Functional Safety PFH vs PFD according to IEC 61508

H

hamze60

#1
Hi,
I have a question about safety definition in high demand/continous and low demand systems ( according to IEC 61508).

- for low demand system safety is defined by PFD. PFD, is affected by two parameters, rate of entering to hazardous state and rate of leaving it (proof test).
- for high demand or continous systems, safety is defined by PFH. PFH, is only defined as rate of entering to hazardous state. Why the rate of leaving of such state is not included? why proof test frequency has not effect in PFH calculation?
Thanks
 
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