Rockwell Automation 1769-PB4 Compact I/O Expansion Power Supplies - Manuel (Page 15)

Compact I/O Expansion Power Supplies 15
Publication 1769-IN028B-EN-P - October 2008
Power Considerations
The following sections explain power considerations for the Compact I/O system.
Disconnect the Main Power
Install the main power disconnect switch where operators and maintenance personnel have
quick and easy access to it. Besides disconnecting electrical power, de-energize all other
sources of power (pneumatic and hydraulic) before you begin working on a machine or
process controlled by a controller.
Isolation Transformer Usage
You may want to use an isolation transformer in the AC line. This type of transformer
provides isolation from your power distribution system to reduce electrical noise and is often
used as a step down transformer to reduce line voltage. Any transformer used with the
Compact I/O system must have a sufficient power rating for its load. The power rating is
expressed in volt-amperes (VA). See
Schematic (Using IEC Symbols) for an example of
circuits using isolation transformers.
Power Supply Inrush
During powerup, the power supply allows a brief inrush current to charge internal capacitors.
Many power lines and control transformers can supply inrush current for a brief time. If the
power source cannot supply this inrush current, the source voltage may sag momentarily.
The only effect of limited inrush current and voltage sag on the system is that the power
supply capacitors charge more slowly. However, the effect of a voltage sag on other
equipment should be considered. For example, a deep voltage sag may reset a computer
connected to the same power source. The following considerations determine whether the
power source must be required to supply high inrush current:
• Power-up sequence of devices in a system
• Amount of the power source voltage sag if the inrush current cannot be supplied
• Effect of voltage sag on other equipment in the system
If the entire system is powered up at the same time, a brief sag in the power source voltage
typically will not affect any equipment.
WARNING
Explosion Hazard - Do not replace components or disconnect equipment unless power
has been switched off.
If you connect or disconnect wiring while the field-side power is on, an electrical arc can
occur. This could cause an explosion in hazardous location installations. Be sure that
power is removed or the area is nonhazardous before proceeding.