Rockwell Automation 1764-xxxx MICROLOGIX 1500 PROGRAMMABLE CONTROLLERS - Manual (Page 31)

Publication 1764-UM001B-EN-P - April 2002
Installing 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).