TYPES OF INSULATION
The insulating materials discussed in the
next paragraphs are commonly used in electrical and electronic equipment.
Rubber
One of the most common types of insulation
is rubber. The voltage that may be applied to a rubber- covered conductor is
dependent on the thickness and the quality of the rubber covering. Other
factors being equal, the thicker the
insulation, the higher may be the applied voltage. Rubber insulation is normally used for low- or medium-range
voltage. Figure 1-7 shows two types of rubber-covered wire.
One is a two-conductor cable in which each
stranded conductor is covered with rubber insulation; the other is a single, solid conductor. In each
case, the rubber serves the same purpose: to confine the current to its conductor.
Referring to the enlarged cross-sectional
view in figure above, note that a thin coating of tin separates the copper conductor from the rubber
insulation. If the thin coating of tin were not used, a chemical action would take place and the rubber would become
soft and gummy where it makes contact with the copper.
When small, solid, or stranded conductors
are used, a winding of cotton threads is applied between the conductors and the rubber insulation.
CODE-GRADED RUBBER.
Code-graded rubber is the standard that the National Electrical
Code (NEC) has adopted as the minimum
requirements for rubber insulation as specified by Underwriters' Laboratories. In this code
system, the letter R indicates the use of a rubber insulator. Type R signifies that the wire is rubber coated.
The NEC codes Type RH and Type RHH signify
a rubber heat-resistant compound. Type RW signifies a rubber moisture-resistant
compound. A Type RHW signifies a rubber heat- and moisture-resistant compound.
Type RHW is approved for use in wet or dry locations at a maximum
conductor temperature of 75º C.
Neoprene, a low-voltage compound, is the one exception to Type RHW.
Although not a rubber compound, neoprene
meets the requirements of Underwriters' Laboratories and was designated Type RHW.
LATEX RUBBER.
Latex rubber is a high-grade compound consisting of 90 percent unmilled grainless rubber. There are two designations for this type of insulation: Type RUH and Type RUW. Type RUH (rubber unmilled heat-resistant) is used in dry locations when the conductor temperature does not exceed 75º C. Type RUW (rubber unmilled moisture-resistant) is used in wet locations when the conductor does not exceed 60º C.
SILICONE
Plastics
Plastic is one of the more commonly used
types of insulating material for electrical conductors. It has good insulating, flexibility, and
moisture-resistant qualities. Although there are many types of plastic insulating materials, thermoplastic is one of
the most common. With the use of thermoplastic, the conductor temperature can be higher than with
some other types of insulating materials without damage to the insulating quality of the material.
Plastic insulation is normally used for low- or medium-range voltage.
The designators used with thermoplastics
are much like those used with rubber insulators. The following letters are used when dealing with
NEC type designators for thermoplastics:
T
Thermoplastic
H
Heat-resistant
W
Moisture-resistant
A Asbestos
N
Outer nylon jacket
M
Oil-resistant
For example, a NEC designator of Type THWN
would indicate thermoplastic heat- and moisture- resistant with an outer nylon
jacket.
Varnished Cambric
Varnished cambric insulation can withstand much higher temperatures than rubber insulation.
Varnished cambric is cotton cloth that has
been coated with an insulating varnish. Figure above shows a cable covered with varnished cambric
insulation. The varnished cambric is in tape form and is wound around the conductor in layers. An oily
compound is applied between each layer of the tape to prevent water from seeping through the insulation.
It also acts as a lubricant between the layers of tape, so they will slide over each other when the cable is
bent.
Cambric insulation is used on extremely
high-voltage conductors used in substations and powerhouses. It is also used in other
locations subjected to high temperatures. In addition, it is used on the coils and leads of high-voltage generators.
Transformer leads also use this insulation because it is unaffected by oils or grease and has high
dielectric strength. Varnished cambric and paper insulation for cables are the two types of insulating
materials most widely used at voltages above 15,000 volts. Such cable is always lead covered to keep out
moisture.
