APPENDIX B – GROUNDING GUIDELINES
B-5
Hardware and Installation User Manual
B.4
MINIMIZING ELECTRICAL NOISE
The following paragraphs provide information to help reduce the possibility of
electrical noise problems.
B.4.1
Definition and Source
Electrical noise is defined as any unwanted electrical signal which enters the
control equipment. Noise signals cover the entire spectrum of frequencies and
may have any waveshape. The largest single difficulty with noise is that it is not
always present. Continuous, or frequent, periodic noises generally are easy to
detect and remedy. Intermittent noise sources that produce short, high-energy
bursts at irregular and widely spaced intervals cause the majority of
difficulties.
Noise has a number of different pathways into control equipment. It can be
conducted through signal or power wiring or can be radiated by electromagnetic
waves. Conducted noise typically is coupled with the signal or power wiring,
either electrostatically or magnetically. Electrostatic coupling occurs through
parasitic capacitance between the noisy line and the signal/power line. This
requires high voltage or high rate of change of voltages in the noisy line and
high parasitic capacitance between lines. This typically would be the case for
long wire runs in the same conduit.
Magnetic coupling occurs through parasitic mutual inductances between lines.
This requires high currents or high rates of change of current, as well as
significant mutual inductance, which may result from proximity of wiring.
Electromagnetically radiated noise typically is high frequency (radio waves).
The control system and its wiring may act as antennas in picking up noise
signals. This pathway is least likely to present problem levels of noise, but its
sources are common in industrial applications.
The primary sources of noise in industry are those devices (and their wiring)
that produce and switch high voltage and current. Typical examples include
large motors, welders and contactors that switch heavily inductive loads such
as brakes or clutches.
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