Failure Rates; Assumptions - turck IM1-12-R Handbuch

4.2.2 Failure rates

The failure rate data used by exida.com in this FMEDA are the basic failure rates from the
Siemens SN 29500 failure rate database. The rates are considered to be appropriate for safety
integrity level verification calculations. The rates match operating stress conditions typical of an
industrial field environment similar to IEC 60654-1, class C. It is expected that the actual
number of field failures will be less than the number predicted by these failure rates.
The user of these numbers is responsible for determining their applicability to any particular
environment. Accurate plant specific data may be used for this purpose. If a user has data
collected from a good proof test reporting system that indicates higher failure rates, the higher
numbers shall be used. Some industrial plant sites have high levels of stress. Under those
conditions the failure rate data is adjusted to a higher value to account for the specific
conditions of the plant.

4.2.3 Assumptions

The following assumptions have been made during the Failure Modes, Effects, and Diagnostic
Analysis of the Isolating Switching Amplifiers IM1-**(Ex)-* and MK13-R-Ex0.
•
Failure rates are constant, wear out mechanisms are not included.
•
Propagation of failures is not relevant.
•
The time to restoration after a safe failure is 8 hours.
•
All modules are operated in the low demand mode of operation.
•
External power supply failure rates are not included.
•
Only one input and one output are part of the safety function
•
Sufficient tests are performed prior to shipment to verify the absence of vendor and/or
manufacturing defects that prevent proper operation of specified functionality to product
specifications or cause operation different from the design analyzed.
•
The two channels on a redundant board are not used to increase the hardware fault
tolerance needed for a higher SIL as they contain common components.
•
The stress levels are average for an industrial environment and can be compared to the
Ground Fixed classification of MIL-HNBK-217F. Alternatively, the assumed environment is
similar to:
o IEC 60654-1, Class C (sheltered location) with temperature limits within the
manufacturer's rating and an average temperature over a long period of time of 40ºC.
Humidity levels are assumed within manufacturer's rating.
© exida.com GmbH
Stephan Aschenbrenner
more@turck.com • www.turck.com • 2015/04
Anhang – EXIDA-Report FMEDA Turck 04/07-14 R002
TURCK 04-07-14 R002 V3R0.doc; February 21, 2014
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