Section 1 - Basic Elements of High-Temperature Insulated Wire

The five basic elements of high-temperature insulated wire are conductor, insulation, protective braid, jacketing and shielding. Not all elements are appropriate for each construction. Simpler constructions may require only a conductor and extruded insulation. Usually, unprotected insulation is a tough thermoplastic or synthetic rubber with good electrical insulating (i.e. dielectric) properties. When severe service is expected at elevated temperatures, more complex constructions may be required. Each element should be discussed with the wire supplier.

Conductor

Critical conductor selection variables are a composition of conductor material, conductor diameter and stranding. Expected service temperature should be considered first because conductor materials vary in heat resistance. Next to be evaluated should be the conductor's capacity to carry current without surpassing the temperature rating of the conductor and insulation. Material and conductor circular mil area are key determinants of this capacity. Finally, the decision should be made on how much flexibility is required, for this determines the stranding configuration.

Insulation

The purpose of primary insulation is to contain and channel the voltage. It is normally extruded over the conductor or applied as a wrapped tape. Materials selected for the primary insulation (plastics, synthetic rubbers and mica) have good dielectric properties as well as heat resistance. For severe applications, it should be determined whether or not a secondary insulation may be required for protection against cutting, tearing or other damage. Because a primary insulation is usually selected for its dielectric strength, a choice may reflect some tradeoff of physical properties.

Braiding

Fiber glass is widely used in braided outer covers for limited mechanical protection. Glass braid almost always is saturated with an appropriate high-temperature finish to prevent fraying or moisture penetration and to improve fiber bonding.

Jacketing

Also called sheathing, jackets generally are extruded from thermoplastics or thermoset materials for mechanical, thermal, chemical and environmental protection. It is also used as a supplemental electrical insulation over metallic shields.

Shielding

Metal shielding, applied as a corrugated or flat tape or as a woven braid used to protect insulation from severe operating conditions. Additionally, it prevents power-generated interference from escaping to the surrounding environment. Metallic shielding also is commonly used in low-voltage communications wiring to protect signal integrity.