Insulating materials, electrical

FIGURE 3.90 Test for DC resistance of insulating materials. Electrical resistance of specimen (ohm) = applied voltage (V)/current measured A). Volume resistivity (ohm-cm) = resistance of specimen (ohm) Xarc of upper electrode (cm )/specimen thickness (cm). Standard test methods ASTM D257, BS 2782 method 202. 

See also insulating materials (thermal) insulating material (electrical). 

Potassium titanate whiskers can be used as thermal insulation material, electrical insulating material, catalyst carriers, filtration material, friction material, and so on based on their special performances. In addition, they are applied as... 

Insulation, electrical Insulation materials Insulation, thermal... 

Some nonhygroscopic materials such as metals, glass, and plastics, have the abiUty to capture water molecules within microscopic surface crevices, thus forming an invisible, noncontinuous surface film. The density of the film increases as the relative humidity increases. Thus, relative humidity must be held below the critical point at which metals may etch or at which the electrical resistance of insulating materials is significantly decreased. 

Sulfur hexafluoride was first prepared in 1902 (1). The discovery in 1937 that its dielectric strength is much higher than that of air (2) led to its use as an insulating material for cables, capacitors (3), and transformers (4) (see Insulation,electric). Sulfur hexafluoride has been commercially available as AccuDri, SF (AUiedSignal Inc.) since 1948. It is also produced by Air Products and Chemicals in the United States and by others in Germany, Italy, Japan, and Russia. 

Phenol—formaldehyde resins are used as mol ding compounds (see Phenolic resins). Their thermal and electrical properties allow use in electrical, automotive, and kitchen parts. Other uses for phenol—formaldehyde resins include phenoHc foam insulation, foundry mold binders, decorative and industrial laminates, and binders for insulating materials. 

Two parallel plates of conducting material separated by an insulation material, called the dielectric, constitutes an electrical condenser. The two plates may be electrically charged by connecting them to a source of direct current potential. The amount of electrical energy that can be stored in this manner is called the capacitance of the condenser, and is a function of the voltage, area of the plates, thickness of the dielectric, and the characteristic property of the dielectric material called dielectric constant. 

R. Bartnikas, "Engineering Dielectrics Vol. II A—Electrical Properties of SoHd Insulating Materials Molecular Stmcture and Electrical Behavior," ASTM Special Technical Publication 783, 1983, Chapts. 1—5, pp. 3—515. 

Insulator Walls. Because of the unavailabiUty of electrically insulating materials which can withstand the harsh environment inside coal-fired channels, the insulator walls of the channel are typically made of metal elements which are insulated from each other to prevent any net flow of current. Like electrode walls, insulator walls are designed to operate with a slag coating. 

Properties desired in cable insulation and flexible circuit substrate materials include mechanical flexibiUty, fatigue endurance, and resistance to chemicals, water absorption, and abrasion. Both thermoplasts and thermosets are used as cable-insulating materials. Thermoplastic materials possess excellent electrical characteristics and are available at relatively low cost. 

Table 14. Mechanical and Electrical Properties of Cable Insulating Materials...

Excellent insulating properties, along with the abiUty to be stmctural components, make plastics the ideal candidate materials for electrical appHcations. Although generally used as insulators, carbon black or carbon fiber can be added to make plastic materials electrically conductive, thereby expanding their usefulness in the electronics area. 

ASTM D256, a. Test Methods for Impact Resistance of Plastics and Electrical Insulating Materials, Vol. 8.01, ASTM, Philadelphia, Pa., 1993. 

Self-Adhering Silicone Rubber Tape" in GeneralElectric Company bulletin. Insulating Materials Data PD 1302, General Electric Co., Schenectady,... 

General-Purpose Polystyrene. Polystyrene is a high molecular weight M = 2 — 3 x 10 ), crystal-clear thermoplastic that is hard, rigid, and free of odor and taste. Its ease of heat fabrication, thermal stabiUty, low specific gravity, and low cost result in mol dings, extmsions, and films of very low unit cost. In addition, PS materials have excellent thermal and electrical properties that make them useful as low cost insulating materials (see Insulation, ELECTRIC Insulation, thermal). 

Other sohd-state apphcations of sihcon carbide include its use as an electroluminescent diode for use in sound recording equipment and photomultipliers and controllers. It has been studied as a reflective surface for lasers. By combining its excellent thermal conductivity and high electrical resistance, sihcon carbide has also found apphcation as an insulating material for integrated circuit substrates. 

Dielectric Constant The dielectric constant of material represents its ability to reduce the electric force between two charges separated in space. This propei ty is useful in process control for polymers, ceramic materials, and semiconduc tors. Dielectric constants are measured with respect to vacuum (1.0) typical values range from 2 (benzene) to 33 (methanol) to 80 (water). TEe value for water is higher than for most plastics. A measuring cell is made of glass or some other insulating material and is usually doughnut-shaped, with the cylinders coated with metal, which constitute the plates of the capacitor. 

The common types of insulating materials in use for electric motors are E and B for small motors and F for medium sized and large ones. General industrial practice, however, is to limit the temperature to class B limits, even if class F insulation is used. 

Test procedure for, evaluation of system of insulating materials for random wound a.c. electrical machines. 

Electrical insulation is continually being improved. The motor manufacturers make use of this and other technological developments to put more power into smaller, lighter, more efficient packages. Modem insulating materials can withstand heat, moisture, and corrosive atmospheres, and new metals can withstand more mechanical punishment. Computer design techniques are also helpful. 

---