OUTSIDE WIRING
In the equipment of lighting and power plants, the cost of the outside
wiring represents a considerable proportion of the total investment,
sometimes costing more than the engines, boilers and dynamos.
A thorough knowledge of outside wiring is therefore necessary to
properly proportion and install the wires so that the system will prove
economical and safe.
=Materials for Outside Conductors.=—Copper wire is now considered to
be the most suitable material not only for the transmission of current
for electric light and power purposes, but also for telegraph and
telephone lines, in place of the iron wire formerly employed.
Hard drawn copper wire is used in outside construction, because its
tensile strength ranges from 60,000 to 70,000 pounds or about twice
that of soft copper. This is desirable to withstand the stresses to
which the wire is subjected which, in the case of long spans, are
considerable.
The table on the next page gives the tensile strength, in pounds per
square inch of cross section, hard drawn copper wire of various sizes
B. & S. gauge.
The metal _aluminum_ possesses certain advantages as a material
for overhead wires. Its conductivity is about .6 that of
copper. The specific gravity of aluminum is about 2.7, while
that of copper is 8.89, so that a given volume of copper will
weigh 3.3 times more than an equal volume of aluminum, and
copper wire of given length and resistance would be about twice
as heavy as an aluminum wire of equal length and resistance.
There are several disadvantages, such as, low tensile
strength, high electro-positive quality of the metal, higher
electrostatic capacity, etc.
TENSILE STRENGTH OF COPPER WIRE
+——————————————-+————————————————+——————————————-+————————————————+
| Size of wire | Tensile | Size of wire | Tensile |
| B. & S. gauge | strength, lbs. | B. & S. gauge | strength, lbs. |
+——————————————-+————————————————+——————————————-+————————————————+
| 0000 | 9971 | 9 | 617 |
| 000 | 7907 | 10 | 489 |
| 00 | 6271 | 11 | 388 |
| 0 | 4973 | 12 | 307 |
| 1 | 3943 | 13 | 244 |
| 2 | 3127 | 14 | 193 |
| 3 | 2480 | 15 | 153 |
| 4 | 1967 | 16 | 133 |
| 5 | 1559 | 17 | 97 |
| 6 | 1237 | 18 | 77 |
| 7 | 980 | 19 | 61 |
| 8 | 778 | 20 | 48 |
+——————————————-+————————————————+——————————————-+————————————————+
=Pole Lines.=—In the majority of cases overhead conductors are
supported by wooden poles. In tropical countries, however, such as
India, Central America, etc., where wood is rapidly destroyed by
the ravages of white ants and other insects, iron poles are almost
exclusively used for telegraph, telephone, and other electric
transmission lines. The form of iron pole generally adopted consists of
tapering shells of sheet iron of convenient length, riveted together at
their ends and set into cast iron base plates which are buried in the
ground.
[Illustration: FIGS. 921 to 929.—Pole construction tools. Fig. 921,
long handled digging shovel; fig. 922, digging bar; fig. 923, crow and
digging bar; fig. 924, tamping and digging bar; fig. 925, wood handle
tamping bar; fig. 926, slick digging tool; fig. 927, post hole augur;
fig. 928, carrying hook; fig. 929, tamping pick.]
=Wooden Poles.=—On account of their size and straightness, various
species of northern pine, cedar and cypress are especially suitable for
large poles. Chestnut, which can be readily sawed and hewed is a very
good material for smaller poles. Sawed redwood is extensively used in
California.
=Preservation of Wooden Poles.=—The preservation of wooden poles
employed in line work is a matter of importance. Decay of the pole at
or near the soil line is caused primarily by various forms of bacteria
or fungi, and in some cases by insects. Bacteria and fungi attack
either dead or living timber. In the case of dead timber, such as that
of poles, they attack the walls of the cells and cause the familiar rot
or decay which eventually destroys the usefulness of the pole.
