ALLEN BRADLEY 1747-L551 PLC Processor

3–8
System Installation Recommendations
Emergency-Stop Switches
Adhere to the following points concerning emergency-stop switches:
• Do not program emergency-stop switches in the controller
program. Any emergency-stop switch should turn off all machine
power by turning off the master control relay.
• Observe all applicable local codes concerning the placement and
labeling of emergency-stop switches.
• Install emergency-stop switches and the master control relay in
your system. Make certain that relay contacts have a sufficient
rating for your application. Emergency-stop switches must be
easy to reach. See the following schematics.
Power Considerations
The following explains power considerations for the SLC 500
processor.
Common Power Source
We strongly recommend that all chassis power supplies have the
same power source as the input and output devices. This helps
reduce the chance of electrical interference due to multiple sources
and grounds as well as helps maintain system integrity if power is
interrupted.
The processor detects the absence of power to any chassis in the
system. If power to any chassis is lost (or not yet applied), the CPU
FAULT LED turns on and all controller outputs are de-energized.
This fault detection makes it necessary that you apply power to the
expansion chassis before you apply power to the chassis containing
the processor to avoid an unwanted fault. Of course, applying power
in sequence is unnecessary if all chassis have a common power
source.
Isolation Transformer
In many industrial applications, a step-down transformer is required
to reduce line voltage to 120 or 240V ac. This transformer also
provides isolation to protect equipment from high voltage transients
that may be generated on your power distribution system.
Publication 1747-6.2