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An Insulator is a material that resists the flow of
electric current. It is an object intended to
support or separate electrical conductors without
passing current through itself. An insulation
material has atoms with tightly bonded valence
electrons. The term electrical insulation has the
same meaning as the term dielectric.
Some materials such as silicon dioxide or Teflon are
very good electrical insulators. A much larger class
of materials, for example rubber-like polymers and
most plastics are still "good enough" to insulate
electrical wiring and cables even though they may
have lower bulk resistivity. These materials can
serve as practical and safe insulators for low to
moderate voltages (hundreds, or even thousands, of
volts).
Electrical insulation
is the absence of electrical conduction. Electronic
band theory (a branch of physics) predicts that a
charge will flow whenever there are states available
into which the electrons in a material can be
excited. This allows them to gain energy and thereby
move through the conductor (usually a metal). If no
such states are available, the material is an
insulator.
Most (though not all, see Mott insulator) insulators
are characterized by having a large band gap. This
occurs because the "valence" band containing the
highest energy electrons is full, and a large energy
gap separates this band from the next band above it.
There is always some voltage (called the breakdown
voltage) that will give the electrons enough energy
to be excited into this band. Once this voltage is
exceeded, the material ceases being an insulator,
and charge will begin to pass through it. However,
it is usually accompanied by physical or chemical
changes that permanently degrade the material's
insulating properties.
Materials that lack electron conduction must also
lack other mobile charges as well. For example, if a
liquid or gas contains ions, then the ions can be
made to flow as an electric current, and the
material is a conductor. Electrolytes and plasmas
contain ions and will act as conductors whether or
not electron flow is involved.
High-voltage
insulators used for high-voltage power transmission
are made from glass, porcelain, or composite polymer
materials. Porcelain insulators are made from clay,
quartz or alumina and feldspar, and are covered with
a smooth glaze to shed dirt. The design of
insulators often includes deep grooves, or sheds,
that provides increased arc-lengths. Insulators made
from porcelain rich in alumina are used where high
mechanical strength is a criterion. Glass insulators
were (and in some places still are) used to suspend
electrical power lines. Some insulator manufacturers
stopped making glass insulators in the late 1960s,
switching to various ceramic and, more recently,
composite materials.
Recently, some electric utilities have begun
converting to polymer composite materials for some
types of insulators which consist of a central rod
made of fibre reinforced plastic and an outer
weathershed made of silicone rubber or EPDM.
Composite insulators are less costly, lighter in
weight, and have excellent hydrophobic capability.
This combination makes them ideal for service in
polluted areas. However, these materials do not yet
have the long-term proven service life of glass and
porcelain.
The first electrical
systems to make use of insulators were telegraph
lines; direct attachment of wires to wooden poles
was found to give very poor results, especially
during damp weather.
The first glass insulators used in large quantities
had an unthreaded pinhole. These pieces of glass
were positioned on a tapered wooden pin, vertically
extending upwards from the pole's crossarm (commonly
only two insulators to a pole and maybe one on top
of the pole itself). Natural contraction and
expansion of the wires tied to these "threadless
insulators" resulted in insulators unseating from
their pins, requiring manual reseating.
Amongst the first to produce ceramic insulators were
companies in the United Kingdom, with Stiff and
Doulton using stoneware from the mid 1840s, Joseph
Bourne (later renamed Denby) producing them from
around 1860 and Bullers from 1868. Utility patent
number 48,906 was granted to Louis A. Cauvet on July
25, 1865 for a process to produce insulators with a
threaded pinhole. To this day, pin-type insulators
still have threaded pinholes.
The invention of suspension-type insulators made
high-voltage power transmission possible. Pin-type
insulators were unsatisfactory over about 60,000
volts.
A large variety of telephone, telegraph and power
insulators have been made; some people collect them. |
