Mitsubishi Electric MELSEC L Series User Manual 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) Emergency stop circuits, protection circuits, and protective interlock circuits for conflicting
operations (such as forward/reverse rotations or upper/lower limit positioning) must be configured
external to the programmable controller.
(2) 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 head
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 head 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 Page 150, Appendix 10.
(3) Outputs may remain on or off due to a failure of a component such as a relay and 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 head module during operation,
configure an interlock circuit in the program of the master station to ensure that the entire system will
always operate safely.
For other controls to a running programmable controller (such as modification of the master station
program or operating status change), read relevant manuals carefully and ensure the safety before
the operation. Especially, when a remote programmable controller is controlled by an external device,
immediate action cannot be taken if a problem occurs in the programmable controller due to a
communication failure. To prevent this, configure an interlock circuit in the program of the master
station, and determine corrective actions to be taken between the external device and head module in
case of a communication failure.
● Analog outputs may remain on due to a failure of the module. Configure an external interlock circuit
for output signals that could cause a serious accident.
2
Downloaded from ManualsNet.com search engine
WARNING