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Hardware Considerations WARNING!
The system designer should be aware that devices in Controller systems could fail and
thereby create an unsafe condition. Furthermore, electrical interference in an operator
interface, such as an MT-600, 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 that 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 Controllers as recommended in the NEMA ICS 3-304
Control Standards.
Programming Considerations
To conform with ICS Safety Recommendations, checks should be placed in the
controller to ensure that all writable registers that control critical parts of plant or
machinery have 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 only
authorized personnel can make such changes.
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
