Use A Master Control Relay - Allen-Bradley 1769-PA2K Installation Instructions Manual

Compact I/O Expansion Power Supplies

Use a Master Control Relay

A hard-wired master control relay (MCR) provides a reliable means for emergency machine
shutdown. Since the master control relay allows the placement of several emergency stop
switches in different locations, its installation is important from a safety standpoint. Overtravel
limit switches or mushroom-head push buttons are wired in series so that when any of them
opens, the master control relay is de-energized. This configuration removes power to input and
output device circuits.
ATTENTION: Never alter these circuits to defeat their function since serious injury and/or machine
damage could result.
TIP If you are using an external DC power supply, interrupt the DC output side rather than the AC line side of the
supply. This process avoids the additional delay of power supply turn off.
Fuse the AC line of the DC output power supply.
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 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, remove power to input and output
devices.
When you use the master control relay to remove power from the external I/O circuits, power
continues to be provided to the power supply of the system. This condition is so that you can still
observe the diagnostic indicators on the processor.
The master control relay is not a substitute for a disconnect to the system. It is intended for any
situation where the operator must quickly de-energize only I/O devices. When you inspect or
install 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.
TIP Do not control the master control relay with the Compact I/O system. Provide the operator with the safety of
a direct connection between an emergency stop switch and the master control relay.
16 Rockwell Automation Publication 1769-IN028I-EN-P - July 2020