Rockwell Automation 1747-ACNR15 ControlNet Adapter Module User Manual - Manual (Page 109)

Publication 1747-UM003A-EN-P
Understanding Your SLC 500/1746 Control System B-17
Isolation
If there is high frequency conducted noise in or around your distribution
equipment, we recommend the use of an isolation transformer in the ac line to
the power supply. This type of transformer provides isolation from your power
distribution system and is often used as a “step down” transformer to reduce
line voltage. Any transformer used with the adapter must have a sufficient
power rating for its load. This power rating is generally expressed in
volt-amperes (VA).
To select an appropriate isolation transformer, you must calculate the power
required by the chassis power supply (or supplies if the system has expansion
chassis) and any input circuits and output loads that are connected through
this transformer.
The power requirement for the input circuits is determined by the number of
inputs, the operating voltage, and the nominal input current. The power
requirement for output loads is determined by the number of outputs, the load
voltage, and load current.
For example, if you have a 1746-P1 power supply, 1746-IV16 16-point dc
input module (0.012A at 24V dc) and a 1746-OV16 16-point dc transistor
sink output module (0.5A at 24V dc), the power consumed would be:
230 VA + (16)(24V)(0.012A) + (16)(24V)(0.5A) = 426.6 VA
In general, we recommend that the transformer is oversized to provide some
margin for line voltage variations and other factors. Typically a transformer
that is 25% larger than the calculated VA is sufficient.
Suppression
Most industrial environments are susceptible to power transients or spikes. To
help insure fault-free operation and protection of equipment, we recommend
suppression devices on power to the equipment in addition to the isolation
equipment.
IMPORTANT
In this case, 0.5A is the maximum rating of the transistor
sink output (at 30° C). If your load draws less than 0.5A,
this figure may be reduced accordingly. The output portion
of the VA calculation should reflect the current
requirements of your loads.