Rockwell Automation 1766-L32AWA, User Manual

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Rockwell Automation Publication 1766-UM001O-EN-P - September 2021 21
Chapter 2 Install Your Controller
disconnecting electrical power, all other sources of power (pneumatic and
hydraulic) should be de-energized before working on a machine or process
controlled by a controller.
Safety Circuits
Circuits installed on the machine for safety reasons, like overtravel limit
switches, stop push buttons, and interlocks, should always be hard-wired
directly to the master control relay. These devices must be wired in series so
that when any one device opens, the master control relay is de-energized,
thereby removing power to the machine. Never alter these circuits to defeat
their function. Serious injury or machine damage could result.
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).
Power Supply Inrush
During power-up, the MicroLogix 1400 power supply allows a brief inrush
current to charge internal capacitors. Many power lines and control
WARNING: Explosion Hazard
Do not connect or disconnect connectors while circuit is live.