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.
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Description and use
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.[1] A common nickname for the guy-strain insulator among US Utility Linemen is "Johnny Ball". [2]
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. [3]
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.[1]
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.[1]
Collectors
Other than their industrial use for which they are produced some people collect insulators, especially antique ones. [4]
Footnotes
- ^ a b c http://www.insulators.info/pictures/?id=137758928 Sediver deadend insulators @ 40000# and 30000# [Suspensions]
- ^ http://www.utilityinnovations.com/lineman-slang-lingo/lineman-slang.html Lineman Slang
- ^ http://ieeexplore.ieee.org/iel5/11056/34941/01668662.pdf Use of Factory-Formed Dead-Ends On High Temperature Conductors
- ^ "The National Insulator Association (NIA) is an international organization of collectors and friends with more than 5900 members interested in communication and electrical insulators, as well as other artifacts connected with insulators, such as telephone, telegraph, power transmission, railroads, and lightning protection devices.". The National Insulator Association (NIA). November 18th, 2009. http://www.nia.org/. Retrieved 16 December 2009.
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External links
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