Electrical insulation

“ 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. ”

“ 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. ”

“ The contact making parts are enclosed in a sheet metal oil tank which has an insulating lining. The leads are brought out at the top. Connections to the outside circuit are made inside the switch and a porcelain insulator is slipped over the joint, thus providing a straight continuous connection from the line with maximum insulation. On the 6,600 volt switch, insulation is obtained by the use of porcelain bases for supporting the live parts. ”

“ All resistance in the rheostat in the low tension circuit should then be inserted and the switch closed. Gradually cut out secondary resistance until the voltmeter shows the same voltage as was recorded previously when the spark jumped across the gap, and apply this voltage to the transformer for one minute. Insulation tests for a period of over one minute are very unadvisable, as transformers with excellent insulation may be seriously damaged by prolonged insulation tests. The longer the strain to which any insulation is subjected, the shorter the subsequent life of the insulation. ”

“ It is, therefore, sometimes important that the insulation resistance between primary and secondary be measured. This can be done, as here shown. Great care should be taken to have all wires thoroughly insulated from the ground, and to have an ammeter placed as near as possible to the terminals of the transformer under test, in order that current leaking from one side of the line to the other, external to the transformer, may not be measured. Great care is required in making this measurement, in order to obtain consistent results. ”

“ The major insulation in a core type transformer consists of several large pieces of great mechanical strength, while in the shell type, there are required an extremely large number of relatively small pieces of insulating material, which necessitates careful workmanship to prevent defects in the finished transformer, when thin or fragile material is used. ”

“ As respects electrical conduction, no advantage is gained by expanding the rod horizontally into a strap or tube—surface does nothing, the solid section is the essential element. [4] There is no occasion for insulation (of the conductor) for this reason. A flash of lightning has an intensity that enables it to break through many hundred yards (perhaps miles) of air, and therefore an insulation of six inches or one foot in length could have no power in preventing its leap to the brickwork, supposing that the conductor were not able to carry it away. ”

“ From (1) it is seen that the flow of the current is proportional to the voltage and inversely proportional to the resistance; the latter depends upon the material, length and diameter of the conductor.
Since the current will always flow along the path of least resistance; it must be so guarded that there will be no leakage. Hence, to prevent leakage, wires are insulated, that is, covered by wrapping them with cotton or silk thread or other insulating material. If the insulation be not effective, the current may leak, and so return to the source without doing its work. ”

“ In general
terms the electro-motive force developed by the secondary stands to that
of the primary terminals in the ratio of the windings. This is only
approximate.
The greatest care is required in the insulating. The secondary is
sometimes wound in sections so as to keep those parts differing greatly
in potential far from each other. This prevents sparking, which would
destroy the insulation.
A make and break, often of the hammer and anvil type, is operated by the
coil. (See Circuit Breaker, Automatic.) As the current passes through
the primary it magnetizes the core. ”

“ The spark indicates the passage of electricity across an insulating medium—dry air—when a good conductor approaches within a certain distance of the charged body. The greater the charge of electricity, the greater is the distance over which the electricity will thus make its escape. Insulation, then, for many miles of wire, and still240 more for a complete system of communication such as we now have, was hopeless, so long as frictional electricity was employed, or considerable electrical intensity required. ”

“ In recent years other methods have been developed for generating electrical power in relatively small quantities for special applications. Semiconductors, which combine heat insulation with good electrical conduction, are used for thermoelectric generators to power isolated weather stations, artificial satellites, undersea cables and marker buoys. ”

“ 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. ”

“ In any other system we have the problem of splicing and cutting of cables, which, at its best, is bad work. What we desire is not such a high insulation as good mechanical protection. As long as we can hold a moderate insulation with good mechanical protection, that is all we want. ”

“ The insulating joint may be omitted in conduit, armored cable, or metal molding systems with straight electric fixtures in which the insulation of conductors is the equivalent to insulation in other parts of the system, and provided that approved sockets, receptacles, or wireless clusters are used of a type having porcelain or equivalent insulation between live metal parts and outer metal shells, if any. ”

“ 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. ”

