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B
GFK-0787B B-3Appendix B Maintenance Override
Recommendations
The following recommendations are given to improve the primary safety as described
by the list:
A program in the DCS that checks regularly that no discrepancies exist between the
override command signals from the DCS and the override activated signals received
by the DCS from the PLC.
The use of the maintenance override function should be documented on the DCS
and on the programming environment if connected. The printout should include:
time stamp of begin and end.
ID of the person who is activating the maintenance override—maintenance
engineer or operator (if the information cannot be printed, it should be entered
in the work-permit).
tag name of the signal being overridden.
The communication packages different from a type-approved MODBUS should
include CRC, address check and check of the communication time frame.
Lost communications should lead to a warning to the operator and maintenance
engineer. After loss of communication a time delayed removal of the override should
occur after a warning to the operator.
hard-wired
switch
Distributed
Control System
(DCS)
Engineering
Workstation
Maintenance Override Handling
(Application Program)
Warning to
the Operator
Safeguarding
Application Program
Sensors
PLC
serial line (e.g. Modbus)
serial line
Actuators
Version History
This version 2.2 supersedes the version 2.1 from 24. Jun 1994