Loading And Installation; Fail-Safe Circuit Concept - Mitsubishi MELSEC-A A2USHCPU-S1 User Manual

8.

LOADING AND INSTALLATION

8 LOADING AND INSTALLATION
To increase the system reliability and fully utilize the functions, procedures and cautions
concerning loading and installation are described below.
8.1

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 stop 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.
Create a safety circuit outside the programmable controller to ensure the whole
DANGER
system will operate safely even if an external power failure or a programmable
controller failure occurs.
Otherwise, incorrect output or malfunction may cause an accident.
(1) For an emergency stop circuit, protection circuit and interlock circuit that is
(2) When the programmable controller detects the following error conditions, it stops
(3) Depending on the failure of the output module's relay or transistor, the output
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.
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 created outside the programmable controller.
the operation and turn off all the outputs.
The overcurrent protection device or overvoltage protection device of the power
supply module is activated.
The programmable controller CPU detects an error such as a watchdog timer
error by the self-diagnostics function.
In the case of an error of a part such as an I/O control part that cannot be detected
by the programmable controller CPU, all the outputs may turn on. In order to make
all machines operate safely in such a case, set up a fail-safe circuit or a specific
mechanism outside the programmable controller.
status may remain ON or OFF incorrectly. For output signals that may lead to a
serious accident, create an external monitoring circuit.
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