Pin insulator

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A pin insulator consists of a nonconducting material such as porcelain, glass, plastic, polymer, or wood that is formed into a shape that will isolate a wire from a physical support (or "pin") on a telegraph, utility pole or other structure, provide a means to hold the insulator to the pin, and provide a means to secure the conductor to the insulator. By contrast to a strain insulator, the pin insulator is directly connected to the supporting pole. The earliest pin insulators predate the strain insulator and were deployed before about 1830. Pin insulators continue in production with manufacturers worldwide.

The pin insulator is designed to secure the conductor to itself. The most common way to do this is to use a wire to tie the conductor to the insulator. Another method is to design the insulator with self-typing features such as complex slots and grooves formed into the insulator. Finally, for heavy conductors, gravity can be used to hold the conductor in place.

Pin insulators are almost always deployed in the open air, so isolation when wet is a major consideration. To combat this problem, pin insulators feature extra skirts or wide shells to increase the surface distance between the conductor and the pin.

The "pin" is typically a wooden or metal dowel of about 3 cm diameter with screw threads. The pin insulator has threads so that it can be screwed onto the pin. A typical pin insulator is more than 10 cm in diameter and weighs one kg or more. Size depends on the voltage to be isolated and the weight of span of wire to be supported.


All glass pintype insulators have been assigned a "Consolidated Design" number - or CD number. This system was first implemented in 1954 by hobbyist, N.R. Woodward. The CD numbers are hobby-specific for collectors, and are not recognized by insulator manufacturers.

Color and shapes[edit]

Company embossing[edit]

Some people collect insulators made only by a certain company that (along with bottles and other glass items) made insulators. Some collectors try to obtain one of each of the variety of interesting shapes (these shapes have been categorized into a system attributing a"Consolidated Design" [or CD] number to each style).

Eye-catching impurities[edit]

Insulators, at the time they were being produced, were viewed simply as another tool and were not meant for spectator analysis. Therefore the production of insulators was not usually the prime concern of a glass company making them. These glass items were commonly "sideline" products. Quality control was not always an important concern.

Companies melted glass and poured it into metal molds to make insulators. The temperature of melted/liquid glass is so high that many foreign objects introduced into the molten glass melt down and diffuse through the "batch." Upon cooling, the impurities cause a discoloration in the finished item. Today, many collectors thrill to this lack of quality control since impurities (adding unique character) make a prized possession from what would normally be a common, low valued insulator. UFO's (Unidentified Floating Objects), amber swirls; "milk" swirls; graphite inclusions; two/three-tone insulators; and (rarely) identifiable objects such as nails, pennies, screws, etc. are known to be imprisoned within the glass of an insulator.

A sparkling CD145 or "beehive" insulator, from the telegraph era, made by the Brookfield Glass Company circa 1882


Possibly the company which produced the most glass insulators during the 19th century and early 20th century in the USA was the Brookfield company. Brookfield must have had poor quality control (or simply did not care!) as Brookfield insulators seem most prone to be found with swirls, etc. .

Amber impurities make beautiful swirls in a CD 162.1 Brookfield
Firebrick from the kiln flaked off into the glass used when making this insulator. The result is a very "snowy" CD145, CRown Embossed Brookfield insulator (CREB) - The "crown" meaning the sides of the dome
The same snowy CREB 145 sitting on its side
Color 1.jpg

The Hemingray Glass company probably made the most variety of colors. A small sampling of colors this company's produced are yellow, golden yellow, butterscotch, glowing orange, amber, whiskey amber, "root beer" amber, orange-amber, red-amber, oxblood, green, lime green, sage green, depression green, emerald green, olive green, Yellow-olive green, aqua, cornflower blue, electric blue, cobalt blue, sapphire blue, glowing peacock blue, and many, many others. These were not produced for their beautiful colors. Rather, the company made these colors so two (or more) companies with their lines strung along the same set of poles would be able to quickly identify which line was theirs by the color of insulator used on those lines. One company may have a string of amber insulators, while another, on the same poles, might have theirs in cobalt blue. They probably had no idea that, close to a hundred years later, these colors would be a main reason why people would collect/display these bright colored pieces of glass.

Other major companies from the USA which can be found embossed on insulators. A few company names found embossed on insulators (and by no means a comprehensive listing) are A T & T, American Insulator, Armstrong, Brookfield, California, Good, Hawley, King City Glass Works (K.C.G.W.), Kerr, Lynchburg, McLaughlin, N.E.G.M Co., Ohio Valley Glass Company (O.V.G.C.), Pyrex, Sterling, Twiggs, Western Flint Glass, Whitall Tatum, and many other names from companies in the States. Canadian companies included Diamond, Dominion, Hamilton Glass Works, G.N.W.TEL. Co.and others. Other manufacturers include Telgraficos Nacionales (Mexico), Zicme (South America), Miva (Italy), Isorex (France), Agee (Australia), and many more.

Although the majority of collectors collect mainly glass insulators, there are a number of people who collect porcelain ones as well. These also come in a vast variety of shapes, sizes and colors.


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