Testing various British insulators for leakage [Book]

Insulators.—There are many substances which insulate well, but which, from their not possessing certain other qualities, are quite unsuitable for purposes of insulation. In choosing a material a primary point is to ascertain whether it will insulate perfectly independently of the nature of its surface, for it must be borne in mind that the process of glazing an insulator is not to improve the insulating qualities of the material, but to give it a hard smooth surface that shall be an advantage in other respects, the object of glazing being chiefly to prevent the wearing of the wire by friction, and the adhesion of dust and dirt. Another condition is that the material shall resist the deposition and the retention of moisture. Thus a porous substance is quite unsuitable. The only material that insulates perfectly and possesses naturally a hard smooth surface is glass; but experience has shown it to possess also certain defects which have rendered its abandonment necessary. Moisture is very rapidly deposited on it, and in such a way as to form a continuous film, and its fragility is such that it cannot be relied upon. It is, however, still extensively used in Switzerland, and a novel form of insulator of blown-glass has been introduced in America. The form is that of a narrow-necked bottle, and several trials mode to test its efficiency are said to have given satisfactory results. Ebonite is for a time the best available material for insulators; it insulates well, it very effectively resists the deposition of moisture, and it is very strong ; but a few months' exposure to atmospheric influences makes its surface rough and spongy, and consequently favourable to the retention of moisture and dust. When in this state, it is inferior to earthenware; but we have only to re-polish it to render its behaviour as good as when first erected. Such а defect, however, renders it unsuitable for general adoption. Next to glass, porcelain possesses the best surface for resisting accumulations of dust arid dirt, and for this reason it is selected wherever the line is exposed to smoke, dust, or salt spray. It does not appear to insulate better than good stoneware, whilst its cost is much greater, brown stoneware is the cheapest, and for ordinary use the best, material for insulators. It is usually made from clay dug on the spot, and as it does not require an admixture of other materials, it is likely to be uniformly good.

The forms of insulators are very various. The most approved, and those which are now most commonly used, are known as Clark's and Varley's. Figs. 7179, 7180, represent Clark's insulators in elevation and section. It consists of a double bell in white porcelain, and is provided with a taper galvanized wrought-iron stem. This form of insulator is extensively used, and when of brown stoneware is found to be both efficient and economical. Figs. 7181, 7182, are of Varley's double insulator. It is of brown stoneware, and is fitted with galvanized wrought-iron shoulder-bolt and nut. These insulators consist of an outer and an inner bell, manufactured and tested separately before union. They are made in three sizes to suit No. 4, No. 8, and No. 11 wires. For efficiency and economy of maintenance this form of insulator is probably unequalled. Figs. 7183, 7184, an iron- protected insulator, consisting of a double bell of either white porcelain or stoneware, protected by a galvanized cast-iron cap, and provided with a galvanized steel, or wrought-iron stem. Fig. 7185 is another iron-protected insulator also consisting of a double bell of either porcelain or stoneware, and protected by a galvanized cast-iron cap perforated to admit of the lower portion of the insulator being washed by the rain. Fig. 7186 is a common form of terminal insulator, known as the umbrella form. It usually consists of a stout insulator of either porcelain or stoneware, and is provided with a strong galvanized bolt, with nut and washers, so as to be suitable for both wooden and iron arms. Bright's double-bell shackle insulator for terminating wires is represented in Fig. 7187. There are of porcelain, and the straps and bolts are either painted or galvanized. This form of insulator may be fitted in pairs, as shown in Fig. 7188, for terminating and loading in wires to stations, or for use on sharp curves and at angles.

The advantages of iron caps have been greatly overrated. Indeed, it seems probable that, except in certain exposed situations, their use is attended with positive disadvantage; for, though they keep the outer surface dry, they do not prevent the accumulation of dust, and they afford a harbour for insects, especially spiders, whose webs conduct when damp. Moreover, they do not altogether prevent the deposition of moisture, so that this advantage which is claimed for them is not wholly real, while, by protecting the surface from the washing action of the rain, they tend to aggravate the evil caused by dust. The only means of protecting insulators against the latter is to make their surfaces hard and smooth, and then to leave them completely exposed to the weather. This is especially the case with coast lines, where the deposition of salt is more troublesome than dust. For this reason an insulator should not be placed under the arm, but above it, as in the former case the surface of the insulator is partially protected, and is consequently always dirty. In some parts insects are very troublesome, and when the line passes near or under trees, it is well to make the openings of the insulators wide so as to be less attractive to insects and more easily cleaned. Paraffin oil is perfectly effectual in repelling insects for a time, and it possesses the additional advantage of improving the insulation; the latter effect will last six months. Creosote in the poles will also repel insects so long as its smell remains.

