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TI Home » TI E2E Community » Support Forums » Microcontrollers » Hercules™ Safety Microcontrollers » Hercules™ Safety Microcontrollers Forum » PFD and PFH
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PFD and PFH

PFD and PFH

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Fei Dai
Posted by Fei Dai
on Sep 07 2011 08:41 AM
Prodigy70 points

What's the difference between PFD and PFH?

I use Markov model to calculate PFD. How to calculate PFH using Markov model?

Thanks

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  • QJ Wang
    Posted by QJ Wang
    on Sep 07 2011 10:29 AM
    Expert4390 points

    Hi Dai Fei,

    Please refer to IEC 61508 for the details regarding PFD and PFH. As far as I understood, PFH is rate or frequency, but PFD is function of time. The relation can be simplified as:

    PFD = PFH(detectable failure in time interval T) * T/2 + PFH(undetectable failures) * t

    Regards,

    QJ

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  • Fei Dai
    Posted by Fei Dai
    on Sep 08 2011 09:45 AM
    Prodigy70 points

    I think, from Markov Model viewpoint, PFH equal to sum of the element of transition-matrix P in  dangerous state.  Right?

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  • QJ Wang
    Posted by QJ Wang
    on Nov 28 2011 15:01 PM
    Verified Answer
    Verified by QJ Wang
    Expert4390 points

    PFH is the average of the matrix P over a period time. After you sum or integrate the matrix P, you need to divide the sum by the time Tm to get the PFH. 

    Regards,

    QJ

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