Rockwell Automation 1768-PA3_PB3 CompactLogix Power Supplies Installation Instructions - Manual (Page 15)

1768 CompactLogix Power Supplies 15
Publication 1768-IN001B-EN-P - July 2006
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.
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.
Periodic Tests of Master-Control Relay Circuit
Periodically test switches to assure they stop machine motion when needed.Any
part can fail, including the switches in a master-control relay circuit. The failure of
one of these switches would most likely cause an open circuit, which would be a
safe power-off failure. However, if one of these switches shorts out, it no longer
provides any safety protection.
Before You Begin
Before you begin the installation of the 1768 power supply, consider the following:
WARNING
Make sure power is disconnected from the power supply before removing
or inserting this power supply from the 1768 CompactLogix system. When
you remove or insert a power supply with power applied, an electrical arc
may occur. An electrical arc can cause personal injury or property damage
by:
• sending an erroneous signal to your system’s field devices, causing
unintended machine motion.
• causing an explosion in a hazardous environment.
Electrical arcing causes excessive wear to contacts on both the power
supply and its mating connector. Worn contacts may create electrical
resistance.