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which is contained either in a protective housing or a switching cabinet and the monitor
can be accessed only by trained personnel. Nevertheless, we recommend that before
the user inserts the parameterisation cable into the safety monitor he perform a
discharge (earthing) at a suitable location.
3.2
Consideration of failure probability according to IEC 61508
To allow calculation of failure probability for the entire system, the AS-i safety monitor
returns a component which depends on the maximum uninterrupted switch-on time of
the output circuit(s).
This results in the following table:
Switch-on time
3 months
6 months
12 months
Table 3.2-1:
The switch-on time describes the period of time until the safety function was requested,
or the maximum period of time between two executed safety checks. During this check,
safe shutdown is tested by actuating every safe sensor.
The total operating time describes the safety system's service life from commissioning
to disassembly; it is used as the basis for calculating the failure probability.
Together with the failure probabilities of the other components used in the safety
system (e.g. AS-interface slaves) it is then possible to determine the overall failure
probability. The resulting value can be used for categorising the safety system under
the respective safety level in accordance with IEC 61508.
78
Total operating time
Failure probability depending on switch-on time
ASM1, ASM1E
10 years
10 years
10 years
PFD
≤ 4 x 10
-5
≤ 6 x 10
-5
≤ 9 x 10
-5
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