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The View Of A Fail-safe Circuit - Mitsubishi Electric MELSEC A Series User Manual

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4.2 The view of a Fail-safe Circuit

When the programmable controller is switched ON/OFF, the outputs may
temporarily be incorrect due to the delay time and difference between the start-up
time of the programmable controller's power supply and that of the external power
supply for process control (especially if it is DC).
For example, if the power to the PC is turned ON after turning ON the external
power supply used for the process control with the DC output module, the DC
output module may make an erroneous output for an instant.
There is a possibility of abnormal operation if the external power supply is abnormal
or a programmable controller fault occurs.
To prevent the abnormal operation of the whole system, machine breakdown, and
accidents, build a fail-safe circuit (emergency stop, protective circuit, interlocking
circuit, etc.) outside the PC.


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