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2.2.4 Safety Guidelines
This section presents recommended installation practices, and procedures. Since no two applications
are identical, these recommendations should be considered as guidelines.
Hardware
Considerations
WARNING!
The system designer should be aware that control devices in
Programmable Controller systems can fail and thereby create an
unsafe condition. Furthermore, electrical interference in an operator
interface, such as the MMI, can lead to equipment start-up, which
could result in property damage and/or physical injury to the
equipment operator.
If you, or your company, use any programmable control systems,
which require an operator or attendant, you should be aware that this
potential safety hazard exists and take appropriate precautions.
Although the specific design steps depend on your particular
application, the following precautions generally apply to installation of
solid-state programmable control devices. In addition, these
precautions conform to the guidelines for installation of Programmable
Controllers as recommended in the NEMA ICS 3-304 Programmable
Control Standards.
Programming
Considerations
To conform with ICS Safety Recommendations, checks should be
placed in the PLC program to ensure that all writable registers that
control critical parts of plant or machinery should have upper and
lower limit checks built into the program, with an out-of-limit safe shut
down procedure to ensure safety of personnel.
ICS 3-304.81 Safety Recommendations:
Consideration should be given to the use
of an emergency stop function which is independent of the programmable controller.
Where the operator is exposed to the machinery, such as in loading or unloading a machine tool, or where
the machine cycles automatically, consideration should be given to the use of an electromechanical override
or other redundant means, independent of the programmable controller, for starting and interrupting the cycle.
If provision is required for changing programs while the equipment is in operation, consideration should
be given to the use of locks or other means of assuring that such changes can be made only by authorized
personnel.
These recommendations are intended as safeguards against the failure of critical components and the
effects of such failures or the inadvertent errors that might be introduced if programs are changed while the
equipment is in operation.*
* The ICS 3-304.81 Safety Recommendations are reproduced by permission of the National Electrical
Manufacturers Association from NEMA ICS 3-304, Programmable Controller Standard.
Summary of Contents for MMI-1 Series
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