Mitsubishi MELSEC-Q Series User Manual page 151

5
SETTINGS AND PROCEDURES BEFORE OPERATION
5.2 Fail-safe Circuit
DANGER
Provide a safety circuit outside the C Controller module to
ensure that the entire system will operate safely even if an
external power failure or C Controller module failure occurs.
Failure to do so could result in accidents due to erroneous
output or operation.
(1) For an emergency stop circuit, protection circuit and
interlock circuit that is designed for incompatible actions
such as forward/reverse rotation or for damage prevention
such as the upper/lower limit setting in positioning, any of
them must be provided outside the C Controller module.
(2) When the C Controller module detects the following
conditions, it will disable the output (Y) from the user
program and writing to buffer memory in the case of both
(a) and (b) and turn off all outputs for (b).
Whether to hold or clear all the outputs is set by a
parameter.
(a) The overcurrent protector or overvoltage protector of
the power supply module is activated.
(b) An error such as a watchdog timer error is detected by
the self-diagnostic function of the C Controller module.
If an error occurs in I/O control parts and the C Controller
module cannot detect the error, all the outputs may turn
ON.
Provide a fail-safe circuit or a preventive mechanism
outside the C Controller module so that machines will
operate safely in such a case.
(3) Output could be left on or off when there is a fault in an
output module relay or transistor.
So build an external monitoring circuit that will monitor any
output signal that could cause serious accidents.
If load current more than the rating or overcurrent due to a short
circuit in the load has flowed in the output module for a long
time, it may cause a fire and smoke. Provide an external safety
device such as a fuse.
Create the circuit so that the external power supply will turn on
after the C Controller system is powered on.
Turning on the external power supply first could result in
accidents due to erroneous output or operation.
5.2 Fail-safe Circuit
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