Rockwell Automation Safety Guidelines for the Application, Installation, and Maintenance of Solid-State Control - Manual (Page 13)

Publication SGI-1.1 - August 2009
Section 3: Application Guidelines 13
3.4 Planning Electrical
Noise Rejection The low energy levels of solid-state controls may cause them to be vulnerable
to electrical noise. This should be considered in the planning stages.
3.4.1 Assessing Electrical Environment
Sources of noise are those pieces of equipment that have large, fast changing
voltages or currents when they are energized or de-energized, such as motor
starters, welding equipment, SCR type, adjustable speed devices and other
inductive devices. These devices, as well as the more common control relays
and their associated wiring, all have the capability of inducing serious current
and voltage transients on their respective power lines. It is these transients
which nearby solid-state controls must withstand and for which noise
immunity should be provided.
An examination of the proposed installation site of the solid-state control
should identify equipment that could contaminate power lines. All power lines
that will be tapped by the proposed solid-state control should be examined for
the presence, severity, and frequency of noise occurrences. If found, system
plans should provide for the control of such noise.
Comments: 3.4.1 — Assessing Electrical Environment
Noise can also occur in the form of electromagnetic radiation, or due to
improper grounding practices. Section C.3.4.3 explains these forms of
noise and precautionary measures that should be taken for protection
against them.
In many instances a system may begin to malfunction some time after it
has been installed and is working properly. This may be due to recent
installation of new equipment capable of inducing noise into presently
operating systems. Thus, it is not sufficient to merely evaluate a system at
the time of installation. Periodic rechecks should be made, especially as
other equipment is moved, modified, or newly installed. When installing a
solid-state system, it is wise to assume various noise sources exist and
install the system to guard against possible interference.
3.4.2 Selecting Devices to Provide Noise Immunity
Installation planning is not complete without examination of the noise
immunity characteristics of the system devices under consideration. Results of
tests to determine relative immunity to electrical noise may be requested from
the manufacturer. Two such standardized tests are the ANSI (C37.90a-1974)
Surge Withstand Capability Test and the NEMA (ICS.1-1983) noise test
referred to as The Showering Arc Test. These are applied where direct
connection of solid-state control to other electromechanical control circuits is
intended. Circuits involving analog regulating systems or high speed logic are
generally more sensitive to electrical noise; therefore, isolation and separation
of these circuits is more critical.
Further information on electrical noise and evaluation of the severity of noise
may be found in ANSI/IEEE Publication No. 518-1982.