Appendix 10 General Safety Requirements - Mitsubishi Electric MELSEC L Series User Manual

Appendix 10
When a programmable controller is powered on or off, normal control outputs may not be performed temporarily. This
is because there is a time lag for the power to be turned on between the programmable controller power supply and
the external power supply (especially, DC) for the control target. Normal control outputs also may not be performed
when the external power supply or the programmable controller fails.
In terms of fail-safe and to prevent any of these abnormal operations from leading to the entire network system failure,
configure safety circuits (such as emergency stop circuits, protection circuits, or interlock circuits, where abnormal
operation may result in mechanical damage or accidents) external to the programmable controller.
(1) Fail-safe measures for head module failure
A head module can detect its own hardware failures by the self-diagnostic function. However, failures which
occur in a part, such as an I/O control part, may not be detected.
In this case, depending on the failure, all input or output points may turn on or off, or normal operation and safety
of the control-target device may not be ensured.
Even though Mitsubishi programmable controllers are manufactured under strict quality control, they may fail due
to some reasons. Provide fail-safe circuits external to the programmable controller so that no machine is
damaged and no accident occurs.
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General Safety Requirements
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