Safety-Related Figures; Determination Of The Safety-Related Figures - AUMA AC 01.2-22X Manual

Safety-related figures

9. Safety-related figures
9.1.

Determination of the safety-related figures

Information
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The calculation of the safety-related parameters is based on the indicated safety
functions. The assessment of mechanical, electrical and electronic components is
based on Failure Modes, Effects and Diagnostic Analysis (FMEDA). FMEDA is a
method to assess the functional safety of a device according to IEC 61508. On the
basis of FMEDA, the failure rates and the fraction of dangerous failures of a device
are determined.
Experience data and data taken from the exida database for mechanical components
is used to calculate failure rates. The electronic failure rates as base failure rates
are taken from the SIEMENS Standard SN 29500.
In compliance with table 2 of IEC 61508-1, the average PFD value for systems with
low demand mode is:
SIL 2 safety functions: ≥ 10
SIL 3 safety functions: ≥ 10
Since actuators only represent a part of the overall safety function, the actuator PFD
should not account for more than approx. 25 % of the permissible total value (PFD
of a safety function. This results in the following values:
Actuator PFD for SIL 2 applications: ≲ 2.5E-03
Electric actuators with actuator controls are classified as type A components with a
hardware fault tolerance of 0. The SFF for the type A subsystem should be between
60 % and <90 % according to table 2 of IEC 61508-2 for SIL 2 (subsystems with a
hardware fault tolerance of 0).
Figure 17: Non-normative failure distribution assumed by AUMA
System power supply has not been considered for calculating the figures for actuator
and actuator controls.
As previously mentioned in the architecture section, safeguarding power supply and
resulting calculations are the responsibility of the plant operator.
The plant operator is responsible for complying with assumed MTTR. Otherwise the
data of the quantitative results is no longer valid.
SA .1 with AC 01.2-22X/-22Y/ACExC 01.2-22X/-22Y
-3 -2
to < 10
-4 -3
to < 10
Multi-turn actuators
)
avg