Alternators used in transmission systems usually conform approximately to a sine curve in the instantaneous values of the pressures they develop, and it is therefore desirable that tests on line insulators be made with voltages whose values follow the sine curve. Either a single transformer or several transformers in series may be employed to step up to the required voltage, but a single transformer will usually give better regulation and greater accuracy. An air-gap between needle points is not a very satisfactory means by which to determine the average voltage on a testing circuit, because, as already pointed out, the sparking distance between the needle points depends mainly on the maximum instantaneous values of the voltage, which may vary with the load on the generator, and the saturation of its magnets. For accurate results a step-down voltmeter transformer should be used on the testing circuit.
An insulator that resists a puncture test may fail badly when subjected to a test as to the voltage that will arc over its surface from line wire to pin. This arc-over test should be made with the outer surface of the insulator both wet and dry. For the purpose of this test the insulator should be screwed onto an iron pin, or onto a wooden pin that has been covered with tinfoil. One wire of the testing circuit should then be secured in the groove of the insulator, and the other wire should be connected to the iron or tin foil of the pin. The voltage that will arc over the surface of an insulator from the line wire to the pin depends on the conditions of that surface and of the air. In light air, such as is found at great elevations, an arc will jump a greater distance than in dry air near the sea-level. A fog increases the distance that a given voltage will jump between a line wire and its insulator pin, and a heavy rain lengthens the distance still further. The heavier the downpour of rain the greater is the distance over the outside surface of an insulator that a given voltage will arc over. The angle at which the falling water strikes the insulator surface also has an influence on the voltage required to arc over that surface, a deviation from a downpour perpendicular to the plane of the lower edge of the petticoat of the insulator seeming to increase the arcing distance for a given voltage.
An insulator should be given an arc-over test under conditions that are approximately the most severe to be met in practice. These conditions can perhaps be fairly represented by a downpour of water that amounts to a depth of one inch in five minutes for each square inch of the plane included by the edge of the largest petticoat of the insulator, when the direction of the falling water makes an angle of forty-five degrees with that plane. A precipitation of one inch in depth on a horizontal plane during five minutes seems to be a little greater than any recorded by the United States Weather Bureau. Under the severe conditions just named, the voltage required to arc over the insulator surface from line wire to pin should be somewhat greater at least than the normal voltage of the circuit where the insulator is to be used. For the transmission line between Spier Falls and Schenectady, on which the maximum voltage is 30,000, the insulators were required to stand a test of 42,000 volts when wet, without arcing over from line wire to pin. In these wet tests the water should be sprayed evenly onto the insulator surface like rain, and the quantity of water that strikes the insulator in a given time should be measured.
When the outside of an insulator is wet with rain, it is evident that most of the resistance between the line wire and the insulator pin must be offered by the inside surface of the petticoat of the insulator. For this reason an insulator that is to withstand a very high voltage so that no arc will be formed over its wet outside surface must have a wide, dry surface under its petticoat. In some tests of line insulators reported in Volume xxi., Transactions A. I. E. E., p. 314, the results show that the voltage required to arc over from line wire to pin depends on the shortest distance between them, rather than on the distance over the insulator surface. Three insulators, numbered 4, 5, and 7 in the trial, were in each case tested by a gradual increase of voltage until a discharge took place between the wire and pin. The pins were coated with tinfoil, and the testing voltage was applied to the tie wire on each insulator and to the tinfoil of its pin. Insulators 4, 5, and 7 permitted arcs from wire to pin when exposed to 73,800, 74,700, and 74,700 volts respectively, the surfaces of all being dry and clean. The shortest distances between wires and pins over insulator surface and through air were 65⁄8, 61⁄4, and 77⁄8 inches respectively for the three insulators, so that the arcing voltages amounted to 11,140, 11,952, and 9,479 per inch of these distances. Measured along their surfaces, the distances between wires and pins on these three insulators were 8, 111⁄4, and 151⁄2 inches respectively, so that the three arcing voltages, which were nearly equal, amounted to 9,225, 6,640, and 4,819 per inch of these distances. These figures make it plain that the arcing voltage for each insulator depends on the shortest distance over its surface and through the air, from wire to pin. It might be expected that the voltage in any case would arc equal distances over clean, dry insulator surface or through the air, and the experiments just named indicate that this view is approximately correct. The sparking distance through air between needle points, which is greater than that between smooth surfaces, is 5.85 inches with 70,000 volts, and 7.1 inches with 80,000 volts according to the report in Volume xix., A. I. E. E., p. 721. Comparing these distances with the shortest distances between wires and pins in the tests of insulators numbered 4, 5, and 7, which broke down at 73,800 to 74,700 volts when dry, it seems that a given voltage will arc somewhat further over clean, dry insulator surface than it will through air. This view finds support from the fact that only a part of each of the shortest distances between wire and pin was over insulator surface, the remainder being through air alone.
The fact that the dry part of the surface of an insulator and the air between its lower wet edge and the pin or cross-arm offer most of the resistance between the line wire and the pin and cross-arm is plainly brought out by the results of the tests above mentioned, in the cases of insulators numbered 4 and 7. While 73,800 volts were required to arc from line-wire to pin when the entire insulator was dry and clean, the arc was formed at only 53,400 volts during a moderate rain-storm, in the case of No. 4 insulator. With insulator No. 7 the arcing voltage was 74,700 when the entire surface was clean and dry, but the arc from wire to pin was started at 52,800 volts during a moderate rain. No. 5 insulator seems to present an erratic result, for when dry and clean the arc jumped from wire to pin at 74,700 volts, and yet during a moderate rain no arc was formed until a voltage of 70,400 was reached. For each of the seven insulators on which tests are reported as above, the voltage required to arc from line wire to pin was nearly or quite as great during a dry snow-storm as when the insulator surface was clean and dry. When the insulators were covered with wet snow their surface insulation broke down at voltages that were within ten per cent above or below the arcing voltages during a moderate rain in five cases. With two insulators the arcing voltages, when they were covered with wet snow, were only about sixty per cent of the voltages necessary to break down the surface insulation between wire and pin during a moderate rain.
