Download warning - LUCKINSlive

A conservative assumption of 10% for the Safety PLC’s contribution to the overall
probability leaves more margin for the user and results in the following required
probabilities of failure for the Safety PLC:
-5
-4
z PFD ≥ 10 to < 10 for low demand
-9
-8
z PFH ≥ 10 to < 10 for high demand
PFD Equation Description
The IEC 61508 assumes that half of the failures end in a Safe state. Therefore, the
failure rate λ is divided into
z
z
λS - the safe failure and
λD - the dangerous failure, itself composed of
z λ DD - dangerous failure detected by the internal diagnostic
z λ DU - dangerous failure undetected.
The failure rate can be calculated by using the mean time between failures (MTBF),
a module specific value, as follows:
λ = 1/MTBF
The equation for calculating the probability of failure on demand is:
PFD(t) = λ DU x t
t represents the time between 2 proof tests.
The probability of failure per hour implies a time interval of 1 hour. Therefore, the
PFD equation is reduced to the following one:
PFH = λDU
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