Allen-Bradley 2080-LC20-20QBB User Manual page 36

Chapter 2 About Your Controller
26
WARNING: Never alter these circuits to defeat their function
since serious injury and/or machine damage could result.
If you are using an external DC power supply, interrupt the DC output
TIP
side rather than the AC line side of the supply to avoid the additional
delay of power supply turn-off.
The AC line of the DC output power supply should be fused.
Connect a set of master control relays in series with the DC power
supplying the input and output circuits.
Place the main power disconnect switch where operators and maintenance
personnel have quick and easy access to it. If you mount a disconnect switch
inside the controller enclosure, place the switch operating handle on the outside
of the enclosure, so that you can disconnect power without opening the
enclosure.
Whenever any of the emergency-stop switches are opened, power to input and
output devices should be removed.
When you use the master control relay to remove power from the external I/O
circuits, power continues to be provided to the controller's power supply so that
diagnostic indicators on the processor can still be observed.
The master control relay is not a substitute for a disconnect to the controller. It is
intended for any situation where the operator must quickly de-energize I/O
devices only. When inspecting or installing terminal connections, replacing
output fuses, or working on equipment within the enclosure, use the disconnect
to shut off power to the rest of the system.
Do not control the master control relay with the controller. Provide the
TIP
operator with the safety of a direct connection between an emergency-
stop switch and the master control relay.
Using Emergency-Stop Switches
When using emergency-stop switches, adhere to the following points:
• Do not program emergency-stop switches in the controller program. Any
emergency-stop switch should turn off all machine power by turning off
the master control relay.
• Observe all applicable local codes concerning the placement and labeling
of emergency-stop switches.
Rockwell Automation Publication 2080-UM005E-EN-E - March 2018