A CABLE
IS A CAPACITOR
A
capacitor is an electrical device consisting
essentially of two conducting surfaces separated
by an insulating material (dielectric) such
as air, paper, oil, glass, or rubber.
A
close look at a cross-section of a shielded
cable reveals a striking resemblance to the
description of a capacitor. The copper strand
is one conducting surface, the cable insulation
is the dielectric, and the shield the second
conducting surface.
The
fact that a cable behaves like a capacitor
must be taken into consideration during cable
design and application. The following characteristics
of a capacitor as related to cable
require special attention:
- A
Capacitor Stores Electrical Energy: The
property of an insulation which determines
how much electrostatic energy can be stored
per unit volume when unit voltage is applied
is called S.I.C. (specific inductive capacity).
In effect, the S.I.C. of an insulation
is the ratio of the amount of energy that
a given capacitor can store when it has
the insulation between its plates to the
amount of energy that the same capacitor
can store when it has air (actually a
vacuum) between its plates. For example,
if a certain air capacitor has a measured
capacitance of one M F (microfarad), but
when the air is replaced with an insulation
and the measured capacitance is 3 MF,
then that insulating material has an S.I.C.
of about 3. S.I.C. is also referred to
as dielectric constant and permittivity.
For a capacitor, whose purpose is to store
electrical energy, a high S.I.C. is desirable;
but for a cable to transport energy, a
low S.I.C. is best. If the S.I.C. of the
dielectric is too high, energy is stored
in the insulation and the cable will not
be an efficient conductor of electricity.
Generally for 600-volt cables, the S.I.C.
is kept below 7. For l5Kv cables below
4 and cables above 15Kv the S.I.C. value
is kept as low as it is possible to obtain
in any given insulation.
- A
Capacitor Permits the Flow of Alternating
Current to a Degree Dependent on the Capacitance
and Frequency: Applying this characteristic
of capacitors to cables one could say
there is no such thing as a perfect insulation
- A-C current appears to flow through
any insulation. The amount or degree is
dependent on the S.I.C. of the insulation
and the frequency (60-cycle, etc.) of
the current.
The
fact that a capacitor permits the flow of
A-C current carries considerable weight in
cable design and application. Some feel for
its importance can be grasped when it is a
major influence in situations such as:
- Should
a cable be shielded or non-shielded?
- A
cable shield should always be grounded
- Cause
for external corona in an installation.
- Significant
factor in choice of cable size.
- Geometry
in cable design.
- Spacing
of conductors during installation.
- Shielding
in data-log cables.
