Section 2 - Application and Specification Guidelines

Selection Approach

When several high-temperature wire applications are being considered simultaneously, cost effectiveness can be improved by standardizing on a single or limited number of wire options. The process can be simplified by first isolating service variables. Sometimes similar conditions, varying only by temperature, for example, can be served by a single wire construction. After the "highest necessary" level of properties is determined, a wire that fulfills this and all lesser requirements can then be selected.

Defining Electrical Requirements

In selecting high-temperature insulated wire, the electrical requirements - operating voltage, conductor temperature rating and current-carrying capacity (ampacity) - must be met. Typical operating voltages for high-temperature lead wire are 300 and 600 V. The temperature rating of wire is determined by a combination of ambient heat and current-generated heat.

Current-generated heat is calculated by matching conductor material and diameter to service amperage. Ambient heat is the additional heat expected from the application. The influence of the diameter can be seen in the effect size has on copper wire temperature for various amperage levels: an AWG 8 wire requires 65 amps of current to reach 80 C an AWG 10 wire reaches 80 C at 47 amps, and so on. Due to the variations in heat dissipation through the insulation, and other factors involved, current-carrying capacity is a difficult selection variable. For this reason, product designers add safety margins. That is, they specify higher capacity conductors than theoretical calculations would indicate.

NOTE: See "Ampacity Ratings," and "Temperature Derating" Tables under the "List of Tables"

Matching Environmental Conditions

After meeting electrical requirements for an application, a careful evaluation should be made of environmental conditions that can damage insulation and thus degrade or destroy circuit integrity.

Ambient Heat

In addition to heat generated by flow of electricity, careful consideration should be given to the potential for high ambient, or surrounding, heat within an enclosed operating environment. Ambient heat can add significantly to total heat by affecting the rate of outward heat release through the insulation. Wire temperature ratings are determined by the total heat - current-generated and ambient heat.

Moisture

Low moisture absorption is a general characteristic of thermoplastics and thermoset materials. Silicone insulation, for example, meets UL and other agency requirements for a "wet environment." Even over prolonged periods, its outstanding insulating properties remain virtually the same.

Abrasion

Materials vary greatly in ability to resist abrasion. Common insulation systems such as fluorocarbon polymers and some synthetic rubber compounds exhibit good abrasion resistance. Other materials with outstanding electrical properties, such as silicone rubbers, must in most cases be protected by an outer cover of braided glass or aramid fiber.

Thermal Stability

Several insulation materials have achieved universal acceptance in part due to excellent thermal stability over time. Mica, for example, is virtually unaffected by long-term high temperatures. Silicone rubber and Teflon* fluorocarbon polymers likewise retain a very high percentage of properties in prolonged service. Fiber glass has excellent thermal and mechanical properties at elevated temperatures.

Chemical Attack

Commonly used insulation system materials such as silicone rubber, thermoplastics and glass are inert in the presence of most chemicals. Oils, grease, hydraulic fluids and other agents, under certain conditions, can attack certain polymers. The wire supplier can recommend specific insulation materials designed to meet these rigorous service conditions.

Mechanical Abuse

Synthetic rubber materials generally show excellent resistance to compression set and cold flow, which are potential causes of insulation thinning. Glass fibers show excellent cut-through resistance.

Low Temperature

Some materials - Teflon polymers, silicone rubber and thermoplastic elastomers, for example - demonstrate excellent low-temperature flexibility and retention of electrical properties.

Flame Resistance

Wire insulating, jacketing and braiding materials are selected, in part, for flame resistance, but vary somewhat in performance in this area. Fluorocarbon polymers are flame resistant, which means they will burn and be consumed when flame is directly applied. Burning ceases, however, when the flame is removed. When exposed to direct flame, silicone rubber decomposes to form a non-conductive ash; if the silicone is contained by a protective glass braid, circuit integrity can be maintained. Mica is virtually noncombustible. Depending on specific mica-tape composition, circuit integrity can be maintained even under catastrophic fire conditions (to 1000 C).

Corona

Silicone rubber demonstrates outstanding resistance to corona discharge. Teflon polymers, on the other hand, cannot be used successfully at corona-inception levels in high-voltage applications.

Ease of Stripping

Stripping ease is of particular importance when automated stripping equipment is used. Generally, thermoset and thermoplastics such as fluoropolymers used in insulation offer excellent stripping characteristics. Glass-braided wire with the correct amount of saturant or binder also strips cleanly.

Terminating

Each method of wire termination - for example, screw terminals, crimping, soldering or welding - presents its own handling problems that must be considered when a conductor, and sometimes insulation, is selected. Silicone and Teflon polymers are attractive materials for use with highly automated equipment, such as that used in appliance assembly, because the required close control over cutting, stripping and handling can be achieved. As another installation aid, both silicone rubber and fluorocarbon polymers have good resistance to soldering temperatures. Teflon TFE tape, for example, is unaffected, so more compact wire spacing becomes possible than can be used with less durable types of insulation.

Soldered Termination	Screw Termination
Crimped Terminations

Routing

Although low coefficients of friction and uniform diameters make handling of wire generally easier in product assembly, care must be taken in choosing wire for various routing paths. Right angle bends, as shown in the drawing of a hair dryer, have little effect on silicone rubber insulation but can lead to failure of fluorocarbon polymer insulation. Abrasion potential must also be considered. When rough handling is anticipated, specification of silicone rubber with a tough glass braid cover can stand up to the abuse.

NOTE: * Teflon is a registered trademark of E.I. DuPont de Nemours & Company.