It is well known that the rapid multiplication or action of the
bacteria and the growth of the fungi are induced by a certain
per cent. of moisture and the heat of the sun, that is, the
portion of the pole at or near the soil line is alternately
moistened and dried. Therefore, in order to protect it against
this action, it is necessary to sterilize the pole by the
application of an antiseptic which will penetrate the pores of
the wood.
=Preservation Processes.=—There are several processes which may be
successfully employed for the preservation of poles or other exposed
timber. The best known of these are the creosoting, burnettizing,
kyanizing, carbolizing, and vulcanizing processes.
In England, creosoted poles showed no sign of decay at the
end of 35 years of service. In the United States they have an
average life of 22 years. In Europe impregnation with copper
sulphate has been extensively used, but this impregnation must
take place within a few days after cutting down the tree.
Uniformly good results have been obtained by impregnation with
corrosive sublimate, involving simply immersion in the liquid
from ten to fourteen days. German authorities state that the
average life of such poles is about 17 years, compared with 14
years for natural or untreated poles.
The application of pitch and tar oftentimes results in more
harm than good. It is authoritatively stated, however, that
in Europe wooden poles are effectively protected by painting
them with tar up to about 2 feet above, and down to about 1½
feet below the soil line. The painted parts of the pole are
then covered with a cloth which after being nailed to the pole,
is also painted with tar. Finally a zinc plated sheet of iron
painted on both sides with tar, is placed around the cloth and
tightened to the pole.
The saving due to the use of sterilized poles is 40 per cent.
of the cost of unsterilized poles. The comparison is made on
the following basis: Cost of pole, $5 each; sterilizing, $1.25;
renewal of sterilized pole in 24 years, unsterilized pole, in
12 years.
[Illustration: FIGS. 930 to 932.—Pole line construction tools. Fig.
930, split wooden handle post hole auger; fig. 931, cant hook; fig.
932. socket peavey.]
=Methods of Setting Wooden Poles in Unsuitable Soil.=—In places where
salt is plentiful and cheap, such as the Great Salt Lake region in
Utah, it has been found that the liberal use of salt mixed with the
dirt filling tamped in around the foot of the pole is very effectual in
preventing decay below the soil line.
Where poles have to be planted in low, swampy ground, or where the
climatic conditions are such that timber decays rapidly, it has been
found advantageous to place the poles in concrete settings. This method
is extensively employed in various parts of the Southern States, square
poles being placed in settings about 7 feet deep and 3½ feet
square. In very soft ground the employment of a concrete setting is
sometimes impracticable. In such cases piles are driven deep into the
soil, and the pole bolted to the part of the pile extending above the
ground.
=Reinforced Concrete Poles.=—The strongest point in favor of concrete
poles is their durability. Untreated wooden telephone and telegraph
poles have to be replaced by new poles about every six or seven
years, depending on the percentage of moisture in the soil, the drier
the soil, the longer being the life of the pole. Concrete poles are
not affected by soil conditions, and if properly made will last
indefinitely.
[Illustration: FIGS. 933 to 935.—Glass insulator and insulator pin and
bracket. The insulator here shown is of the pony double petticoat type.
Insulator pins are used with cross arms, brackets are attached direct
to the pole.]
One form of reinforced concrete pole consists of a skeleton
frame work of four corrugated iron rods covered with ordinary
concrete. The pole is octagonal in shape, 30 feet long, and
provided with mortises for cross arms, the latter being
fastened in place by means of iron bolts. It is stated that
they are less expensive than pine poles, and that each pole can
be manufactured at the point on the line at which it is to be
installed or planted.
In Canada, reinforced concrete poles are made square on account
of the ease of making, and also on account of the steel economy
permitted thereby. All poles are made at the point of erection.
They are moulded in wooden forms, in a horizontal position,
the top side being left open and finished with a trowel. The
concrete is composed of one part of Portland cement, two parts
clean sharp sand, and four parts broken stone. A 35 foot pole
for ordinary line work weighs about 2½ tons and a 50 foot
pole about 5 tons.
