Rockwell-automation 1758-RTU202 DataSite Hardware User Manual User Manual download pdf (Page 17)

Publication 1758-UM001D-EN-P - June 2011

Installing the Controller 17

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 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.

ATTENTION

Never alter these circuits to defeat their function since serious

injury and/or machine damage could result.

TIP

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 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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Rockwell-automation 1758-RTU202 DataSite Hardware User Manual User Manual Page 17