To select wire the instrumentation engineer specifies the conductor to be used, ie copper, or silver wire, or a type of thermocouple wire. The next consideration is the insulation. (Insulation is sometimes referred to as "dielectric"). The types include Teflon, PVC, polyethylene, rubber, etc. Each material has specific physical, electrical, thermal and chemical properties, and price. Vinyl may cost $ .60/lb, $8/lb for Teflon FEP and Tefzel, and $ 38/lb for Kapton.
Vinyl offers low Cost, good flexibility, fair electricals, a good general purpose material.
Irradiated PVC offers moderate cost, heat resistance, good electrical and physical properties, excellent abrasion, cut-through and solder resistance.
Teflon offers high cost, high temperature rating, excellent chemical resistance, excellent electrical properties, poor cut through resistance.
Tefzel offers high cost, good electrical and physical properties, excellent chemical resistance.
Kynar offers high cost, poor electrical properties, excellent physical properties.
Kapton offers very high cost, lightweight, small outside diameter,, excellent cut through properties, good electrical properties.
Polyethylene offers low cost, excellent electrical properties, high flammability, stiffer than vinyl.
Irradiated Polyethylene offers moderate cost, excellent electrical, physical, and chemical properties, flame retardant, resistant to environmental stress cracking and ozone.
Rulan offers the same as polyethylene except flame retardant.
Polysulfone offers low cost, good electrical properties, excellent physical properties, good handling properties, poor chemical resistance.
A conductor jacket over the primary insulation must be considered when additional mechanical protection is required. The standard jacket for vinyl Insulation Is nylon, and for polyethylene insulation vinyl or nylon. While causing a slight decrease in the flexibility of the Insulated wire, nylon 16 tough, has high tensile, Impact and flexural strength, excellent resistance to abrasion, good electrical properties, and resistance most solvents, alkalies, oils, gasolines, and most organic fluids. A nylon jacket is recommended when a shield is .to be applied over less than 15 mils of insulation. Its purpose is to act as a mechanical barrier and prevent shorting between the conductor and shield.
For special applications, a fire resistant tape Is used as a flame barrier. Cables without tapes may show the convolutions of the cabled conductors.
Overall jackets or sheaths are used to cover and protect the cable against mechanical damage, chemical or electrochemical attack, f Ire, or any other detrimental external influence. The rule of thumb to determine jacket thickness is that it should be approximately 10% of the cable core diameter. Standard cable jackets are vinyl, polyethylene, neoprene and polyurethane.
The jacket type may be a matter of individual preference or may depend upon cable use. Vinyl jackets which afford normal mechanical protection are generally specified for indoor applications and general purpose installations. A neoprene jacket, or thermoplastic rubber, especially the reinforced type, is specified when it is known that the cable will be susceptible to abuse and severe handling conditions. Flame retardant neoprene or thermoplastic rubber is used for jacketing cables installed in critical areas.
The increasing number of high frequency interference sources has emphasized the necessity for shielding insulated wires or components in electronic equipment. Shields, however, are used not only for screening a signal from external excitation, but also as a safety measure on high voltage lines and as an actual conductor in a circuit.
If a shield is required, the end user has a choice among several options - braided copper wire; spiral (served) copper wire; copper and aluminum tapes; laminates of aluminum/polyester and aluminum/glass/ aluminum with spiral drain wires for ease of termination; semi-conductive plastics; combinations of conductive glass/copper; and other variations and combinations.
The most commonly used type and the most effective in high frequency applications is a braided copper shield. The degree of protection from external Interference increases with the amount of shielding.
The most economical shield is aluminum polyester laminated tape used in conjunction with a drain wire applied either spirally or longitudinally, directly adjacent to the aluminum side of the tape.
In electronic cables, the accepted standard for overall shields are those made from braided round copper wire, or aluminum / polyester tape and drain wire.
Having selected the conductor and insulation, means of circuit Identification should w be considered. The end User has the choice of all white; solid colors; solid colors with bands, stripes, printed numbers or legends; or colored textile braids. Solid colors are most economical and colored textile braids most expensive. Each end user may have reasons for specifying a particular type of color coding, but It would be to his advantage to discuss the price differential each with sales and marketing personnel.
A wire design cannot be complete unless the voltage rating (a.c. or d.c.) and temperature rating (°C or °F) are specified. Voltage rating is related to Insulation wall thickness; temperature rating to the type of insulation .
An extremely reliable high temperature, low voltage insulation often chosen for its non-aging characteristics, thin wall insulating capability, resistance to chemicals and abrasion resistance. Also important are its low dielectric constant and low power factor. There are two (2) types-Tetrafluorethylene TFE, Fluorinatedethylenepropylene FEP. Teflon is not damaged by normal soldering operations. TFE insulation in tape form (often fused) is widely used and can be provided in very long lengths. Type FEP can be extruded in long, continuous lengths and is readily color coded for use in control and instrumentation cables.
Tefzel ETFE, rated at 150°C, Is a melt processible fluorocarbon thermoplastic combining many of the desirable properties of Teflon and Kynar. Mechanically it is tough, and has excellent flex life, and Impact, cut-through and abrasion resistance. Electrically It Is an excellent low loss dielectric and has outstanding electrical properties. Additionally, It is weather resistant, Inert to most solvents and chemicals and is hydrolytically stable. Like irradiated polyethylene It has excellent resistance to high-energy radiation. Insulation thicknesses from 5 to 100 mils are available.
An excellent dielectric with unusual heat resisting characteristics up to 2500°C. Silicone rubber has high dielectric strength, excellent ozone and corona resistance, is extremely flexible and fire resistant, burning to an insulating ash. Certain compounds of silicone rubber remain seviceable after exposure to Gamma radiation in excess of 108 Rads. It also has long storage life and good bonding properties so necessary in electrical and electronic applications.
Polyethylene is a good general purpose insulation that has excellent toughness, good brittleness characteristics at low temperatures, resistance to chemicals and moisture at a relatively low price. Polyethylene has good processibility and makes an excellent high frequency insulation which can be rated at, 750°C. High density polyethylene has superior heat resistant characteristics (900°C) and mechanical toughness.
This thermosetting polyethylene material can be compounded to retain much of the excellent electrical properties of polyethylene with the improved physical and thermal properties of thermosetting materials. Vulcanized polyethylene is rated at 900°C or 1250°C continuous operating temperatures. Cross linked polyethylene can also withstand higher temperatures for short periods of time (in case of emergency). It has high dielectric strength, excellent abrasion resistance, is crack resistant, and exhibits a good balance between physical, electrical, and mechanical properties.
A Vulcanized chlorosulphonated polyethylene (usually jacketing) compound which is a true thermosetting Neoprene-like material. Hypalon is very tough, has excellent flame resistance, chemical resistance, ozone and corona resistance, radiation resistance and is suitable for very rugged applications including direct burial. Hypalon is usually rated at 900°C.
Perhaps the mose common wire and cable material used today. There are many desirable formulations tailored to fit the needs of a particular application. Temperature ratings vary from 600°C to 1050°C. PVC resin will not support combustion; has good moisture, chemical and abrasion resistance. PVC can be custom compounded for low temperature flexibility, extreme flame retardance, low chloride content, or for a wide variety of specialized applications.
A light weight, thin wall 900°C insulating compound with characteristics similar to high density polyethylene with improved heat resistance, tensile strength and abrasion resistance. Polypropylene also has a lower specific gravity and lower dielectric constant than polyethylene. Polypropylene has good impact strength, low moisture absorption, excellent chemical resistance, high creep resistance, and is useful in high frequency applications. It retains these excellent properties in cellular constructions.
