Rockwell Automation 1762-Lxxxx MicroLogix 1200 Programmable Controllers - Manual (Page 19)

Publication 1762-UM001G-EN-P - March 2011
Install Your Controller 2-5
Power Distribution
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. It is recommended that the controller remain 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.
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.
Power Considerations The following explains power considerations for the micro controllers.
Isolation Transformers
You may want to use an isolation transformer in the ac line to the controller.
This type of transformer provides isolation from your power distribution
system to reduce the electrical noise that enters the controller and is often used
as a step-down transformer to reduce line voltage. Any transformer used with
the controller must have a sufficient power rating for its load. The power
rating is expressed in volt-amperes (VA).