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An Introduction to Programming
3.5
Safety First!
PLCs have many advantages over hard-wired controllers. However, when it comes to safety it
is important to understand that you cannot trust a PLC blindly.
Emergency STOP devices
It is essential to ensure that errors in the control system or program cannot cause hazards for
staff or machines. Emergency STOP devices must remain fully functional even when the PLC
is not working properly – for example to switch off the power to the PLC outputs if necessary.
Never implement an Emergency STOP switch solely as an input that is processed by the PLC,
with the PLC program activating the shutdown. This would be much too risky.
Safety precautions for cable breaks
You must also take steps to ensure safety in the event that the transmission of signals from the
switches to the PLC are interrupted by cable breaks. When switching equipment on and off via
the PLC always use switches or pushbuttons with make contacts for switching on and with
break contacts for switching off.
EMERG.
OFF
X001
0
Motor ON
X002
2
Motor OFF
This ensures that if there is a cable break the drive is switched off automatically and it is not
possible to activate the drive. In addition to this, switching off has priority because it is pro-
cessed by the program after the switch on instruction.
Interlock contacts
If you have two outputs that should never both be switched on at the same time – for example
outputs for selecting forward or reverse operation for a motor – the interlock for the outputs
must also be implemented with physical contacts in the contactors controlled by the PLC. This
is necessary because only an internal interlock is possible in the program and an error in the
PLC could cause both outputs to be activated at the same time.
FX Beginners Manual
+24 V
ON
OFF
X000 X001
X002
COM Y000
Y001
0 V
SET Y000
Motor ON
RST Y000
Motor ON
In this example the contactor for a drive sys-
tem can also be switched off manually with
an Emergency OFF switch.
In the program for this installation the make
contact of the ON switch is polled with an LD
instruction, the break contact of the OFF
switch with an LDI instruction. The output,
and thus also the drive, is switched off when
the input X002 has a signal state of "0". This
is the case when the OFF switch is operated
or when the connection between the switch
and input X002 is interrupted.
Safety First!
3 – 21
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