BASIC DIELECTRIC THEORY OF CABLE
Theodore A. Balaska and Carl C. Landinger 12-1, 2 4
Whether being used to convey electric power or signals, it is the purpose of a
wire or cable to convey the electric current to the intended device or location. In
order to accomplish this, a conductor is provided which is adequate to convey
the electric current imposed. Equally important is the need to keep the current
from flowing in unintended paths rather than the conductor provided.
Insulation is provided to largely isolate the conductor from other paths or
surfaces through which the current might flow. Therefore, it may be said that
any conductor conveying electric signals or power is an insulated conductor.
2. AIR INSULATED CONDUCTORS
A metallic conductor suspended from insulating supports, surrounded by air,
and carrying electric signals or power may be considered as the simplest case of
an insulated conductor. It also presents an apportunitY to easily visualize the
Location of Voltage and Current
In Figure 2-1, clearly the voltage is between the conductor and the ground [2-3,
2-41. Also, because of the charge separation, there is a capacitor and a large
resistance between conductor and ground. Finally, as long as ground is well
Copyright © 1999 by Marcel Dekker, Inc.
away from the conductor, the electric field lines leave the conductor as
reasonably straight lines emanating from the center of the conductor. We know
that all bend to ultimately terminate at ground.
Air is not a very good insulating material since it has a lower voltage
breakdown strength than many other insulating materials. It is low in cost if
space is not a constraint. As the voltage between the conductor and ground is
increased, a point is reached where the electric stress at the conductor exceeds
the breakdown strength or air. At this point, the air literally breaks down
producing a layer of ionized, conducting air surrounding the conductor. The
term for this i