Power Considerations - Allen-Bradley 1769-PA2K Installation Instructions Manual

Compact I/O Expansion Power Supplies

Power Considerations

The following sections explain power considerations for the Compact I/O system.
Disconnect the Main Power
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.
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 that is
controlled by a controller.
Isolation Transformer Usage
You can 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 that is 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) on page 17
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 can 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 must be
considered. For example, a deep voltage sag can reset a computer that is 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 simultaneously, a brief sag in the power source voltage typically
does not affect any equipment.
14 Rockwell Automation Publication 1769-IN028I-EN-P - July 2020
for an example of circuits using isolation