Telecommunications Protection
Telecommunications Protection
Because early telecommunications equipment was constructed with components such as
mechanical relays, coils, and vacuum tubes, it was somewhat immune to lightning and
power cross conditions. But as cross bar and step-by-step switches have given way to more
modern equipment such as digital loop carriers, repeater amplifiers, and multiplexers, an
emphasis has been put on protecting this equipment against system transients caused by
lightning and power cross conditions.
Lightning
During an electrical storm, transient voltages are induced onto the telecommunications
system by lightning currents which enter the conductive shield of suspended cable or
through buried cables via ground currents.
As this occurs, the current traveling through the conductive shield of the cable produces an
equal voltage on both the Tip and Ring conductors at the terminating ends. Known as a
longitudinal voltage surge, the peak value and wave-form associated with this condition is
dependent upon the distance the transient travels down the cable and the materials with
which the cable is constructed.
Although lightning-induced surges are always longitudinal in nature, imbalances resulting
from terminating equipment and asymmetric operation of primary protectors can result in
metallic transients as well. A Tip-to-Ring surge is normally seen in terminating equipment
and is the primary reason most regulatory agencies require telecom equipment to have
both longitudinal and metallic surge protection.
Power Cross
Another system transient that is a common occurrence for telecommunications cables is
exposure to the AC power system. The common use of poles, trenches, and ground wires
results in varying levels of exposure which can be categorized as direct power cross, power
induction, and ground potential rise.
Direct power cross occurs when a power line makes direct contact to telecommunications
cables. Direct contact is commonly caused by falling trees, winter icing, severe
thunderstorms, and vehicle accidents. Direct power cross can result in large currents being
present on the line.
Power induction is common where power cables and telecommunications cables are run in
close proximity to one another. Electromagnetic coupling between the cables results in
system transients being induced onto the telecommunications cables, which in turn can
cause excessive heating and fires in terminal equipment located at the cable ends.
Ground potential rise is a result of large fault currents flowing to Ground. Due to the varying
soil resistivity and multiple grounding points, system potential differences may result.
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