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Safety Precautions 3
3 Safety Precautions
!WARNING Never attempt to disassemble any Units while power is being supplied. Doing
so may result in serious electrical shock or electrocution.
!WARNING Provide safety measures in external circuits (i.e., not in the Programmable
Controller), including the following items, to ensure safety in the system if an
abnormality occurs due to malfunction of the PLC or another external factor
affecting the PLC operation. Not doing so may result in serious accidents.
!WARNING Input only the specified range of voltage or current to a Unit. A current or volt-
age exceeding the specified range may cause malfunction or fire.
!WARNING Provide safety measures in external circuits (i.e., not in the Programmable
Controller), including the following items, to ensure safety in the system if an
abnormality occurs due to malfunction of the PLC or another external factor
affecting the PLC operation. Not doing so may result in serious accidents.
• Emergency stop circuits, interlock circuits, limit circuits, and similar safety
measures must be provided in external control circuits.
• The PLC will turn OFF all outputs when its self-diagnosis function detects
any error or when a severe failure alarm (FALS) instruction is executed.
As a countermeasure for such errors, external safety measures must be
provided to ensure safety in the system.
• The PLC outputs may remain ON or OFF due to deposits on or burning of
the output relays, or destruction of the output transistors. As a counter-
measure for such problems, external safety measures must be provided
to ensure safety in the system.
• When the 24-V DC output (service power supply to the PLC) is over-
loaded or short-circuited, the voltage may drop and result in the outputs
being turned OFF. As a countermeasure for such problems, external
safety measures must be provided to ensure safety in the system.
!WARNING The CPU Unit refreshes I/O even when the program is stopped (i.e., even in
PROGRAM mode). Confirm safety thoroughly in advance before changing the
status of any part of memory allocated to Output Units, Special I/O Units, or
CPU Bus Units. Any changes to the data allocated to any Unit may result in
unexpected operation of the loads connected to the Unit. Any of the following
operations may result in changes to memory status.
• Transferring I/O memory data to the CPU Unit from a Programming
Device
• Changing present values in memory from a Programming Device
• Force-setting/-resetting bits from a Programming Device
• Transferring I/O memory files from a Memory Card or EM file memory to
the CPU Unit
• Transferring I/O memory from a host computer or from another PLC on a
network
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