a. The simple grounding of elements of a
communications facility is only one of several measures necessary to achieve a
desired level of protection and electrical noise suppression. To provide a low
impedance path for
(1) the flow of ac electrical current
to/from the equipment and
(2) the achievement of an effective
grounding system, various conductors, electrodes, equipment, and other metallic
objects must be joined or bonded together.
Each of these bonds should be made so that
the mechanical and electrical properties of the path are determined by the
connected members and not by the interconnection junction. Further, the joint
must maintain its properties over an extended period of time, to prevent
progressive degradation of the degree of performance initially established by
the interconnection.
Bonding is concerned with those techniques
and procedures necessary to achieve a mechanically strong, low-impedance
interconnection between metal objects and to prevent the path thus established
from subsequent deterioration through corrosion or mechanical looseness.
b. The ability of an electrical shield to
drain off induced electrical charges and to carry sufficient out of-phase
current to cancel the effects of an interfering field is dependent upon the
shielding material and the manner in which it is installed.
Shielding of sensitive electrical circuits
is an essential protective measure to obtain reliable operation in a cluttered
electromagnetic environment. Solid, mesh, foil, or stranded coverings of lead,
aluminum, copper, iron, and other metals are used in communications facilities,
equipment, and conductors to obtain shielding.
These shields are not fully effective unless
proper bonding and grounding techniques are employed during installation.
Shielding effectiveness of an equipment or subassembly enclosure depends upon
such considerations as the frequency of the interfering signal, the
characteristics of the shielding material, and the number and shapes of
irregularities (openings) in the shield.
Interference-causing signals are associated
with time-varying, repetitive electromagnetic fields and are directly related
to rates of change of currents with time. A current-changing source generates
either periodic signals, impulse signals, or a signal that varies randomly with
time.
To cause interference, a potentially
interfering signal must be transferred from the point of generation to the
location of the susceptible device. The transfer of noise may occur over one or
several paths. There are several modes of signal transfer (i.e., radiation,
conduction, and inductive and capacitive.
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