Rockwell Automation 1766-L32AWA, User Manual

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Rockwell Automation Publication 1766-UM001O-EN-P - September 2021 23
Chapter 2 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. See
Figure 3 on page 24 and Figure 4 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.
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.
Do not bring in unfiltered outside air. Place the controller in an enclosure to protect it
from a corrosive atmosphere. Harmful contaminants or dirt could cause improper
operation or damage to components. In extreme cases, you may need to use air
conditioning to protect against heat build-up within the enclosure.
ATTENTION: 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.