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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.
EFFECTS OF ELECTRICAL NOISE
The predominant effect of noise on the system is to cause "soft" failures; that is, failures which do not damage the
system but do cause it to function improperly. Three main types of soft failures are encountered:
1. Logic memory alteration.
2. Register alteration.
3. Momentary I/O failure.
Logic memory alteration presents the most significant potential problem in that it may, in some cases, result in
improper machine operation. Register and temporary I/O failure, normally result only in nuisance failures such as
operation in the wrong mode, machine glitches or incorrect or non-existent error messages.
GROUNDING
A good grounding system is a major consideration in planning any electrical system. However, it is essential for
proper operation of the electronics that a low-impedance path to earth ground exist. All filtering devices internal to
the PLC require a good earth ground return. The structural ground present in many industrial environments does
not provide an adequate ground return. A supplementary grounding electrode should be used to reduce the
impedance of the earth ground return when direct-wire connection to the power system is not feasible. As a
minimum, a No. 12 AWG stranded copper wire should be used to connect to the copper grounding electrode.
The connection should exhibit very low DC resistance (0.05 OHM) and low high-frequency impedance (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.
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