“ In pole and over-house lines, the wires are kept from each other and the current from escaping to the earth by insulators of glass. In Europe white porcelain and brown stoneware insulators are used, and the former substance, when of good quality, well glazed and well burned, is perhaps the most perfect of all insulating materials, and does not deteriorate with age. The fewer the poles on which the wires are suspended, the better is the insulation and the less the cost, but the liability to accident is greater. ”

“ As insulating materials, whether rubber, cotton, or paper, may be impaired or destroyed by heat, it is necessary that the temperature of underground cables under full load be kept within safe limits. Rubber insulation can probably be raised to 125° or 150° Fahrenheit without injury, and paper and cotton may go a little higher. For a given size and make of leaded cable the rise of temperature in its conductors above that of the surrounding air, for a given loss in watts per foot of the cable, may be determined by computation or experiment. ”

“ 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. ”

“ As it is easier for the current to circulate in a short path than in a long one, there is a continual tendency for it to jump from the overhead wire through the insulation to the ground, but this is effectually prevented by the very perfect character of the insulation. ”

“ If the insulation per mile, i.e., the total insulation multiplied by the mileage of the wire loop, is found to be less than 200,000 ohms, the wire is considered to be faulty. The climatic conditions in the British Islands are such that it is not possible to maintain, in unfavourable weather, a higher standard than that named, which is the insulation obtained when all the insulators are in perfect condition and only the normal leakage, due to moisture, is present. ”

“ 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. ”

“ Whatever gauge wire be selected, it must be carefully insulated, to avoid all chance contact with nails, staples, metal pipes or other wires. The best insulation for wires employed indoors is gutta-percha, surrounded with a coating of cotton wound over it, except only in cases when the atmosphere is excessively dry. ”

“ On land, where wires are carried on the tops of poles, the air itself is a sufficient insulator. A few glass rings at the points where the wire passes through the iron staples by which it is supported, and the insulation is complete. But in the sea the electricity would be instantly dissipated, unless some material could be found which should insulate a conductor sunk in water, as completely as if it were raised in air. But what could thus inclose the lightning, and keep it fast while flying from one continent to the other? ”

“ As the transmission line leaves the power house it crosses a highway and runs in a perfectly straight line to the house. Over the highway insulated wires were used as a safety precaution, but bare aluminum wire was used for the remainder because it was cheaper.
The buildings are all in a cluster and a branch from the transmission line runs into each one where the current is used. All the wires which are inside of any of the buildings, or are close to the woodwork, are covered with insulation, and, where concealed, are further protected by being placed in twisted metal tubes. ”

“ When it's pushed its way through all of them and done the work you want it to do, it's tired out and goes away by the other wire. But if you cut off the insulation down in the basement, where those two wires are close together, and make it possible for the current to jump straight across without doing any work, it will take the short circuit instead of the long one and you won't have any lights in your house. ”

“ Ordinary insulated wires are ineffectually protected by the rubber compounds used. Glass, mica, and better still, a large air gap are the only insulations that will serve, for the tremendous potential or voltage of the current must be carefully considered whenever insulation is necessary. The coil is better provided with a spring key in the primary circuit than a vibrator, it gives better control of the circuit and probably a larger and better spark. ”

“ 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. ”

“ INSULATION But when wire is finished it isn’t always finished. Since electricity spread itself over the earth in a million services, insulation in various forms has come to be almost as important as the wire itself. Insulation in its more advanced forms is a complex affair, gauged to accord with specific conditions and multiplying processes to secure the maximum of protection, both from electric current to life and property and from dampness and abrasion to the wire itself. ”

“ The expansion of the interior cable parts and insulating material, accompanying the rise in temperature, forces the oil from the joints along the cable and also causes it to exude at the joints. As the cable cools, the fluid insulation, particularly at the joint, may not be sufficient to fill it and voids result. This occurs the more readily if the insulating compound tends to solidify at cooling temperatures. These consequences are avoided by the use, in conjunction with the reservoir, of oil which is sufficiently fluid to flow freely at ordinary cable temperatures. ”