Glass or Porcelain Insulators, article originally published in the Age of Steel

[Trade Journal]

Publication: China, Glass & Lamps

Pittsburgh, PA, United States
vol. 22, no. 19, p. 8-9, col. 1-2



A writer in the Age of Steel handles the subject of glass or porcelain insulators in the following manner:

Considering the purely electrical part of long distance transmission, with which we are most directly concerned for the present and directing our attention to the insulators, which present the first and most difficult problem, we find at the outset a royal battle waged between the insulating properties of glass and porcelain. In this country glass for many years has been the king of the insulators; abroad, for an equal period of time, porcelain has remained pre-eminent.

For potentials up to about 25,000 volts, where an insulator of seven inches diameter is sufficient, there seems not much reason for the employment of anything but glass, unless the lines reach a size above 1-2 inch in diameter, when its strength is deficient. For these lower potentials and lighter lines the good properties of glass are easily summarized. In the first place it is cheaper by far; while not so strong as porcelain, it is, as has already been said, sufficiently strong for the purpose; its inspection is easier and surer, since it requires only a visual examination and a few taps of a hammer to ascertain its soundness in place of the tedious high potential test necessary in the examination of porcelain. But having said this much all has been said that is possible in favor of glass.

It is not true that glass is a better insulator than sound porcelain, nor is its surface so good in damp weather against surface leakage. The mechanical strength does not equal one-half that of porcelain, as has been proved by a long series of mechanical test performed by dropping a steel ball from a height upon insulators of the two materials; temperature also affects it much more than porcelain, and it is often found to crack simply from the effect of extreme changes of temperature.

The insulators of high voltage lines must be very large, for the reason that with a striking distance through the air of from 4 to 6 inches great gaps must be provided in the path of the current. But above all a great creeping distance is essential.

Insulators that are a perfect protection in the hardiest rains fail utterly in clear weather when covered by smoke or soot, which allows small amounts of currents to escape and char away the pins or cross arms. The compound insulator is very attractive, for the reason that it is easier and cheaper to make and handle two or three small parts rather than one large one, and in some cases the additional advantage has been sought of securing the good properties of both glass and porcelain by making parts of an insulator of each; but there is both a theoretical and practical fallacy in this type of design. In the first place, no dielectric is so strong as one entirely homogeneous. As the stress is passed from glass to porcelain, or vice versa, at the two surfaces the stress piles up as it were, and it is better if one believes in glass to rely upon it; or, on the other hand, to show faith in the porcelain used by employing it entirely.

From the pure standpoint of practice the insulator which must be cemented together is troublesome. The shrinkage of the mastic leaves voids and produces strains; if cement be used, its slowness in hardening delays the work and requires large areas for setting out to harden any considerable number of insulators. The one other mastic in use is molten sulfur, which, while better as a mastic than cement is still more liable to crack the insulators while they are being joined, and, finally when exposed to light and air, frequently decomposes on the surface, producing sulphuric acid; and, finally, when current creeps over its surface following the acid, perhaps catches fire and often results in the rupture and fall of the insulator. The insulator which will be entirely satisfactory for this use has not yet been found, but the diligent search made for it by some of our best engineers is resulting in its definition, and will probably soon result in its production, not as an inspiration, but as a careful engineering conception.


Researcher notes: 
Supplemental information: 
Researcher:Bob Stahr
Date completed:January 30, 2005 by: Elton Gish;