Electrical insulator

“ Another of these reasons was that I was led to recognize the transmission of electrical energy to any distance through the media as by far the best solution of the great problem of harnessing the sun's energy for the uses of man. For a long time I was convinced that such a transmission on an industrial scale, could never be realized, but a discovery which I made changed my view. I observed that under certain conditions the atmosphere, which is normally a high insulator, assumes conducting properties, and so becomes capable of conveying any amount of electrical energy. ”

“ If mechanical displacement of particles or transportation of conducting bodies seem inadmissible, it is probable that there is a modification of the insulator itself, the modification persisting for some time by virtue of a sort of “coercive force.” An electric current of high potential, which would be completely stopped by a thick insulating sheet, may be supposed to gradually traverse the very thin dielectric layers between the conducting particles, the passage being effected very rapidly if the electric pressure is great, and more slowly if the pressure is less. ”

“ Such suspension is necessary, for if we were to handle the spheres or lay them on to the table their charges would leak away; if we wish a body to preserve its charge for a sensible time we must support it by an insulator—such as silk, glass, mica, ebonite, which does not allow electricity to flow along or through it. Metals offer a very easy path for the flow of electricity, and are therefore called conductors. There is no sharp line of demarcation between insulators and conductors. Dry wood, for instance, is not a perfect insulator; and when damp it is not a perfect conductor. ”

“ Iron expands at a much higher rate than glass or porcelain, and it is desirable to cement iron pins into insulators rather than to screw them in. There seems to be some reason to think that an insulator will puncture more readily when it is exposed to severe mechanical stress by the expansion of the iron pin on which it is mounted.
Tests of insulators are usually made with alternating current, and the form of the voltage curve is important, especially where the test is made to determine what voltage will arc over the surface of the insulator from the line wire to the pin. ”

“ Ordinary insulators display the characteristics of the dielectric of a capacitor formed by the wire and ground. As the frequency increases, the overall impedance decreases. A better high-frequency insulator is a quarter-wave section of transmission line shorted at one end. Such an insulator is shown in figure 3-20. The impedance of a shorted quarter-wave section is very high at the open-end junction with the two-wire transmission line. This type of insulator is known as a METALLIC INSULATOR and may be placed anywhere along a two-wire line. ”

“ Its insulating property is a positive advantage, since we may hold one end in our hand and yet electrify the surface at the other end. The charge is not able to slip about freely over the whole surface, as is the case with a charged conductor; hence, if we touch a particular point of the electrified part, we take off a little of the charge at that point, but the rest of the rod remains charged. We thus see that a charge can reside on the surface of an insulator, and it can be proved experimentally that the charge even penetrates a little way into the body of the insulator. ”

“ All insulators, whether made of glass or porcelain, should be tested electrically to determine their ability to resist puncture, and to hold back the surface leakage of current, before they are put into practical use on high-tension lines. Experience has shown that inspection alone cannot be depended on to detect defective glass insulators. Electrical testing of insulators serves well to determine the voltage to which they may be subjected in practical service with little danger of puncture by the disruptive passage of current through their substance. ”

“ Wooden insulator pins are among the weakest elements in electric transmission systems. As line voltages have gone up it has been necessary to increase the distances between the outside petticoats of insulators and their cross-arms and to lengthen the insulators themselves in order to keep the leakage of current between the conductors within permissible limits. ”

“ Mechanical failures of wooden insulator pins have been far more common than those of poles, both as a direct result of the line strains and because such pins are often charred and weakened by the leakage of energy from the conductors. For these reasons the general use of iron or steel pins for the insulators of long lines operating at high voltages seems desirable. ”

“ An insulator is a non-conducting substance, impervious, so to speak, to electricity. It is the theoretical antipodes of a conductor. While the conductor is a substance through or over which electricity flows freely, the insulator will neither{282} permit electricity to pass through its mass nor over its surface. It can therefore be used as a means of confining electricity to a conductor, and preventing it from escaping to other conductors in the neighbourhood. In short it can be made to insulate or isolate the particular conductor from all other conductors. ”

“ When the outgoing and incoming wires are both placed upon or underground the strain upon the insulation is very much increased, for then instead of the two lines being separated by fifteen or twenty feet of pole, which is a very fair insulator, they are separated by only a few inches of earth or perhaps metal, the first of which is a fairly good conductor, while the last is a nearly perfect one. It is evident, therefore, that the insulation proper in an underground or surface system must be of the highest order. ”

“ Where there is room for wooden struts, as on a private right of way, they should be used instead of guys because of their more substantial character and the higher security of insulation thus obtained. Ordinary strain insulators cannot be relied on with lines that operate at very high voltages, and where guys must be used a timber four by six inches and [256] ten to twenty feet long may have the guy twisted about each end of it and be used as a strain insulator. A guy or strut should come well up under the lower cross-arm on a pole to avoid breaking of the pole at the point of attachment. ”

“ It may therefore be confidently asserted that no insulator can possibly be made that would be capable of preventing the electric discharge leaving the lightning conductor provided it could find an easier path leading to the earth. Mr. Phin, in his work on ‘Lightning-Rods’ says very pertinently:—But not only are insulators worthless—they are positively dangerous if the principle upon which they are adopted is fully carried out, which, however, is but rarely done. ”

“ Since no substance yet known is absolutely a non-conductor of electricity, perfect insulation is impossible. If, however, the supports on which a telegraph wire rests present, on each part and on the whole, so great a resistance to electric conduction as to allow only a small portion of the electricity sent in, in the actual working, at one end to escape by lateral conduction, instead of passing through the line and producing effect at the other end, the insulation is as good as need be for the mode of working adopted. ”

