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Standoff Insulator Guide

Standoff Insulators are primarily used to insulate and support busbar or grounding bars used in high-voltage power transmission, including panel boards and switchboards.

Types of Insulators

The most common materials for standoff insulators used with power transmission are ceramic or porcelain, and composite polymer materials.

Porcelain insulators are made from clay, quartz or aluminum and feldspar, and then covered with a smooth glaze to reduce the accumulation of dirt. Insulators made from porcelain rich in aluminum are used where high mechanical strength is a necessity. While porcelain insulators have high strength ratings, they are easily chipped and broken, so care must be taken during installation.

Composite insulators are less costly, lighter in weight, and have excellent hydrophobic capability. This combination makes them ideal for service in indoor areas affected by industrial residue like machine oil and grease.

Choosing the Right Insulator

Standoff insulators come with high voltage ratings, which is determined by the material used and the thickness of the body and to some extent the shape, since the design of insulators often includes deep grooves, or sheds.

Switchboard-Panel

High-Strength

Center Post

Heavy-Duty Apparatus

Large Center Post

15 kV

The first and most obvious variable is height, which determines the distance that will separate a source of voltage from ground or other components. Since an electric current will jump across a gap, the greater the voltage, the greater the distance required for safety.

The next issue is thread size and insert material. Insulators are manufactured with threaded metal inserts that are used to mount them onto walls or cabinets. Several different thread sizes are offered for each size insulator, accommodating numerous applications and attachment options.  By far, the most popular metal for inserts is aluminum, because it doesn't rust. Steel inserts are generally used only for applications requiring a high torque on mounting bolts. The downside to steel is high humidity and rust.

Other Physical Considerations

  • Tensile strength is the amount of stress that the insulator will withstand when pressure is applied pulling straight up on the bolt in the insert.
  • Cantilever strength is the amount of weight an insulator will support when something is hung on an insulator that is mounted on the wall.
  • Compression strength is the amount of weight an insulator will support in a direct vertical application of pressure.
  • Dielectric strength is another term for insulating properties. 
  • Arc resistance, in accordance with ASTM-D495, is expressed as the number of seconds that a material resists the formation of a surface-conducting path when subjected to an intermittently occurring arc of high voltage low-current characteristics.
  • Creep is the distance from potential to ground measured along a ceramic insulator’s surface path. This rating is dependent on environment as well as the surface condition and overall length.