Mitsubishi MELSEC L Series User Manual page 4

[Design Precautions]
(4) When the programmable controller detects an abnormal condition, it stops the operation and all
outputs are:
• Turned off if the overcurrent or overvoltage protection of the power supply module is activated.
• Held or turned off according to the parameter setting if the self-diagnostic function of the CPU
module detects an error such as a watchdog timer error.
Also, all outputs may be turned on if an error occurs in a part, such as an 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 "General Safety Requirements" in the MELSEC-L CPU
Module User's Manual (Hardware Design, Maintenance and Inspection).
(5) Outputs may remain on or off due to a failure of a component such as a transistor in an output
circuit. Configure an external circuit for monitoring output signals that could cause a serious
accident.
● In an output circuit, 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.
● Configure a circuit so that the external power supply is turned off first and then the programmable
controller. If the programmable controller is turned off 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
each network. Incorrect output or malfunction due to a communication failure may result in an
accident.
● When changing data from a peripheral device connected to the CPU module to the running
programmable controller, configure an interlock circuit in the program to ensure that the entire
system will always operate safely. For other controls to a running programmable controller (such as
program modification or operating status change), read relevant manuals carefully and ensure the
safety before the operation. Especially, in the case of a control from an external device to a remote
programmable controller, immediate action cannot be taken for a problem on the programmable
controller due to a communication failure. To prevent this, configure an interlock circuit in the
program, and determine corrective actions to be taken between the external device and CPU
module in case of a communication failure.
● An absolute position restoration by the positioning function may turn off the servo-on signal (servo
off) for approximately 20ms, and the motor may run unexpectedly. If this causes a problem, provide
an electromagnetic brake to lock the motor during absolute position restoration.
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WARNING