Rockwell Automation System Design for the Control of Electrical Noise - Manual (Page 114)

Publication GMC-RM001A-EN-P — July 2001
A-12 Noise Control Supplement
AC Line Filters
Always install a suitably rated AC line filter on the main panel as close
as possible to a switch-mode PSU.
Using Separate DC Power Supplies
It is often assumed that the use of separate DC PSU's will isolate noise.
In fact, noise will travel in either direction through a power supply. It
can be demonstrated that a noise transient caused by switching an
inductive load connected to one PSU is easily detected in the load
circuit of a second PSU that shares the same AC line supply.
Suppressing inductive loads and feeding each PSU via a line filter will
reduce the effect.
In the figure below, 7V noise spikes are seen at the AC line terminals
of the PSU.
Figure A.11
Noise spikes on PSU AC line terminals
-10V
-8
-6
-4
-2
0
2
4
6
8
10V
7.2V pk
Source Omron PSU AC Neutral
DC floating, Unsuppressed Relay
µs -1 0 1 2 3 4 5 6 7 8 9