Publication 1763-UM001E-EN-P - June 2015
26 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 28 and 29.
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.
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.
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.
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