Rockwell Automation 1769-PB4 Compact I/O Expansion Power Supplies - Manuel (Page 6)

6 Compact I/O Expansion Power Supplies
Publication 1769-IN028B-EN-P - October 2008
Before You Begin
There are some points about power distribution that you should know.
• The master control relay must be able to inhibit all machine motion by removing
power to the machine I/O devices when the relay is de-energized. We recommend
that the controller remains powered even when the master control relay is
de-energized.
• If you are using a DC power supply, interrupt the load side rather than the AC line
power. This avoids the additional delay of power supply turn-off. The DC power
supply should be powered directly from the fused secondary of the transformer.
Power to the DC input and output circuits should be connected through a set of
master control relay contacts.
Perform Periodic Tests of Master Control Relay Circuit
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. These switches should be tested periodically to assure they will stop machine
motion when needed.