Rockwell Automation 1762-Lxxxx MicroLogix 1200 Programmable Controllers - Manual (Page 22)

Publication 1762-UM001G-EN-P - March 2011
2-8 Install 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. Refer to the figures on pages 2-10 and 2-11.
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.
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 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.