failure mode. When there is a fault with your equipment, you want the failure to cause a minimum amount of damage to people, itself, and the environment.

When a "fail-safe" module and a "standard" module are compared, the difference between them is the "fail-safe" module is designed differently to reduce it's fault(dangerous fault) probability when properly configured. It is able to detect a larger number of faults that a standard module cannot. Consider a "single-strand" fault where two I/O points on the module right next to each other become shorted, or an I/O point becomes stuck on. A standard module won't care and will happily let your press continue crushing whatever is in the way.

On any safety system, there are multiple components involved:

-The sensor itself
-The module registering the signal
-The programming interpreting the signal
-The contactor performing the safety function
-All the connections between them

All of these components can fail or have a problem that can disrupt the function of the safety system. Redundancy(2/3-channel circuits) plays a part in this in that additional faults can be detected and not compromise the operation of the safety system.

In the situation you describe, using a standard vs. a "fail-safe" module: The intended functionality is the same, however your probability of failure is orders of magnitude greater.

All systems should have a risk assessment performed where hazards and safety zones are identified. Only then can you determine what reduction measures you need to take.

Safety is one thing where shortcuts and cost-cutting measures should never be taken.


Pi

Thank you Pireciter for your response