Preface
MicroLogix 1000 Programmable Controllers User Manual
1–4
Master Control Relay
A hard-wired master control relay (MCR) provides a reliable 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 1–6.
Never alter these circuits to defeat their function, since serious injury and/or
machine damage could result.
Note
If you are using an external dc output 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 external 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.
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.
Note
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.
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