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The mean time to dangerous
failure (MTTF d ) is initially
determined for each redundant
channel. Next, a total MTTF d
value is determined for both
channels. This value is given in
years and is a qualitative
statement of the safety function.
In line with the applicable
standard, the technical safety
measure is assessed and given
one of three classifications: low,
medium and high.
Defining the mean time to dangerous failure (MTTF d )
Input signal
Input
Control signal
Logic
Output
MTTF
d
Evaluation
Low
Medium
High
Source: DIN EN ISO 13849-1 Chapter 4.5.2
MTTF
d
3 years ≤ MTTF d < 10 years
10 years ≤ MTTF d < 30 years
30 years ≤ MTTF d < 100 years
Characteristic service life values from the technical data
B 10
MTTF
d
MTTF
d
1______
MTTF
d
=
N
i=1
1_______
MTTF
d,i
Application
data