© exida.com GmbH TURCK 04-07-14 R001 V2R0.doc; February 8, 2013
Stephan Aschenbrenner Page 6 of 25
1 Purpose and Scope
Generally three options exist when doing an assessment of sensors, interfaces and/or final
elements.
Option 1: Hardware assessment according to IEC 61508
Option 1 is a hardware assessment by
exida.com
according to the relevant functional safety
standard(s) like DIN V VDE 0801, IEC 61508 or EN 954-1. The hardware assessment consists
of a FMEDA to determine the fault behavior and the failure rates of the device, which are then
used to calculate the Safe Failure Fraction (SFF) and the average Probability of Failure on
Demand (PFD
AVG
).
This option for pre-existing hardware devices shall provide the safety instrumentation engineer
with the required failure data as per IEC 61508 / IEC 61511 and does not consist of an
assessment of the software development process
Option 2: Hardware assessment with proven-in-use consideration according to IEC 61508 /
IEC 61511
Option 2 is an assessment by
exida.com
according to the relevant functional safety
standard(s) like DIN V VDE 0801, IEC 61508 or EN 954-1. The hardware assessment consists
of a FMEDA to determine the fault behavior and the failure rates of the device, which are then
used to calculate the Safe Failure Fraction (SFF) and the average Probability of Failure on
Demand (PFD
AVG
). In addition this option consists of an assessment of the proven-in-use
documentation of the device and its software including the modification process.
This option for pre-existing programmable electronic devices shall provide the safety
instrumentation engineer with the required failure data as per IEC 61508 / IEC 61511 and justify
the reduced fault tolerance requirements of IEC 61511 for sensors, final elements and other PE
field devices.
Option 3: Full assessment according to IEC 61508
Option 3 is a full assessment by
exida.com
according to the relevant application standard(s)
like IEC 61511 or EN 298 and the necessary functional safety standard(s) like
DIN V VDE 0801, IEC 61508 or EN 954-1. The full assessment extends option 1 by an
assessment of all fault avoidance and fault control measures during hardware and software
development.
This option is most suitable for newly developed software based field devices and
programmable controllers to demonstrate full compliance with IEC 61508 to the end-user.
This assessment shall be done according to option 1.
This document shall describe the results of the hardware assessment carried out on the
Isolating Transducers IM33-**(Ex)-Hi/24VDC and Analog Signal Transmitters
IM35-**Ex-Hi/24VDC.
It shall be assessed whether the described Isolating Transducers and Analog Signal
Transmitters meet the average Probability of Failure on Demand (PFD
AVG
) requirements and
the architectural constraints for SIL 2 sub-systems according to IEC 61508.
It does not consider any calculations necessary for proving intrinsic safety.
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