Rockwell Automation Safety Guidelines for the Application, Installation, and Maintenance of Solid-State Control - Manual (Page 14)

Publication SGI-1.1 - August 2009
14 Section 3: Application Guidelines
Where severe power line transients are anticipated or noted, appropriate filters
such as commercially available line filter, isolation transformers, or voltage
limiting varistors, should be considered.
All inductive components associated with the system should be examined for
the need for noise suppression.
Comments: 3.4.2 —Selecting Devices to Provide Noise Immunity
Inductive devices are capable of generating high voltage transients when
switched off. In addition to possibly causing damage to solid-state devices
by exceeding the semiconductor voltage rating, the high voltage transient
can be coupled to other portions of a system where it appears as noise.
Fortunately, it is fairly easy to limit the effects of this type of noise with
some form of suppression device. When necessary, in addition to
suppression devices often provided in solid-state equipment, an external
suppressor should be connected as close as possible to the source of the
transient for maximum attenuation.
NOTE: A surge suppressor increases drop-out time of an
electromechanical device.
3.4.3 Design of Wiring for Maximum Protection
Once the installation site and power conductors have been examined, the
system wiring plans that will provide noise suppression should be considered.
Conducted noise enters solid-state control at the points where the control is
connected to input lines, output lines, and power supply wires.
Input circuits are the circuits most vulnerable to noise. Noise may be
introduced capacitatively through wire to wire proximity, or magnetically from
nearby lines carrying large currents. In most installations, signal lines and
power lines should be separate. Further, signal lines should be appropriately
routed and shielded according to manufacturer's recommendations.
When planning system layout, care must be given to appropriate grounding
practice. Because design differences may call for different grounding, the
control manufacturer's recommendations should be followed.
C.3.4.3 Design of Wiring for Maximum Protection
Noise can also occur in the form of electromagnetic radiation. Examples
include radio frequency (RF) energy emanating from portable transceivers
(walkie-talkie) and fixed station transmitters of various types. Close
coupling is not required; the various lines entering the system act as
receiving antennas. Tests described in 3.4.2 may not be sufficient to
demonstrate noise immunity to radio frequency signals. Variations in
building construction and equipment installation make it impossible for
equipment manufacturers to perform meaningful tests of radio frequency
sources. RF fields are affected by concentrating masses of metal such as
steel beams, piping, conduit, metal enclosures, and equipment used in
production such as fork lift trucks and products being transported on
conveyors.