THE
MAJOR PREREQUISITE OF AN INSULATED CABLE IS THAT IT
REMAIN ELECTRICALLY STABLE WITHIN THE LIMITS OF THE
INTENDED APPLICATION
The
basic function of an insulated cable can be summed up
in two words - transmit power. To perform this function
effectively, the electrical characteristics of the insulated
cable must remain stable and predictable within given
limits. This fundamental requirement is the basis for
a major share of research and development effort.
In
a broad sense, the environments, which define the limits
of a cable application, can be divided into four areas:
(1)
Physical Environment
Both
cable installation and actual operation must be
evaluated. Generally, stationarytype cables are
exposed to damage during installation and portable
cables during actual service. Severe bending, compression,
cutting, abrasion, and excessive tension can all
contribute to a mechanical type of damage, which
reduces the reliability of a cable installation.
(2)
Chemical Environment
Cable
components are chemical compounds or mixtures of
compounds. As such their reactions are predictable
and subject to some control. Neoprene and polyethylene,
for example, are chemically as different as sugar
and salt. Chemical environments, such as free chlorine,
oil, ozone, etc., can influence the choice of materials
for insulations and jackets.
(3)
Thermal Environments
A
basic law of chemistry - the speed of a chemical
reaction is doubled with a 10C rise in temperature
- plays a vital part in cable application. Elevated
conductor temperature or high ambients will accelerate
the degradation of insulations and jackets.
(4)
Electrical Environment
Magnetic
and static electrical fields can influence the desired
stability of a cable - for example, causing interference
of signals in data logging control cables requiring
adjustments in cable design.
It
is readily apparent that no two applications are exactly
alike and that few will operate within the framework
of all the limits described. This allows latitude in
choice of material and cable design to obtain a balance
between economy and sound engineering.
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