ELECTRICAL INSULATORS
Any materials that retard the flow of electricity and are used to prevent the passage or escape of electric current from conductors. No materials are absolute nonconductors; those rating lowest on the scale of conductivity are therefore the best insulators. An important requirement of a good insulator is that it not absorb moisture which would lower its resistivity. Glass and porcelain are the most common line insulators because of low cost. Pure silica glass has an average dielectric strength of 500 V/mil (20 X 106 V/m), and glass-bonded mica about 450 V/mil (17.7 X 106 V/m), while ordinary porcelain may be as low as 200 V/mil (8 X 106 V/m), and steatite about 240 V/mil (9.4 X 106 V/m). Slate, steatite, and stone slabs are still used for panelboards, but now a great variety of insulating boards are made by compressing glass fibers, quartz, or minerals with binders, or standard laminated plastics of good dielectric strength may be used. Vulcoid, of Budd Co., is typical. For slots and separators, natural mica is still valued because of its heat resistance, but because of the irregular quality of natural mica and the difficulty of handling the small pieces, it has been largely replaced by synthetic mica paper, polyester sheet, or impregnated papers or fabrics. The impregnated fish paper called Armite comes in thicknesses down to 0.004 in (0.010 cm) and has a dielectric strength of 500 V/mil (20 X 106 V/m).
Synthetic rubbers and plastics have now replaced natural rubber for wire insulation, but some aluminum conductors are insulated only with an anodized coating of aluminum oxide. Wires to be coated with an organic insulator may first be treated with hydrogen fluoride, giving a coating of copper fluoride on copper wire and aluminum fluoride on aluminum wire. The thin film of fluoride has high dielectric strength and heat resistance. The AIEE classification for wire insulation is by heat resistance, from Class O insulation, for temperatures to 195°F (90°C), to Class C insulation, for temperatures above 355°F (179°C).
Insulating oils are mineral oils of high dielectric strength and high flash point employed in circuit breakers, switches, transformers, and other electrical apparatus. An oil with a flash point of 285°F (140°C) and fire point of 310°F (154°C) is considered safe. A clean, well-refined oil will have high dielectric strength, but the presence of as low as 0.01% water will reduce the dielectric strength drastically. The insulating oils, therefore, cannot be stored for long periods because of the danger of absorbing moisture. Impurities such as acids or alkalies also detract from the strength of the oil. Since insulating oils are used for cooling as well as insulating, the viscosity should be low enough for free circulation, and they should not gum. Askarel is an ASTM designation for insulating fluids which give out only nonflammable gases if decomposed by an electric arc. They are usually chlorinated aromatic hydrocarbons such as trichlorobenzene (TCB), but fluori-nated hydrocarbons are also used. They have high dielectric strength, and a dielectric constant below 2. Askarel also refers to dielectric fluids containing polychlorinated bi-phenyls (PCBs), which had been widely used in transformers. These fluids, which may contain as much as 50% PCBs, have been replaced because of environmental concerns regarding PCBs. Insulating gases are used to replace air in closed areas to insulate high-voltage equipment. Sulfur hexafluoride for this purpose has a dielectric strength 2.35 times that of air. The insulating oil, fluids, and gases are generally referred to as dielectrics, although this term embraces any insulator.
Insulation porcelain, or electrical porcelain, is not usually an ordinary porcelain except for common line insulators. For such uses as spark plugs they may be molded silica, and for electronic insulation they may be molded steatite or specially compounded ceramics, more properly called ceramic insulators. Insulation porcelains compounded with varying percentages of zirconia and beryllia have a crystalline structure and good dielectric and mechanical strengths at temperatures as high as 2000°F (1093°C). These porcelains may have some magnesia, but are free of silica. However, zircon porcelain is made from zirconium silicate, and the molded and fired ceramic is equal to high-grade steatite for high-frequency insulation. Vitrolain, of Star Porcelain Co., is an electrical porcelain of high strength and density with porosity of only 0.25%. Thyrite, of General Electric Co., is a porcelain that possesses the property of being an insulator at low potentials and a conductor at high potentials. It is used for lightning arresters. The German Hartporzellan, or hard porcelains, are specially compounded ceramics having good resistance to thermal shock. The material called Nolex by the Naval Surface Weapons Center (NSWC) is made by hot-molding finely powdered synthetic fluorine mica. The molded parts are practically pure mica. They can be machined, have high-dimensional precision because they need no further heat treatment, and have high dielectric strength. Beryllia is a valued insulator for encapsulation coatings on heat-generating electronic devices as it has both high electrical resistivity and high heat conductivity.
Most ceramics are electrical insulators and are used widely for insulation of electric power lines. The applications range from structural power insulators to electronic packaging and substrates. Of particular interest is the use of ceramics as substrates. The ceramics serve as the structural and insulating base on which electronic components are deposited or attached. The requirement for good surface finish has led to the development of fine-grained alumina material that can be prepared with a very good finish. The requirement for high thermal conductivity in some applications has led to the use of beryllia, high-purity alumina, and more recently, aluminum nitride as substrate materials. Other materials are silicon carbide doped with beryllia to give electrical insulation and glass-ceramics which can easily be produced in the complex shapes often needed. They can also be produced with a tailored thermal expansion coefficient multilayer ceramic (MLC) substrate for highspeed computer processing modules.

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