Insulating and Jacketing Materials
- Neoflon (FEP, PFA, ETFE)
- Neoprene (blown on)
- Polyethylene/ Cross-Link Polyethylene (XLPE)
- Irradiated Cross-Link PVC (XLPVC)
- Teflon® (FEP or PFA)
- Tefzel® (ETFE)
Insulating & Jacketing
There are wide range of plastic insulating materials to choose from, and yet the selection of a particular dielectric for a specific application frequently is a trade-off in properties. Each plastic has both desirable characteristics and practical limitations. In some cases, a solution lies in using composite insulations, where two or more materials are combined to take advantage of the desirable characteristics of each. The combination is usually better than the sum of its parts. But even with composites, the result often represents a compromise between what is theoretically desirable and what is commercially available and economically supportable.
Types of Conductors
Most frequently used to transmit electrical energy on the types of conductors are copper, copper-covered steel, high strength copper alloys, and aluminum. For more unusual applications, conductors are fabricated from pure nickel, pure silver, copper-covered aluminum, and a host of metals, metal alloys, and metal combinations.
Material may be coated with rubber, polyethylene, asbestos, thermoplastic, or varnished cambric material, which are called insulators as they have very low electron mobility (few or no free electrons), all of which depend on the voltage of the circuit, the temperature, and whether the circuit is exposed to water or chemicals.
Not all conductive metals have the same level of conductivity – some obviously being better than others – and not all insulators are equally resistant to electron motion.
The following section will go over some of these differences.
Copper is the most widely used conductor material. Its physical properties are high electrical and thermal conductivity, ductility, malleability and solderability, high melting point, and high resistance to corrosion, wear, and fatigue.
- Copper-covered steel
Copper-covered steel combines the conductivity and corrosion resistance of copper with the strength of steel. Three types are presently available, differing primarily in method of producing the composite metal. In one type, molten welding permanently bonds the two components; in another, a copper layer is electroplated over a steel rod; and in the third, the copper and steel are metallurgically bonded.
- High Strength Alloys
Copper alloy conductors are specified because they permit significant size and/or weight reductions especially important in computer and aerospace cable and wire applications. It offer high breaking strength and greater flex life with only a small increase in DC resistance. Cadmium-chromium copper, cadmium copper, chromium copper, and zirconium copper are most frequently used.
- Stainless Steel
Stainless steel is used for medical lead wires and cables. Stainless steel has poor conductivity compared to copper and may have to be gold plated to improve the conductivity.
- Bare Copper
Bare copper slowly combines with oxygen at room temperatures to form copper oxide. Raising the temperature accelerates this reaction, and at about 180°C and higher, bright copper wire turns black in just a few minutes. Oxide film is a poor conductor of electricity and must be either removed or prevented from forming in order to assure reliability of connections. This is usually accomplished by coating the copper wire with another metal which oxidizes more slowly at operating and processing temperatures. Thus, a coating is sometimes used to facilitate termination (soldering); sometimes as a processing aid (preventing oxidation of the copper at Teflon® TFE extrusion temperatures); and sometimes to offer a lower-resistance connection (“Wire-Wrap” termination). Bare copper is satisfactory at temperatures up to about 100°C.
- Tinned Copper
Tinned copper conductors are a soldering aid and are usually specified where this terminating method is to be used. Suitable for conductors continually exposed to temperatures not exceeding 150°C, tinned copper conductors are slightly more expensive than bare copper wires. However, the labor savings gained by using tinned copper more than offset the additional expense, especially when manual twisting and solder dipping of the stripped lead is required.
- Silver Coated Copper
Silver plated copper is made by electro-plating pure silver on 18 AWG wire which then is cold drawn to size and finally annealed. Minimum silver thickness is 40 micro-inches. Though higher in cost than tinned copper, silver-coated conductors are recommended for wires operating from above 150°C to about 200°C and in high frequency applications where, because of skin effect, higher conductivity of silver is desirable. They are readily wet by solder, permitting rapid soldering with hand irons. Care must be taken, however, to prevent solder wicking under the insulation, which may reduce conductor flex life. Silver coated copper will oxidize after a few hundred hours at 250°C.
- Nickel Coated Copper
Nickel plated conductor (50 micro-inches minimum nickel thickness) is recommended for Teflon® TFE hook-up wire operating for prolonged periods at temperatures of from 200° to 260°C, and where silver coating is objectionable because of possible solder wicking. Ordinary soft solder does not wet nickel as readily as it does tin or silver. It adheres well enough to make a good termination, but will not wick into the stranded conductor beyond the joint, thereby leaving flexibility unimpaired. Connections exposed to temperatures above the melting point of soft solder require special soldering techniques. The term “nickel clad” refers to a much thicker coating – 10% to 30% of the radius of the strand.
Stranded conductors are composed of uninsulated “strands” of wire twisted together. The advantages of conductor stranding over a single strand of equal cross-section are increased flexibility and flex-fatigue life. Stranded conductor can be manufactured in a variety of configurations, the most common being concentric (true concentric, equilay concentric, unidirectional concentric, and unilay concentric), bunched and rope.
When the term “concentric stranding” is used, it refers to the definition of the word “concentric”, which is having a “common center”. Concentric conductor may be defined as: a central wire (strand) surrounded by one or more layers of helically laid wires in a geometric pattern.
The geometric pattern requires that concentric constructions can only be produced with 7, 19, 37, 61, (etc.) strands or members, following the pattern that each successive layer has 6 more strands than the layer below it.
In all types of concentric constructions, the geometric pattern of the strands is consistent for the entire length of the conductor. That is, the central strand, and the strands in each layer remain in their respective positions from the beginning to the end of its length.
The main advantage of concentric constructions is the close/tight diameter tolerances that can be maintained on the conductor. Concentric constructions have very smooth uniform surfaces that are suited for thin wall insulation in high performance applications.
- Concentric Stranding
There are four common types of “concentric” constructions manufactured for the high performance wire and cable industry. Although there are four distinct types, the industry normally refers to “Concentric” as “True Concentric” and will use the terms interchangeably. The other types are referenced as noted.
Concentric – or True Concentric characterized by a central wire surrounded by one or more layers of helically laid wires in a geometric pattern, with alternately reversed lay direction and increasing lay length.
Equilay – or Equilay Concentric characterized by a central wire surrounded by one or more layers of helically laid wires in a geometric pattern, with alternately reversed lay direction and the same lay length.
Unidirectional – or Unidirectional Concentric characterized by a central wire surrounded by one or more layers of helically laid wires in a geometric pattern, with the same lay direction and an increasing lay length.
Unilay – or Unidirectional Equilay Concentric characterized by a central wire surrounded by one or more layers of helically laid wires in a geometric pattern, with the same lay direction and the same lay length.
Bunched Stranding – Bunch strand wire contains any number of strands in random pattern. Twisted in one operation, all strands have the same lay direction and same lay length, however, the result is a rougher surface and lower dimensional tolerance than the concentric constructions. The number of strands is determined by the size of the individual strands and the total cross-sectional area required.
Rope Stranding – Wire constructions consist of single strands assembled together into concentric or bunched configurations. Rope stranding has the advantage of increasing flexibility by using a larger number of finer strands while maintaining a tighter diameter tolerance than a simple bunched construction. Ropes are more evident in the larger AWG sizes, such as 8 AWG and larger, but there are also many applications that require the flexibility of rope constructions in the smaller gauges. Constructions vary and can contain hundreds or thousands of strands.
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