Safety Considerations Guide for Triconex General ... - ICEWeb

76 Appendix B HART Communication
2008-04-01
Picture 1: HART usage within SIS
SIS
DCS, AMS,
Diagnostic
Monitor, or
Maintenance
System
RS 485
HART Protocol
Isolator
Isolator
HART MUX
4 – 20 mA
HART Master
HART
Enabled
Field Device
SIS = Safety Instrumented System
MUX = Multiplexer connectd to AMS one one side, and to
4-20 mA signal on the other side.
AMS = Asset Management System that is connected to
MUX using RS 485. The AMS uses the HART protocol to
communicate with the HART enabled device.
DCS = Dstributed Control System
In order to ensure that the HART Master/MUX connection to the 4-20 mA signal (input/output)
connected to the SIS and use of HART protocol is non-interfering the 4-20 mA safety signal, the
rules/guidelines specified in the following must be followed by the end user.
Note, that the end user could incorporate some of the guidelines in the management of safety,
management of change, and management of maintenance policies and procedures.
4. Rules and Guidelines for the use of a HART protocol within SIS
AMS/MUX connection to 4-20 mA signal must be isolated and decoupled such that a failure
of the MUX does not affect the 4-20 mA signal
A detailed FMEA of the MUX interface to the 4-20 mA signal must be performed to show that
the normal operation or the failure of MUX won’t affect the 4-20 mA signal. In this case, the
MUX and associated AMS are non-interfering the 4-20 mA safety signal. If non-interfering
can not be shown by FMEA the probability of failure must be considered to determine the
safety parameters in accordance to IEC 61508 to include them into safety loop
consideration.
The HART enabled devices used with SIS must be suitable for the SIL rating of the safety
loop as per IEC 61511 guidelines. HART enabled field devices shall be documented to be
non-interfering the 4-20 mA safety signal. In compliance with IEC 61511 -1 clause 11.5.2,
the HART enabled devices used within an SIS (Safety Instrumented System) shall be
documented to be in accordance with IEC 61508-2 and IEC 61508-3 as applicable to the SIL
(Safety Integrity Level) of the individual SIF (Safety Instrumented Function), or else they
shall meet the requirements of IEC 61511-1 clause 11.4 for hardware fault tolerance and
clauses 11.5.3 to 11.5.6 for prior use, as appropriate.
HART_in_Safety_Related_Applications_pdf.doc
Revision 1.0 Page 6 of 7
Safety Considerations Guide for Triconex General Purpose v2 Systems