To test insulators in the trough, place the insulating cup in a vessel of water made slightly acid; some acidulated water being poured into the cup, smear a little turps on the rim, and let it remain twenty-four hours. At the expiration of this time immerse the poles of a battery in the water, one inside and the other outside the cup, a very sensible galvanometer having been included in the circuit. If the needle exhibits any deflection, the circuit is complete, and consequently the insulator is imperfect. The imperfection usually arises from porosity and cracks in the material.

Culley gives the following method of ascertaining approximately the comparative value of different shapes;—

"The best method of testing is to fix as largo a number as convenient, certainly not less than ten of each kind, upon a pole, connecting them by a wire to the present line wire, and fixing a second independent wire to their bolts, to represent the earth, and to determine the leakage from, one wire to the other.

"To obtain good results, the following precautions are necessary:—Fix a pole not less than 20 ft. high in an open place not sheltered in any way on any side, place the arms on which the insulators are to be fixed at least 2 ft. apart, the insulators themselves a foot apart. Do not place all of the same kind together, but rather alternate or mix them as much as possible so as to obtain an average of exposure for all.

"Take special care that the wire representing the line touches each one closely all round and uniformly as to the number of turns; let it be all the same gauge. The object is to ensure equal surfaces of metal in contact with the porcelain, because the leakage takes place from each point of the wire over the surface of the insulator, and consequently if the wire does not touch uniformly, the leakage will vary with the variation of the surfaces in contact. If possible, divide each set of ten in half, making duplicate sets of five each. If those do not test alike, there is probably a defective insulator among them, which will vitiate the test, and which must be removed. Never use separate gutta-percha or other insulated leads from each set to the testing room, for these will become more or less damp on their surface, and will vary in insulation, one from the other, more perhaps than the insulators themselves. Use only one wire, employing a man to shift it from one set to the other.

"It is seldom that rain falls steadily and uniformly, so that if the testing lasts even five minutes there will often be a considerable difference in the amount of moisture during the interval. After testing all the sets, commence again in the reverse order; if the two series agree, the observation is a good one. As the object of such tests is not so much to find the absolute resistance of the several specimens as to ascertain their relative value, it is better not to occupy time in testing units, but rather to read a simple deflection so that the tests may be made as rapidly as possible, ensuring the greatest possible uniformity of circumstance; but, with the greatest precaution, the teats will frequently be extremely anomalous.

Speaking generally, the best insulator is not that which tests best when quite new, but that which bears exposure the best; and therefore no tests are of any value which have not extended over several months. However carefully this experimental testing may be conducted, it is not altogether satisfactory; the only true method is to insulate two wires on the same poles for a distance of ten miles or more, and to test them every damp day for six mouths. It is useless in any ease to test in dry weather."

"I will give the result of tests made in a situation quite free from insects, but somewhat smoky —Gloucester Road, Regent's Park, during twelve months, 1868-69;—

Mean Comparative Loss.

Brooks' double cup (American glass) ............. 5 parts leakage

Varley's ordinary size ................................... 20   "          "

Porcelain ..................................................... 26   "          "

Double cones, porcelain ................................ 51   "          "

Shackles ...................................................... 82   "          "

"The porcelain insulators were very much wider than the Varley's to render them less liable to be blocked up by insects. The test, therefore, does not show the difference between stoneware and porcelain so much us that between a wide and a narrow opening. ' "The effect of iron caps may be judged from the following ;—

Porcelain, without caps .................................... 38 parts leakage.

        "          with open caps (or cages) ............. 44   "          "

        "          with closed caps ........................... 55   "          "

Small earthenware insulator, without caps .......... 40   "          "

      "              "              "        with open caps ... 50   "          "

"The tests also show that with a wide cup a bracket gives a better result than an arm, probably because splashing is avoided; the leakage being 26 and 38 respectively."

"New insulators may be dipped while dry and hot 'in melted paraffin with excellent results, but it is not certain whether the surface may not retain dirt more easily."

On short lines, good insulation is of less importance than on long lines. When, therefore, the line is a short one, simple and cheap insulators may be used, and their maintenance need not be very carefully looked after; but when the line is long, it becomes necessary to employ the best kinds, and to keep them in a perfect condition. If the insulation is defective, and there are no practicable means for improving it, the resistance must be lessened. This may be effected either by using a larger wire, or by diminishing the resistance of the apparatus. A method of reducing the resistance on long lines by the use of shunts has recently been tried with good results.