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Question: When Circuit breaker fails due to “Does not open” then, in case of electric discharge, PLC can be damaged but probability of that damage is 0,1 so our Critically number will be: Cm = 0,05 x 0,49 x 0,1 x 100%. I think this is correct but… Electrical discharge is very rare – I can estimate that 1 per year. What to do with that ?

Answer: Remember, Beta is your estimation of the probability that the failure mode will result in the associated severity. I think the above would be OK as-is. But, if you can reason that the probability is lower due to duty cycle or some other rationale, you could lower the Beta to what you believe is reasonable.

Question:. One of temperature sensor Failure Modes is “improper output”. In our case “improper output” can has different effects for example.
If “improper output” (wrong signal – wrong temperature indication) does exceed the regulator limits (< -100oC i >100oC) then PLC know about the sensor failure and everything is OK. If “improper output” (wrong signal – wrong temperature indication) does NOT exceed the regulator limits (>-100oC i <100oC) then there is a problem! If in that case temperature sensor understatement temperature then it is OK but if overestimates then this can be CRITICAL. So how to calculate beta in that case? beta = probability of exceeding the limits x probability of overestimates In my opinion if can be always 50/50 So… beta = 0,5 X 0,5 = 0,25 for CRITICAL effect?