Extruded Polytetrafluoroethylene
Extruded polytetrafluoroethylene is a
high-temperature insulation used extensively in aircraft and equipment installations. It will not burn,
but will vaporize when subjected to intense heat. Conductors for high temperatures use a nickel coating rather
than tin or silver to prevent oxidation. Nickel-coated wire is more difficult to solder, but makes
satisfactory connections with proper soldering techniques.
Fluorinated Ethylene Propylene
(FEP)
FEP has properties similar to extruded
polytetrafluoroethylene, but will melt at soldering temperatures. It is rated at 200º C and is,
therefore, considered a high-temperature insulation. There are no known toxic vapors from FEP. Common-sense
practice, however, requires that you provide adequate ventilation during any soldering operation.
Asbestos
Asbestos insulation was used extensively in
the past for high-temperature insulation. Today, it is seldom used by the Navy. Many naval ships and
aircraft, however, still contain asbestos-insulated wiring.
Aboard ship, this is particularly true in
galley and laundry equipment. The reason for discontinuing the use of asbestos as an insulator is that
breathing asbestos fibers can produce severe lung damage. It can render you disabled or cause fatal fibrosis
of the lungs. Asbestos is also a factor in the development of cancer in the gastrointestinal tract. Safety
precautions concerning asbestos will be covered in more detail.
One type of asbestos-covered wire is shown
in figure above. It consists of stranded copper conductors covered with felted asbestos. The wire is, in
turn, covered with asbestos braid. This type of wire is used in motion-picture projectors, arc lamps, spotlights,
heating element leads, and so forth.
Another type of asbestos-covered cable is shown in figure above. It is combination of asbestos and varnished cambric. This type of insulation serves as leads for motors and transformers that sometimes must operate in hot, damp locations. The varnished cambric covers the inner layer of felted asbestos. This prevents moisture from reaching the innermost layer of asbestos. Asbestos loses its insulating properties
when it becomes wet. It will, in fact,
become a conductor. Varnished cambric prevents this from happening because it resists moisture.
Although this insulation will withstand some moisture, it should not be used on conductors that may at times
be partially immersed in water. Under those circumstances, the insulation must be protected with an
outer lead sheath.
Paper
Paper has little insulation value alone.
However, when impregnated with a high grade of mineral oil, it serves as a satisfactory insulation for
extremely high-voltage cables. The oil has a high dielectric strength, and tends to prevent breakdown of
the paper insulation. The paper must be thoroughly saturated with the oil. The thin paper tape is wrapped
in many layers around the conductors, and then soaked with oil.
Silk and Cotton
In certain types of circuits (for example,
communications circuits), a large number of conductors are needed, perhaps as many as several hundred.
Next Figure shows a cable containing many conductors.
Each is insulated from the others by silk
and cotton thread. Because the insulation in this type of cable is not subjected to high voltage, the use of
thin layers of silk and cotton is satisfactory.
Silk and cotton insulation keeps the size
of the cable small enough to be handled easily. The silk and cotton threads are wrapped around the
individual conductors in reverse directions. The covering is then impregnated with a special wax compound.
Enamel
The wire used on the coils of meters,
relays, small transformers, motor windings, and so forth, is called magnet wire. This wire is insulated
with an enamel coating. The enamel is a synthetic compound of cellulose acetate (wood pulp and magnesium).
In the manufacturing process, the bare wire is passed through a solution of hot
enamel and then cooled. This process is repeated until the wire acquires from
6 to 10 coatings. Thickness for
thickness, enamel has higher dielectric strength than rubber.
It is not practical for large wires because
of the expense and because the insulation is readily fractured when large wires are bent.
Mineral Insulated
Mineral-insulated (MI) cable was developed
to meet the needs of a noncombustible, high heat- resistant, and
water-resistant cable. MI cable has from one to seven electrical conductors.
These conductors are insulated in a
highly compressed mineral, normally magnesium oxide, and sealed in a liquidtight, gastight metallic tube, normally
made of seamless copper .
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