Section 4 - Insulation, Braiding, Jacketing, Shielding
Insulation
Primary Insulation - Primary insulation contains and channels the voltage. Placed next to the conductor as an extruded coating or tape wrap, its primary requirements are good dielectric - or synonymously, insulating-properties. Primary insulations are selected from several classes of materials: thermoplastics, including extruded and tape-wound Teflon*; synthetic rubbers; mica; and fiber glass.
For service below
Secondary Insulation - Primary insulation, usually selected for superior dielectric properties and heat resistance, sometimes may require a secondary insulation to protect against cutting, tearing or other physical abuse.
The secondary insulation may or may not have good dielectric properties, and is generally applied as a tape wrap or serving. Standard constructions that employ proven glass servings or fluorocarbon tapes achieve the greatest cost-effectiveness. Exotic materials (such as Kapton* film, which resists all chemicals except strong bases), may be used to meet special requirements of more unusual conditions.
Braiding
Fiber glass is the principal material used in braiding. To prevent fraying, improve moisture resistance and improve fiber bonding, wire manufacturers almost always saturate and finish braided fiber glass with high-temperature lacquers, Teflon PTFE dispersions or silicone varnish. Braided wires suit applications involving high heat.
For mechanical protection, an aramid material commonly referred to as K-fiber (Kevlar*) is used on large size single conductor silicone cables, or for the outer coating for multi-conductor high- temperature cables.
Jacketing
Jackets are protective covers extruded over insulation. Also called sheaths, jacketing materials resist abrasion, chemicals, and a range of environmental hazards, such as high ambient heat, oils and grease.
A jacket provides additional mechanical protection to wire insulation, but may also serve as electrical insulation to isolate shield materials such as braided copper from the exterior environment.
To select the right jacket material - operating conditions and cost should be evaluated carefully with the aid of the wire supplier.
Shielding
Shielding is a metal cover - braided or served strands of tinned-copper, silver-plated copper, nickel-plated iron or stainless steel - that gives mechanical protection in harsh conditions.
Shielding refers also to protection for electronic circuits from electrically or electronically generated interference.
Shielding for high-temperature insulated wire is used to prevent electrical interference from escaping through the wire insulation to disrupt sensitive low-voltage electronic circuits or for mechanical protection.
Properties of Insulating, Braiding and Jacketing Materials
NOTE: * Teflon, Kapton, and Kevlar are registered trademarks of E.I. DuPont de Nemours & Company.