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ELECTRICAL NOISE
Electrical noise refers to stray electrical signals
in the atmosphere at various signal strengths
and frequencies that can affect the operation
of equipment. One of the best ways to prevent
this is to shield the equipment and cables within
a continuous ground envelope, such that any
incident noise will be conducted to earth ground
before it can affect the circuit conductors.
For conductors inside the control panel the ground-
ed enclosure provides this envelope.
For factory supplied control cables shielded cable
has been used. The shield typically consists of an
overall foil shield in combination with an overall
braided shield. This provides the most effective
shielding, as the foil covers the "holes" in the braid,
and the braid allows for practical 360° termination
at both ends of the cable.
The AC input cord is not shielded, but instead is
directed to an AC line filter as soon as it enters the
cabinet. This filter filters out any noise that comes
in on the AC line. For maximum noise immunity
the AC line should connect to the filter as soon
as it enters the cabinet with as short of leads as
pos-sible. Additional noise protection is provided
by running the AC input line to the control panel
in grounded conduit, which is the recommended
method and is required by most codes.
For maximum noise protection any user supplied
input/output (I/O) wiring should be made using
shielded cable or conduit which is connected to
earth ground in a continuous 360° fashion at both
ends. The best way to do this is to use a connector/
fitting at each end of the cable/conduit that makes
contact to the grounded enclosure in this manner.
Connecting the drain wire of a shield to a ground
point on or in the cabinet (usually referred to as
pigtailing) is not an effective method of shielding
and actually making things worse (see Figure 2).
Cable is recommended for the DC I/O. Again, for
maximum noise immunity the cabling must contain
overall foil and braided shields and be terminated
in a continuous 360° manner as previously de-
scribed. Special fittings have been provided on
the control panel for termination of these cables
where they enter the cabinet. The use of these
fittings is described under "Electrical I/O" in the
"Installation" section of this manual.
Figure 2: Cable Connection Examples
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