When the outside surface of an insulator is wet, as during a moderate rain, it seems that the under surface of the insulator, and the distance through air from the lower wet edge of the insulator to the pin or cross-arm, make up most of the insulation that prevents arcing over from the wire to the pin or cross-arm. It further appears that it is useless to extend the distance across the dry under surface of the insulator indefinitely without a corresponding increase of the direct distance through air from the lower wet edge of the insulator to the wood of cross-arm or pin. Insulator No. 7 in the tests under consideration had a diameter at the lower edge of its outer petticoat of seven inches, and was mounted on a standard wooden pin. The diameter of this pin in the plane of the lower edge of the insulator was probably about 11⁄4 inches, so that the radial distance through air from this edge to the pin must have been 27⁄8 inches approximately. During a moderate rain the surface insulation of this insulator broke down and an arc was formed from wire to pin with 52,800 volts. The sparking distance between needle points at 50,000 volts is 3.55 inches, according to Volume xix., A. I. E. E., p. 721, and must be shorter between smooth surfaces, such as the wire and pin in question, so that nearly all of the 52,800 volts in this case must have been required to jump the 27⁄8 inches of air, leaving very little to overcome the slight resistance of the wet outside surface of the insulator. On this insulator the surface distance from wire to pin was 15-1⁄2 inches, while the shortest breaking distance was only 77⁄8 inches, so that the distance across the dry under surface of the insulator must have been 151⁄2 - (77⁄8 - 27⁄8) = 101⁄2 inches approximately. It is evidently futile to put a path 101⁄2 inches long across dry insulator surface in parallel with a path only 27⁄8 inches long in air, as an arc will certainly jump this shorter path long before one will be formed over the longer. The same line of reasoning applies to No. 3 insulator in this test, which had a diameter of 63⁄4 inches, a surface distance from wire to pin of 13 inches, and a minimum distance of 71⁄4 inches, and whose surface insulation broke down at 48,600 volts during a moderate rain. The necessity of increasing the distance between the lower wet edges of insulators and the pins and cross-arm, as well as the distance across the dry under surfaces of insulators, led to the adoption of the so-called umbrella type for some high-voltage lines. In this type of insulator the main or outer petticoat is given a relatively great diameter, and instead of being bell-shaped is only moderately concave on its under side. With an insulator of this type mounted on a large, long pin, the lower edge of the umbrella-like petticoat may be far removed from the pin and cross-arm. Beneath the large petticoat of such insulators for high voltages there are usually one or more smaller petticoats or sleeves that run down the pin, and increase the distance between it and the lower edge of the largest petticoat.
Insulators on Transmission Lines.
| Location of Line. | Voltage of Line. | Material of Insulator. | Inches Diameter of Insulator. | Inches Height of Insulator. | ||
|---|---|---|---|---|---|---|
| Electra to San Francisco | 60,000 | Porcelain | 11 | 11 | 1⁄4 | |
| Colgate to Oakland | 60,000 | Porcelain | 11 | 11 | 1⁄4 | |
| Cañon Ferry to Butte | 50,000 | Glass | 9 | 12 | ||
| Shawinigan Falls to Montreal | 50,000 | Porcelain | 10 | 13 | 1⁄2 | |
| Provo around Utah Lake | 40,000 | Glass | 7 | 5 | 3⁄4 | |
| Santa Ana River to Los Angeles | 33,000 | Porcelain | 6 | 3⁄4 | 4 | 7⁄8 |
| Spier Falls to Schenectady | 30,000 | Porcelain | 8 | 1⁄2 | 6 | 3⁄4 |
| Apple River Falls to St Paul | 25,000 | Glass | 7 | 5 | 3⁄4 | |
| Chambly to Montreal | 25,000 | Porcelain | 5 | 1⁄2 | 6 | 1⁄2 |
| Niagara Falls to Buffalo | 22,000 | Porcelain | 7 | 1⁄2 | 7 | |
| Portsmouth to Pelham, N. H. | 13,000 | Porcelain | 5 | 1⁄4 | 3 | 3⁄4 |
| Garvins Falls to Manchester, N. H. | 12,000 | Glass | 5 | 4 | 3⁄4 | |
The inner petticoat or sleeve that runs down over the pin and sometimes reaches nearly to the cross-arm, of course becomes wet on its outside surface and at its lower edge during a rain; but between this lower wet part of the inner petticoat, or sleeve, and the lower wet edge of the larger outside petticoat, there is a wide, dry strip of insulator surface. A result is that an arc over the surface of the outside petticoat can reach the wet edge of the sleeve only by crossing the strip of dry under surface or jumping through the air.
The same type of insulator is used on the 60,000-volt lines between Electra and San Francisco and between Colgate and Oakland, each insulator having an outer petticoat 11 inches in diameter and one inner petticoat or sleeve 61⁄2 inches in diameter. This inner petticoat runs down the pin for a distance of 71⁄2 inches below the outer petticoat. Slightly different pins are used for mounting the insulators on the two transmission lines just named, so that on the former the distance through air from the lower edge of the outer petticoat to the cross-arm is 11 inches, and on the latter the corresponding distance is 111⁄2 inches. On the Electra line the lower edge of the inner petticoat of each insulator is about 31⁄2 inches, and on the Colgate line about 4 inches above the cross-arm.