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Rockwell Automation Publication 2080-UM002N-EN-E - November 2022
Chapter 2 About Your Controller
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
controller 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.
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.
• Install emergency-stop switches and the master control relay in your system. Make
certain that relay contacts have a sufficient rating for your application. Emergency-
stop switches must be easy to reach.
• In the following illustration, input and output circuits are shown with MCR protection.
However, in most applications, only output circuits require MCR protection.
The following illustrations show the Master Control Relay wired in a grounded system.
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
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.
Do not control the master control relay with the controller. Provide the
operator with the safety of a direct connection between an emergency-
stop switch and the master control relay.
In most applications input circuits do not require MCR protection;
however, if you need to remove power from all field devices, you must
include MCR contacts in series with input power wiring.