Rockwell Automation 1769-PB4 Compact I/O Expansion Power Supplies - Manuel (Page 17)

Compact I/O Expansion Power Supplies 17
Publication 1769-IN028B-EN-P - October 2008
Use a 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.
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 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 system’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 system. It is intended for
any situation where the operator must quickly de-energize only I/O devices. 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
• 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.
TIP
Do not control the master control relay with the Compact I/O system. Provide the
operator with the safety of a direct connection between an emergency-stop switch and
the master control relay.