Publication: The University of Colorado Journal of Engineering
Boulder, CO, United States
AN INSULATOR TEST.
BY STANLEY D. COFFIN, B.S. (E.E.), 1903.
High-tension transmission at the present time is receiving a great deal of attention in the engineering world. Its importance is indicated by the large amount of special work done in this line by some of the large electrical companies. Since results in high-tension work cannot be accurately prophesied, the data to be relied upon are obtained from tests made under the most adverse working conditions.
A test made by an electrical company to show the relative value of different types of insulators under very high voltage was on a line 800 feet long, situated near a railroad and a power house, so that smoke and dirt were nearly always present — a bad condition for high-voltage lines. The wires were No. 5 B. & S. (copper), placed 4 feet apart and supported by 42 insulators of various types. One hundred and twenty thousand volts was put on the line and kept there, most of the time, for a month; during which time observations were made of the line losses and the behavior of the line and insulators under the various conditions of the weather.
The first two weeks it rained nearly every day and the line was noisy and luminous. At night the brush discharge from both wire and insulators was nearly a foot long. Noise and glow first appeared at about 60,000 volts and increased rapidly as the voltage was raised. When it was raining, waves traveled back and forth along the wires between the poles at a high frequency and disappeared only when the voltage was lowered to about 60,000 volts. As the voltage was raised they appeared again at 96,000 volts and were strong enough at 120,000 to shake the poles and tug at the insulators on the building at the end of the line.
The first five poles were equipped with Locke's No. 329 porcelain insulators, and gave no trouble. There was a little noise and glow, which was confined to the head of the pin; but when the design was changed, placing the first petticoat as in the Stone & Webster, all noise and glow disappeared. Next on the line was Locke's No. 25 glass insulator. Four of this type were on iron pins with porcelain bushings and six were on dry wooden pins, but the result was apparently the same on the two pins. They made a loud buzzing sound and gave a great glow. It was necessary to replace many of them, as they were continually breaking.
Locke's No. 316 insulators were tried, mounted on iron pins with porcelain bushings, but proved to be too small for such a voltage, and after much trouble were replaced by one of Thomas' No. 60 porcelain, one of Richard Ginori's (Italian), four of Locke's No. 33 and four of Stone & Webster's No. 4.
Thomas' insulator was punctured, but the fault was in the material and not the design, which is good. The Richard Ginori, a fine white porcelain insulator, stood the voltage well. Locke's No. 337 and Stone & Webster's No. 4 are large types and gave no trouble. Other tests show the efficiency of these two types to be about the same. Stone & Webster's No. 4 is less complicated and weighs 24 lbs., while Locke's No. 337 weighs 32 lbs.
Muncie, Provo No. O, Locke's No. 318 and Locke's No. 331 were tried, but all four types are evidently too small for such voltage. The Muncie, a good design, and Provo No. O, are of Hemingray glass.
As a result of this test, and many others, an insulator has been designed called the Chesney Thomas, now manufactured by Thomas, which is a combination of the good qualities of many others. It has no static discharge and will stand 120 kilo volts, between an iron pin and the wire, under a shower of a 3/4-inch stream of water. The material is porcelain. All tests show that porcelain is better than glass for high-voltage insulators, for when the glass breaks it goes all to pieces and allows the wire to drop. On the other hand, the porcelain simply punctures and after the insulator has broken down it offers just as good a support as ever.
It has also been demonstrated that thick porcelain is no better than thin. Two insulators the same size, one a half-inch in thickness and the other an inch, will break down at the same voltage.