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DG · Edition 08.13
28
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Glossary
9 .9 Mean dangerous failure rate
D
Mean rate of dangerous failures during operation time (T
10d
).
Unit: 1/h.
from EN ISO 13849-1:2008
9 .10 Safe failure fraction SFF
Fraction of safe failures related to all failures, which are as-
sumed to appear
from EN 13611/A2:2011
9 .11 Probability of dangerous failure PFH
D
Value describing the likelihood of dangerous failure per hour
of a component for high demand mode or continuous mode.
Unit: 1/h
from EN 13611/A2:2011
9 .12 Mean time to dangerous failure MTTF
d
Expectation of the mean time to dangerous failure
from EN ISO 13849-1:2008
9 .13 Demand rate n
op
Mean number of annual operations
from EN ISO 13849-1:2008
9 .14 Average probability of dangerous failure on
demand PFD
avg
(LDM = 1 – 10 switching cycles/year)
Average probability of a dangerous failure of the safety function on
demand
see EN 61508-6