“ The wire to be wound on the bobbins is sold by all dealers in electrical apparatus. It is copper wire, covered with cotton or [Pg 68] with silk, to ensure insulation. Mention has already been made of what is meant by insulation at § 3, but, in order to refresh the reader's memory, Mr. G. Edwinson's words are quoted here. "To insulate, as understood by electricians, means to protect from leakage of the electric current, by interposing a bad conductor of electricity between two good conductors, thus insulating [11] or detaching them from electric contact." ”

“ One end of the stem is firmly attached to the pole, and the other bears the wire. The best idea of a single telegraphic insulator may be got from a common umbrella, with its stem of insulating substance attached upright to the top of a pole and bearing the wire supported in a notch on the top outside. The umbrella may be either of the same substance as the stem—all glass or all glazed earthenware, for instance—or of a stronger material, such as iron, with an insulating stem fitted to it to support it below. Very good insulators may be made of continuous glass ”

“ Good insulators or non-conductors are paraffine, solid or liquid, turpentine, silk, sealing wax, india rubber, dry glass or porcelain. The best electrical conductors are the best thermal conductors, and a “red hot” temperature converts insulators into fairly good conductors. ”

“ You can use any size of copper or aluminum wire that is not smaller than No. 16 Brown and Sharpe gauge. When you buy the wire get also the following material: (1) two porcelain insulators as shown at A in Fig. 5; (2) three or four porcelain knob insulators, see B; (3) either (a) an air gap lightning arrester, see C, or (b) a lightning switch see D; (4) a leading-in porcelain tube insulator, see E, and (5) a ground clamp, see F.
To make the aerial slip each end of the wire through a hole in each insulator and twist it fast ”

“ A main conveying alternating currents of very high tension may be injured merely by a blow hole or small crack in the insulation, the more so as a blowhole is apt to contain gas at low pressure; and as it appears almost impossible to completely obviate such little imperfections, I am led to believe that in our future distribution of electrical energy by currents of very high tension, liquid insulation will be used. ”

“ The first attempts at transmitting messages through wires laid in water were made about 1839. These early experiments were not very successful, because the art of wire-insulation had not attained any degree of perfection at that time. It was not until gutta-percha began to be used as an insulator for submarine lines that any substantial progress was made. ”

“ Fig. 206.—The Battery.
It should be observed that the degree of value as a conductor or non-conductor depends somewhat upon the atmosphere. For instance, glass is an excellent insulator, or non-conductor, when dry, but when wet it changes to a conductor. So insulators are at times covered with a solution of shell-lac, or fat, to keep away moisture. We may reasonably conclude that bodies [Pg 204] which are good and bad conductors are good and bad conductors of electricity. ”

“ Wires are usually covered with a material called an insulator, in order to prevent the loss of electric current due to the wires coming into contact with other bodies or circuits. Insulators are substances which do not conduct electricity.
Wires which are insulated by heavy braids of cotton fiber and then impregnated with some compound, such as creosote, are called weather-proof wires, and are best adapted to outside service, where they must be exposed to the action of the elements. ”

“ When a solid dielectric is used, it matters little how thick and how good it is; if air be present, streamers form, which gradually heat the dielectric and impair its insulating power, and the discharge finally breaks through. Under ordinary conditions the best insulators are those which possess the highest specific inductive capacity, but such insulators are not the best to employ when working with these high frequency currents, for in most cases the higher specific inductive capacity is rather a disadvantage. The prime quality of the insulating medium for these currents is continuity. ”

“ Since any dielectric, even air, is not a perfect insulator, a small current known as LEAKAGE CURRENT flows between the two wires. In effect, the insulator acts as a resistor, permitting current to pass between the two wires. Figure 3-5 shows this leakage path as resistors in parallel connected between the two lines. This property is called CONDUCTANCE (G) and is the opposite of resistance. Conductance in transmission lines is expressed as the reciprocal of resistance and is usually given in micromhos per unit length. ”

“ Oil Insulated Transformers.—High voltage transformers are insulated with oil, as it is very important to maintain careful insulation not only between the coils, but also between the coils and the core. In the case of high voltage transformers, any accidental static discharge, such as that due to lightning, which might destroy one of the air insulated type, might be successfully withstood by one insulated with oil, for if the oil insulation be damaged it will mend itself at once. ”

“ For future work at still higher voltages, the advantage as to both first cost and insulating qualities seems to lie with insulators that are very long in an axial direction, and which have their petticoats arranged one below the other and all of about the same diameter, rather than with insulators of the umbrella type, like those on the Electra and Colgate lines. ”

“ The art of insulating electric wires has been known almost as long as that of conducting electricity for practical purposes by means of wires. Prior to the use of electricity for lighting, however, the feeble character of the currents conveyed upon these wires did not require that the insulating material should be non-combustible, and the skill of the inventor was directed towards a method of insulation which should protect the wire from moisture and other external injury. ”

“ Insulation is the covering by which the escape of [8] electricity through the wire is made impossible. Always see to it that the insulation is in perfect condition.
All wires must be insulated. In damp places rubber covered wire must be used.
Wires must always be protected with porcelain tubes passing through partition walls, girders, and where they pass over pipes, and other wires, etc.
Incandescent lights are merely globes with a vacuum in which a filament of tungsten or some other highly resistant material meets the electric current and glows through its very resistant power. ”