GE GFK-0787B Noise Reduction Machine User Manual


 
B
section level 1 1
figure_ap level 1
table_ap level 1
B-1
GFK-0787B
Appendix B Maintenance Override
The information in this appendix is reprinted by permission of TUV.
Abstract
Suggestions are made about the use of maintenance override of safety relevant sensors
and actuators. Ways are shown to overcome the safety problems and the inconvenience
of hardwired solutions. A checklist is given.
Maintenance Override
There are basically two methods used now to check safety relevant peripherals
connected to PLCs:
Special switches connected to inputs of the PLC. These inputs are used to deactivate
actuators and sensors under maintenance. The maintenance condition is handled as
part of the application program of the PLC.
During maintenance sensors and actuators are electrically switched off of the PLC
and checked manually by special measures.
In some cases, e.g. where space is limited, there is the wish to integrate the maintenance
console to the operator display, or to have the maintenance covered by other strategies.
This introduces the third alternative for maintenance override:
Maintenance overrides caused by serial communication to the PLC.
This possibility has to be handled with care and is introduced in this paper.
Maintenance Override Procedures
Connecting to PLC via serial lines is possible mainly in two ways:
A. The serial link is done via the MODBUS RTU protocol or other approved serial
protocols. The maintenance override may not be performed by the engineering
workstation or programming environment.
B. The engineering workstation or programming environment is allowed to be
connected to the PLC to perform maintenance override. That requires additional
safety measures inside the associated PLC to prevent a program change during
maintenance intervals. These measures shall be approved, e.g. by TUV.