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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 physi-
cal 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 precau-
tions generally apply to installation of solid-state program-
mable 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 indepen-
dent 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 compo-
nents 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.
2.3 Communications Connections
This section presents recommended installation practices and procedures. Since
no two applications are identical, these recommendations should be consid-
ered as guidelines.
Hardware
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