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How to calculate PFD of redundant sensors
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IBM DUT
Moderated by: Michel Houtermans
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Friday 19 January 2007 9:10:26 am
How to calculate PFD of redundant sensors
Calculation of a measuring system with redundant sensors,
if PFD=1.5E-03 (sensor A), PFD=1.5E-03 (sensor B).
Then, how to determine the PFD value for the redundant connection of the two sensors?
Please tell me the detail of the key.
Thanks in advance.
Email:2983625@163.com
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Krishna Moorthy
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Tuesday 23 January 2007 12:08:44 pm
Re: How to calculate PFD of redundant sensors
For a 1oo2 arrangement
PFD (1oo2) = PFD (A)*PFD(B)+PFD(common cause A and B)
where
PFD(common cause A and B) = beta*PFD(of one sensor)
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håkan landsberg
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Monday 17 December 2007 10:19:03 am
Re:
Is it really so? (Assuming we are talking IEC 61508). According to the standard you shall only calculate PFD for one of the channels and then use the formula (a pretty long one!) from part 6 of the standard under "B.2.2.2" for low demand mode, or "B.3.2.2" for high demand mode.
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Michel Houtermans
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Thursday 20 December 2007 9:47:19 pm
Re: How to calculate PFD of redundant sensors
Ok, lets make 1 thing clear. Part 6 of IEC 61508 is NOT normative. So the formulas in that part are not required....
Actually the formulas in that part are simplified because they are derived from Markov models.
If you want to make the most accurate calculations I suggest you make a real Markov model. If you do not care and just want a simple calculation then the formulas from IEC 61508 are sufficient. They are conservative and do the job. Markov just gives you more accurate and more detailed results. For the people that really care.
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