Mitsubishi Electric MELSEC-A0J2H Series Handbook page 4

[Design Precautions]
● Configure safety circuits external to the programmable controller to ensure that the entire system
operates safely even when a fault occurs in the external power supply or the programmable
controller. Failure to do so may result in an accident due to an incorrect output or malfunction.
(1) Configure external safety circuits, such as an emergency stop circuit, protection circuit, and
protective interlock circuit for forward/reverse operation or upper/lower limit positioning.
(2) When the programmable controller detects an abnormal condition, it stops the operation and all
outputs are:
Overcurrent or overvoltage protection of the power supply
module is activated.
The CPU module detects an error such as a watchdog
timer error by the self-diagnostic function.
All outputs may turn on when an error occurs in the part, such as I/O control part, where the CPU
module cannot detect any error. To ensure safety operation in such a case, provide a safety
mechanism or a fail-safe circuit external to the programmable controller. For a fail-safe circuit
example, refer to the QCPU User's Manual (Hardware Design, Maintenance and Inspection).
(3) Outputs may remain on or off due to a failure of an output module relay or transistor. Configure
an external circuit for monitoring output signals that could cause a serious accident.
● In an output module, when a load current exceeding the rated current or an overcurrent caused by a
load short-circuit flows for a long time, it may cause smoke and fire. To prevent this, configure an
external safety circuit, such as a fuse.
● Configure a circuit so that the programmable controller is turned on first and then the external power
supply.
If the external power supply is turned on first, an accident may occur due to an incorrect output or
malfunction.
● For the operating status of each station after a communication failure, refer to relevant manuals for
the network.
Incorrect output or malfunction due to a communication failure may result in an accident.
A
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WARNING
Q series module
All outputs are turned off
All outputs are held or turned off
according to the parameter setting.
A series module
All outputs are turned off
All outputs are turned off