22 Rockwell Automation Publication 2080-UM002G-EN-E - March 2015
Chapter 2 About Your Controller
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 removes power to input and output
device circuits. See illustrations Schematic – Using IEC Symbols
on page 24 and
Schematic – Using ANSI/CSA Symbols)
on page 25.
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.
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.
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