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.
[Illustration: 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.
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.
[Illustration: 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.
[Illustration: 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 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: FIG. 1,044.—Ground connection for lightning arrester.]
[Illustration: 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.]
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.