Strain insulator
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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. 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.
When the application requires more insulation than a single insulator can supply, strain insulators are used in series: A set of insulators are connected to each other. The series can support the same strain as a single insulator, but the series provides a much higher effecive insulation.
Strain insulators are typically used outdoors in overhead wiring. In this environment they are exposed to rain. 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 therefore incorporate flanges to shed water, and strain insulators intended for vertical mounting are often bell-shaped.
The first insulators were manufactured in about 1830. There is an active community of antiquarians devoted to identiying and collecting these devices.
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