Electrical Insulation

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 

Silicone is a rubber compound that does not carry the "R" designator for many of its  applications. An example of this is Type SA (silicone-asbestos). In Type SA, the insulator around the  conductor is silicone rubber, but the outer covering must consist of heavy glass, asbestos-glass, or  asbestos braiding treated with a heat, flame, and moisture-resistant compound.

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  .