
8. LOADING AND INSTALLATION
8 - 1
MELSEC-A
8.
LOADING AND INSTALLATION
This chapter describes the procedure for loading and installation and gives relevant
precautions to ensure that the system performs with high reliability and that its
functions are used to best effect.
8.1
Consideration for Safety
When the power to the system is turned ON or OFF, the process output may not
perform normally at times due to the difference between the delay time and the rise
time of the power supply of the PLC CPU main module and the external power
supply (especially DC). Also, if there is an error in the external power supply, the
output process may malfunction.
For example, if the power supply to the PLC is switched on after switching on the
external power supply for the sequence program operation at a DC output module,
the DC output module may temporarily output erroneous signals when the power to
the PLC is switched on. A circuit that allows the power to the PLC to be switched on
first must therefore be provided.
In addition, if there is an abnormality in the external power supply or trouble in the
PLC, this could cause malfunction.
To (a) prevent erroneous operation of the entire system, and (b) ensure safety,
prepare circuits (such as an emergency stop circuit, protection circuit, and interlock
circuit) that prevent machine damage and/or accidents due to erroneous operation
of peripheral devices. An example system design circuit based on this concept is
shown on the following page.
!
DANGER
•
Safety circuitry must be so designed and constructed externally that an entire system stays in
safe in case of a external power supply failure and/or PLC failure. In particular, the following
safety circuitry are required to constructed outside of the PLC.
(1) Emergency stop circuit, protection circuit, interlocking circuit for contrary operations such as
forward and reverse movement , and hardware stroke limit circuit for positioning controls
must be constructed externally.
(2) In case of hardware failure which PLC CPU cannot detect occurs, all or some output
signals could be turned on without program instructions. An external safety circuitry must be
so constructed that safety of equipment or machine can be protected from such case.
Please refer to Sub-clause 4.3.2 for details.
(3) In some cases, relays or transistors used in output modules stay always ON or OFF as
failure symptoms. If such failure could cause serious damage on persons or properties,
those safety critical output signals must be externally monitored.
•
If the power to the PLC is turned ON after turning ON the external power supply used for the
process control with the DC output module, the DC output module may make an erroneous
output for an instant. Take the following procedures for power up of the equipment, in order to
prevent such erroneous input and output to/from the PLC.
Содержание A1SH
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