=Cross Arms.=—The familiar cross arms for stringing wires are usually
attached to the poles before they are erected. They are commonly made
from yellow pine wood, generally 3¼ x 4¼ inches, and are freely
coated with good mineral paint as a preservative. Attachment is made to
the pole by cutting a _gain_ one inch deep and of sufficient breadth to
allow the longest side of the cross arm to fit accurately. It is then
secured in place by a lag screw, with a square head, so that it may be
driven into place with a wrench.
[Illustration: FIG. 936.—Cross arm which carries the insulator pins.
The standard cross arm is 3¼ x 4¼ inches, double painted, and
bored for 1½ inch pins and two ½ inch bolt holes. Telephone arms
are 2¾ x 3¾ inch, bored for 1¼ inch pins and two ½ inch
bolts.]
The cross arm is further secured to the pole with braces. These are
flat strips of wrought iron or low carbon steel, 30 inches long, ¼
inch thick and 1¼ inches wide, according to standard specifications.
Holes are bored at points one inch from either end, one for attaching
to the pole, the other for attaching to the cross arm; two braces
forming a triangle with the cross arm for the base and with the apex
at the point of connection to the pole. Like all other iron work used
on pole lines, the braces are carefully "galvanized," so as to stand
three immersions of one minute each in a saturated solution of copper
sulphate without showing copper deposits, the color being black at the
completion of the test.
Before the cross arm is set in place the gain is carefully painted with
white lead. As it is important that cross arms on a line of poles,
particularly when there are several on each pole, should be at equal
distances from the ground as well as being uniformly spaced, it is
necessary that some measuring instrument should be used to accomplish
this. Such an instrument is the ordinary _template_, which is a length
of board carrying a pointed block at one end, to correspond exactly
with the top of the pole, and also cross cleats nailed at precisely the
same intervals below it as it is proposed attaching the cross arms. The
template, laid upon a pole, shows where to cut the gains.
In planting the poles it is customary to so arrange them that the cross
arms on alternate poles shall face in opposite directions, for the
purpose of equalizing the strain on the line. On curves, however, all
cross arms are placed on the side of the pole facing the middle of the
curve.
=Ques. What provision is made for attachment of the wires?=
Ans. The cross arms are bored with holes for the insertion of the
insulator pins, which are made of locust wood and threaded at the upper
end to receive the glass insulator.
The cross arm is made of such a length as to accommodate the
number of pins to be inserted. An arm for two pins is made
three feet long, according to the standard usually followed,
with holes for the pins at center points three inches from
either end and a space of 28 inches between them in the center.
=Ques. How must electric light and power wires be placed when wired on
telephone or telegraph poles?=
Ans. They must not be put on the same cross arm with the telegraph,
telephone, or similar wires, and when placed on the same pole with such
wires the distance between the two inside pins of each cross arm must
not be less than twenty-six inches.
[Illustration: FIG. 937.—Portable platform with rigging as used by
linemen in wiring and making repairs.]
=Poles for Light and Power Wires.=—In selecting the style of
pole necessary for a certain class of work, the conditions and
circumstances should be considered. Poles may be divided into
three classes, the size of wire to be carried being one of the
important considerations.
_First Class._—Main line of poles should range in length of
from 30 to 35 feet with 6 inch tops. The height of trees, of
course will have to be considered in many cases.
_Second Class._—Town lighting by arc lights. All poles should
have at least 6 inch tops. The corner poles should have 6½
inch tops, and wherever the cross arms are placed on a pole at
different angles, the pole should have at least a 6½ inch
top. A 30 foot pole is sufficiently long for the main line, but
it would be advisable to place 35 foot poles on corners.
_Third Class._—Where heavy wire, such as No. 00, is used for
feeder wire, the poles should have at least 7 inch tops. Where
mains are run on the same pole line the strain is somewhat
lessened, and poles of smaller size will answer.
