Rockwell Automation 1768-PA3_PB3 CompactLogix Power Supplies Installation Instructions - Manual (Page 14)

14 1768 CompactLogix Power Supplies
Publication 1768-IN001B-EN-P - July 2006
Other Types of Line Conditions
Occasionally the power source to the system can be temporarily interrupted. Also,
it is possible that the voltage level may drop substantially below the normal line-
voltage range for a period of time. Both the interruption and the voltage sag are
considered power losses for the system.
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. Wiring this way removes power to input and output device circuits.
• 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 input line of the dc output power supply should be connected to an
external fuse.
• 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 enclosure, place the switch operating handle on the outside of the enclosure, so
that you can disconnect power without opening the enclosure.
When any of the emergency-stop switches are opened, remove power to input and
output devices.
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.
ATTENTION
Never alter these circuits to defeat their function since serious injury and/or
machine damage could result.