CHAPTER XLIII

LIGHTNING PROTECTION

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; 2. Air gaps; 3. Sharp turns.

=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 to the earth. As soon as the charge on the point becomes strong enough to break apart the molecules of the air in front of it, a stream of electrified particles, opposite in sign to that of the charge on the cloud, passes from the neighborhood of the rod to the cloud and thus neutralizes the charge of the cloud.

=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.

=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.

A lightning rod with defective insulation or broken ground connection is a danger rather than a protection.

=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.

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.

=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 one plate to the other thus quickly destroying the plates and causing other more serious damage.

=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.

=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.

=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.

Ground connections may be made with a one inch galvanized iron pipe driven about 8 feet or until it reaches permanently moist earth, and extending at least 7 feet above ground. The ground wire should be securely soldered to a brass plug firmly screwed into the pipe, and both strongly stapled to the pole so there will be little danger of the connection being broken.

A good ground is important, as the efficiency of the protection would be impaired if the ground connection were poor. Wherever the earth is dry and a good ground cannot surely be obtained, an excavation 4 or 5 feet deep should be made, and after placing the copper ground plate or iron pipe in the hole, it should be filled with crushed coke or charcoal about pea size. This improves the electrical connection between pipe or plate and 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.