_Cull Poles._—All poles that are smaller at the top than the
sizes agreed upon, are troubled with dry rot, large knots
and bumps, have more than one bend, or have a sweep of over
twelve inches, should certainly be classed as cull poles.
Specifications for electric light and power work should be,
and in many cases are, much more severe than those required by
telegraph lines. A cull pole, one of good material, is the best
thing for a guy stub, and is frequently used for this purpose.
A cedar pole is always preferable to any other, owing to the
fact that it is very light in comparison to other timber, and
is strong, durable, and very long lived.
_Pole Setting._—In erecting poles, it seems to be the
universal opinion of the best posted construction men that a
pole should be set at least five feet in the ground, and six
inches additional for every five feet additional length above
thirty-five feet; also additional depths on corners. Wherever
there is much moisture in the ground, it is of much value to
paint or smear the butt ends of the pole with pitch or tar,
allowing this to extend about two feet above the level of
the ground. This protects the pole from rot at the base. The
weakest part of the pole is just where it enters the ground.
Never set poles further than 125 feet apart; 110 feet is good
practice.
_Painting._—When poles are to be painted, a dark olive
green color should be chosen, in order that they may be as
inconspicuous as possible. One coat of paint should be applied
before pole is set, and one after pole is set. Tops should be
pointed to shed water.
=Spacing the Poles.=—In general, the spacing of poles, like their
dimensions, is regulated by the weight of the lines they are designed
to carry—the heavier the lines the greater the number of poles. The
spacing of poles also depends on their liability to injury from storms
and wind in any given locality, and the nature of the service. Poles
for a telephone line may be spaced twenty to fifty to the mile—that
is, from about 260 to 100 feet apart.
[Illustration: FIGS. 938 to 941.—Pole line construction tools. Fig.
938, pike pole; fig. 939, raising fork; fig. 940, mule pole support;
fig. 941, jenny pole support.]
=Erecting the Poles.=—Since each pole on a properly constructed line
is sawed to the right length and carefully shaped before it is finally
inserted in the ground, it is necessary that the holes be dug to as
nearly the required depth as possible. Holes for poles are dug very
little wider than their diameter at the butt, and the depth is usually
computed according to the nature of the soil and the weight of the
proposed line. Excavation, while sometimes accomplished with patent
post hole augers, or even dynamite, is usually done with a long handled
digging shovel, and the earth removed with a spoon shovel, such as is
shown in fig. 921.
[Illustration: FIG. 942.—Guy anchor log in position.]
[Illustration: FIG. 943.—Stombaugh guy anchor. It is made of cast iron
and can be screwed into the ground like an auger.]
Wherever required by the nature of the soil, a "grouting" or foundation
of loose stones is formed in the bottom of the hole, and, in marshy or
springy ground, a base of concrete and cement is laid, with filling of
the same material around the pole, when raised.
=Ques. How are the poles transported to the holes?=
Ans. They are rolled or carried on hooks similar to those used for
carrying blocks of ice, except for a long handle for lifting the load
at either side.
[Illustration: FIG. 944.—Method of raising a pole. When the pole has
been properly placed, it is seized by several linemen. As soon as the
top of the pole is raised high enough to permit the pikes to be thrust
into the pole, it is then raised to a vertical position. At about 50°
the butt end slides into the hole. The earth is then filled in around
the pole and firmly tamped down. Eight or ten poles are about as many
as can be set by the average gang in a day.]
=Ques. How are the poles raised and placed in the holes?=
Ans. A piece of timber is inserted in the hole as a slide to prevent
crumbling of the earth as the pole is slid into place. The end is
raised by hand sufficiently to allow the "dead man," or pole hoist, to
be placed beneath, and this is moved along regularly as the pole is
lifted with pike poles, until it slides into place through the force of
gravity.
