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Sicherheitshandbuch – Signaltrenner
4.2 Methodology – FMEDA, Failure rates
4.2.1 FMEDA
A Failure Modes and Effects Analysis (FMEA) is a systematic way to identify and evaluate the
effects of different component failure modes, to determine what could eliminate or reduce the
chance of failure, and to document the system in consideration.
A FMEDA (Failure Modes, Effects, and Diagnostic Analysis) is a FMEA extension. It combines
standard FMEA techniques with extension to identify online diagnostics techniques and the
failure modes relevant to safety instrumented system design. It is a technique recommended to
generate failure rates for each important category (safe detected, safe undetected, dangerous
detected, dangerous undetected, fail high, fail low) in the safety models. The format for the
FMEDA is an extension of the standard FMEA format from MIL STD 1629A, Failure Modes and
Effects Analysis.
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 645-1, 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 Transducers IM33-**(Ex)-Hi/24VDC and Analog Signal Transmitters
IM35-**Ex-Hi/24VDC.
•
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.
•
The HART protocol is only used for setup, calibration, and diagnostics purposes, not during
normal operation.
•
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 645-1, 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
26
TURCK 04-07-14 R001 V2R0.doc; February 8, 2013
Hans Turck GmbH & Co. KG • Tel. +49 208/4952-0 • Fax +49 208/4952-264
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