Rockwell Automation 1747-L40 SLC 500 Fixed Hardware Style - Manual (Page 44)

Chapter 2
System Installation Recommendations
2–5
A hard–wired master control relay (MCR) provides a convenient means for
emergency controller 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 figure on page
2–6 .
!
ATTENTION: Never alter these circuits to defeat their function,
since serious injury and/or machine damage could result.
Important: If you are using a DC output power supply, interrupt the output
side rather than the AC line to avoid the additional delay of
power supply turn–on and turn–off. The power supply should
receive its power directly from the fused secondary of the
transformer. Connect the power to the DC input and output
circuits through a set of master control relay contacts.
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 is stopped.
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.
Important: The operator must not control the master control relay with the
SLC 500 controller. Provide the operator with the safety of a
direct connection between an emergency–stop switch and the
master control relay.
Master Control Relay