Strain insulator

Strain insulator

A strain insulator is an insulator that provides both large electrical insulation and a large load-bearing capacity. Strain insulators were first used in telegraph systems to isolate the signal wire from ground while still supporting the wire. Strain insulators are used to support radio antennas and overhead power lines.

A typical strain insulator is a piece of glass or porcelain that is shaped to accommodate two cables or a cable shoe and the supporting hardware on the support structure (hook eye, or eyelet on a steel pole/tower). The shape of the insulator maximizes the distance between the cables while also maximizing the load-bearing transfer capacity of the insulator. In practice, for light loads such as radio antennas, the strain insulator is usually in tension. For heavy loads such as guy lines and overhead power lines, the strain insulator is usually in compression. A common nickname among Utility Linemen is "Johnny Ball", which is in reference to the guy-strain insulator.

When the line voltage requires more insulation than a single insulator can supply, strain insulators are used in series: A set of insulators are connected to each other using special hardware. The series can support the same strain as a single insulator, but the series provides a much higher effective insulation.

If one string is insufficient for the weight of a cable(s), a heavy steel plate is used to effectively bundle all the insulator strings together mechanically. One plate is on the "hot" end and another is located at the support structure. This setup is almost universally used on long spans, such as when a power line crosses a river, canyon, lake, or other terrain requiring a longer than nominal span.

Strain insulators are typically used outdoors in overhead wiring. In this environment they are exposed to rain and in urban settings, pollution. As a practical matter, the shape of the insulator becomes critically important, since a wetted path from one cable to the other can create a low-resistance electrical path.

Strain insulators intended for horizontal mounting (often referred to as Dead-Ends) therefore incorporate flanges to shed water, and strain insulators intended for vertical mounting (referred to as Suspension Insulators) are often bell-shaped.

The first insulators were manufactured in about 1830. There is an active community of antiquarians devoted to identifying and collecting these devices.


* [ Glass Strain Insulators]
* [ Timeline for US insulators]

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