  A lightning arrester is an apparatus designed to provide a path by which lightning disturbances or other static discharges may pass to earth. Lightning arresters may be divided into three classes, according as their action depends upon the effects of: 1. Sharp points; Lightning Rods.—This form of arrester consists of a conducting rod or cable erected on the outside of a building and connected to earth, in order to afford protection from lightning by carrying the lightning discharge into the ground; or to prevent lightning by leading the electricity from the earth to the cloud without disturbance. Ques. Why do lightning rods terminate in sharp points? Ans. The action of the rod depends on the discharging effect of a sharp point as follows: When an electrically charged cloud approaches a building provided with a lightning rod, it induces an opposite charge in the earth and in the rod which is connected Ques. How should a lightning rod be erected? Ans. The conductor should be carried to all high points of the building it is to protect and should be well insulated from the latter and grounded in deep wet earth, independent of gas or water pipes. Sharp bends and corners should be avoided. Fig. 1,039.—Diagram showing principle of air gap arrester: lightning discharges more readily at sharp points than along flat surfaces. Ques. Why have lightning rods fallen into disfavor? Ans. On account of numerous failures due to faulty installations, and non-maintenance of the rod in good condition, also because of the excessive prices charged by unscrupulous dealers for rods and their erection.
Air Gap Arresters.—Many of the lightning arresters used for the protection of electrical apparatus depend upon the fact that lightning discharges will jump across air spaces that are good insulators for the regular working current, while they find difficulty in passing through circuits containing electromagnets. Fig. 1,040.—Union lightning arrester and ground wire switch for telegraph lines. Two line wires are attached to the two plates provided with points. The ground wire being connected to the third or central plate. The pin serves as a ground wire switch and cut out. This is a good form for short lines. The principle of air gap arresters is illustrated in fig. 1,039. There are two brass plates slightly separated; one is connected to the line and the other is grounded. The air gap between the plates is very small and the resistance thus interposed, while sufficient to prevent the regular working current jumping across, is not great enough to interfere with a lightning discharge which readily jumps the gap and passes off to earth. Ques. Why are teeth provided on the plates? Ans. For the same reason that points are used on lightning rods. That is, when electricity at high pressure accumulates at such points the surrounding air is electrified and the charge escapes by means of the charged air particles. Fig. 1,041.—Mason multi-discharge lightning arrester. The construction of this arrester is based on the well known principle that lightning discharges more readily at points or angles than elsewhere. The wire is wound around square carbon rods, which are connected to the ground, the line being insulated from the rods by sheets of mica. The wire itself being square, instead of round, adds to the efficiency of the arrester, by increasing the number of points or angles. Ques. For what kind of service is the form of arrester just described used? Ans. It is suitable for telegraph and telephone lines where currents of very low voltage are employed. Ques. Why is it not used on lines employing higher voltage, such as in electric light and power stations? Ans. Current at high pressure would follow the lightning across the gap and establish an arc or continuous flame from Ques. What provision is made to prevent the destruction of arresters by the line current? Ans. Lightning arresters used on heavy duty circuits are designed to rupture the arc as soon as formed. Fig. 1,042.—Diagram showing operation of variable gap arc breaker used on heavy duty lightning arresters. When a lightning discharge passes across the gap to earth, the dynamo current follows it and energizes the magnet M, which attracts the short arm of the double lever, thus quickly jerking the terminal B away from C. The wider air gap thus interposed between B and C greatly increases the resistance which breaks the arc. Ques. How is this done? Ans. There are several methods, of which may be mentioned the variable gap method described in fig. 1,042, and the magnetic blow out method shown in fig. 1,043. Ques. Where should lightning arresters be placed? Ans. They should be placed as near as possible to the point where wires enter a building, and in an easily accessible place away from combustible material. Ques. What should be avoided in installing lightning arresters? Ans. Kinks and sharp bends in the wire running from the outdoor lines to the arresters and from arresters to ground should be avoided as far as possible. Ques. Why should kinks and sharp bends be avoided? Ans. Because they offer resistance to the lightning discharge. Fig. 1,043.—Horn type lightning arrester. In this type of arrester, two wires, after approaching within a short distance of one another, are bent divergently. These wires are supported on insulators. One of them is connected to the line to be protected and the other is earthed. The normal line pressure is insufficient to bridge the gap, even at its narrowest portion, but an extra high pressure whether due to lightning or to other disturbing phenomena, will bridge the gap at its narrowest point and establish a path to earth. When, however, the main current attempts to flow across, phenomena of electromagnetic repulsion force the arc upward along the horns, lengthening and attenuating it, until it finally becomes extinguished. Ques. How should lightning arresters be grounded? Ans. They should be connected to ground with No. 6 B. & S. gauge copper wire or larger. Gas pipes within a building must not be used for a ground connection. Fig. 1,044.—Ground connection for lightning arrester. Fig. 1,045.—Carbon lightning arrester with fuses as used on telephone lines. The arrester consists of two blocks of carbon separated a small distance by a thin sheet of insulating mica, which is perforated with one or more holes; a high voltage charge on the line will jump through the hole in the mica from the carbon on the line side to the lower carbon, which is connected with the ground; the fuses protect the instruments against foreign currents which might damage, although not of sufficiently high pressure to jump to earth; sometimes the connections are reversed so that the fuse is between the line and the earth.
Ques. Does lightning often strike telephone or electric light lines? Ans. No, the lines become charged to a high pressure by induction from lightning flashes or from the passing of clouds that are highly charged. |
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