Insulation -Electrical

Live-front switchboards upto the early part of the 20th century were made of slate or marble. Some high voltage equipment is designed to operate within a high pressure insulating gas such as sulphur hexafluoride. Insulation materials that perform well at power and low frequencies may be unsatisfactory at radio 

Rubber (natural, butyl, silicone) Polyamide (nylon) 

Polyvinyl chloride (PVC) Polymethylmetachrylate (PMMA) Polycarbonate (PC) Polytetrafluoroethylene (PTFE) 

When the electric field is high enough a dielectric may suddenly lose its property of non-conduction, permanently or temporarily, showing an electrical breakdown. 

It consists in the abrupt rise of electrical current under the effect of an electric field. Its causes depend on the medium, the geometry and the type and amplitude of the electric field. 

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Electrical insulation with oxidized bitumen electrical cabling, condensers, batteries. 

Figure C2.11.4. A commercial spark plug witli its electrically insulating ceramic body comprised of alumina and glass (white portion).

Ethylene. Under the influence of pressure and a catalyst, ethylene yields a white, tough but flexible waxy sohd, known as Polythene. Polyethylene possesses excellent electric insulation properties and high water resistance it has a low specific gravity and a low softening point (about 110°). The chemical inertness oi Polythene has found application in the manufacture of many items of apparatus for the laboratory. It is a useful lubricant for ground glass connexions, particularly at relatively high temperatures. 

Tetrafluoroethylene. Emulsion polymerisation of tetrafluoroethylene, catalysed by oxygen, yields polytetrafluoroethylene (Tejlon) as a very tough horn-hke material of high melting point. It possesses excellent electrical insulation properties and a remarkable inertness towards all chemical reagents, including aqua regia. 

The isotope boron-10 is used as a control for nuclear reactors, as a shield for nuclear radiation, and in instruments used for detecting neutrons. Boron nitride has remarkable properties and can be used to make a material as hard as diamond. The nitride also behaves like an electrical insulator but conducts heat like a metal. 

As the demand for rubber increased so did the chemical industry s efforts to prepare a synthetic sub stitute One of the first elastomers (a synthetic poly mer that possesses elasticity) to find a commercial niche was neoprene discovered by chemists at Du Pont in 1931 Neoprene is produced by free radical polymerization of 2 chloro 1 3 butadiene and has the greatest variety of applications of any elastomer Some uses include electrical insulation conveyer belts hoses and weather balloons... 

Insulation, electrical Insulation materials Insulation, thermal... 

Electrical Behavior. The resistivity of acetate varies significantly with humidity with typical values ranging from 10 ohm-cm at 45% rh to 10 ohm-cm at 95% rh (16). Because of the high resistivity both acetate and triacetate yams readily develop static charges and an antistatic finish is usually apphed to aid in fiber processing. Both yams have also been used for electrical insulation after lubricants and other finishing agents are removed. 

Polyesters are also used in continuous filament spunbonded nonwovens (see Nonwoven fabrics). Reemay spunbonded fabric is composed of continuous filament PET with a polyester copolymer binder. These spunbonded fabrics are available in a wide range of thicknesses and basis weights and can be used for electrical insulation, coated fabric substrates, disposable apparel for clean rooms, hospitals, and geotextiles (qv). 

Polyimide. Polyimide is a biaxiaHy oriented high performance film that is tough, flexible, and temperature- and combustion-resistant. Its room temperature properties compare to poly(ethylene terephthalate), but it retains these good characteristics at temperatures above 400°C. Its electrical resistance is good and it is dimensionally stable. The principal detriment is fairly high moisture absorbance. The main uses are for electrical insulation, particularly where high temperatures are prevalent or ionizing radiation is a problem. The films may be coated to reduce water absorption and enhance... 

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

Polytetrafluoroethylene (PTFE) provides the most satisfactory electrical insulation. Concentric rings of PTFE and PTFE impregnated with calcium fluoride are used for the packing glands which support the anode and cathode posts. Rubber is used as the gasket material to form a seal between the cover... 

J. K. Olthoff and co-workers. Conference Kecord of the IEEE International Symposium on Electrical Insulation, 1990, pp. 248—252. 

Electrical Properties. Polytetrafluoroethylene is an excellent electrical insulator because of its mechanical strength and chemical and thermal stabihty as well as excellent electrical properties (Table 6). It does not absorb water and volume resistivity remains unchanged even after prolonged soaking. The dielectric constant remains constant at 2.1 for a temperature range of —40 to 250°C and a frequency range of 5 Hz to 10 GHz. 

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