CBE 520, Безопасность, Установка, Эксплуатация, Обслуживание

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(such as copper lugs). Minimum wire sizes, color coding, and general safety practices should
comply with American National Standards and the National Electrical Code.
Other earth ground sources are not as desirable as a copper ground stake. Green wire earth grounds
(safety grounds) brought into the control system from plants distribution networks tend to be very
noisy. In most plants, the green wire earth ground system is characterized by ground loops, multiple
terminations to different references, and long wire runs adjacent to motor power and other high-
power wiring. Conduit as an earth ground has even more problems. Though many installations use
these ground references successfully, the practice should be avoided.
ELECTRICAL NOISE CONSIDERATIONS
A major consideration in the installation of a successful system is the problem created by electrical
noise. The following paragraphs provide information to help the user avoid electrical noise
problems. Though many potential problems are presented, few, if any, will be encountered in an
actual application using a suitably installed PLC system, even in a relatively harsh industrial noise
environment.
BACKGROUND NOISE
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 wave shape. The largest
single difficulty with noise is that it is not always present. Continuous, or frequent, periodic noises
are generally easy to detect and accommodate. Intermittent noise sources that produce short, high
energy bursts at irregular and widely spaced intervals cause the majority of problems. Noise has a
number of different pathways into the control equipment. It can be conducted through signal or
power wiring or it can be radiated by electromagnetic waves. Conducted noise is typically coupled
into the signal or power wiring either electro-statically or magnetically. Electrostatic coupling
occurs through parasitic capacitance between the noisy line and the signal/power line. 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 currents or high rate
of change of current as well as significant mutual inductance, which may result from proximity or
wiring. Electromagnetically radiated noise is typically 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 to a PLC, and its sources are rare industrial applications. The
dominate sources of noise in industry are those devises (and their wiring) that produce and switch
high voltages and currents. Typical examples include large motors, welders, and contactors that
switch heavily inductive loads such as brakes or clutches. Other examples of noise sources are
Triac/SCR motor control and power invertors.