Mitsubishi MELSEC-A A1SCPUC24-R2, User Manual

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4.2 Fail-Safe Circuit Concept
When the programmable controller is powered ON and then OFF,
improper outputs may be generated temporarily depending on the delay
time and start-up time differences between the programmable controller
power supply and the external power supply for the control target
(especially, DC).
For example, if the external power supply for the control target is
powered ON and then the programmable controller is powered ON, the
DC output module may generate incorrect outputs temporarily upon the
programmable controller power-ON. Therefore, it is required to build the
circuit that energizes the programmable controller by priority.
The external power failure or programmable controller failure may lead
to the system error.
In order to eliminate the possibility of the system error and ensure fail-
safe operation, build the following circuit outside the programmable
controller: emergency circuit, protection circuit and interlock circuit, as
they could cause machine damages and accidents due to the
abovementioned failures.
An example of system design, which is based on fail-safe concept, is
provided on the next page.