Rockwell Automation 1764-xxxx MICROLOGIX 1500 PROGRAMMABLE CONTROLLERS - Manual (Page 34)

Publication 1764-UM001B-EN-P - April 2002
2-8 Installing 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
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.