[Illustration: FIG. 945.—Method of pulling an anchor into place before
the guy wire is fastened to the top of the pole, thus obviating the
liability of pulling the pole out of plumb.]
This accomplished, the pole is held in a perpendicular position
by pikes in the hands of assistants, or planted in the ground
around it, while the earth is carefully shoveled into the hole
and thoroughly packed down with a tamper.
=Guys for Poles.=—Where poles are subject to severe strains which
might throw them down and break the wires, guy cables are largely
employed, these being attached near the top and secured either to the
base of the next pole, to a suitable guy stub or post, or to a guy
anchor, which is buried about eight feet in the earth and held down by
stones and concrete.
=Ques. Under what conditions is it necessary to guy poles?=
Ans. They are guyed at corners in order to thoroughly secure the poles
so that no strain may come on the cornerwise span. It is also necessary
to guy a line where it is to be deflected from a straight path, as when
rounding a hill, water course or railway curve, in order to neutralize
the pull of the wires, tending to incline the poles toward the center
on which the arc is described; also when descending a hill.
[Illustration: FIGS. 946 to 948.—Methods of guying corner poles. The
proper guying of corner and terminal poles is especially important; on
corners and curves, the guys should be stronger and more frequent and
should be placed on the outer side as shown in the diagrams.]
[Illustration: FIG. 949.—Head and foot guying of a pole line in
descending a hill.]
=Guy Stubs and Anchor Logs.=—In guying a line under such conditions,
each pole is connected by a suitable cable to a guy post or "stub," or
to an anchor log. Standard rules specify stubs between 18 and 25 feet,
with exact limits as to circumference measures at the top and at a
point 6 feet from the butt, according to the kind of wood used.
[Illustration: FIGS. 950 to 952.—Lineman's tools. Figs. 950 and
951, Eastern pole climbers, with and without strap for attaching to
legs; fig. 952, portable vise with strap for pulling up the slack in
splicing.]
Thus, guy stubs of cedar or juniper, either 18 or 25 feet in
length, must have a circumference of 22 inches at the top and
of 32 inches 6 feet from the butt; stubs of chestnut must
measure 24 inches in the first, and 34 in the second, while
those of cypress require 28 in the first, and in the second,
39 inches for an 18 foot length, and at least 41 for a 25 foot
length. In planting guy stubs the same rules are followed as
hold for poles, every means being adopted to promote security
of construction except that the stub is raked or tilted against
the strain on the guy cable.
=Wiring the Line.=—The erection and guying of the poles of a line as
well as the attachment of the cross arms and the screwing on of the
insulators are completed before the stringing of the line is begun. It
is particularly essential that the pull on poles of a given line be
accurately calculated, and that each one be guyed accordingly before
the line is strung, in order to avoid the danger of an undue strain
upon the wires in attempting to rectify the condition afterward. It is
a good working rule that the wires should be subjected to no stress
other than the weights of their own spans after they have been attached
to the poles.
[Illustration: FIGS. 953 and 954.—Pay out reels. Fig. 953, type used
for telephone or telegraph work; fig. 954, type used for electric light
work.]
=Ques. Describe how the wires are strung.=
Ans. In stringing the lines, either one or the full number of wires
may be put up at the same time. When one line only is to be strung,
the operation consists simply in reeling the wire and running it off
from a hand reel, such as is shown in fig. 953 or 954. At each pole the
wire is drawn up to its place, pulled out to the desired tension, and
attached to the insulator.
In the operation of stringing a number of lines at once, the
method is different. The reels are placed at the beginning
of a section, each wire being inserted and secured through a
separate hole in a board, which is perforated to correspond
with the spacing of the insulators on the cross arms. A rope
is then attached to this running board, which is drawn by a
team of horses through the stretch to be wired, being lifted
over each pole top in turn. When a certain length has thus been
drawn out the wires are drawn to the required tension between
each pair of poles and secured to the insulators.
[Illustration: FIG. 955.—One form of "come along." The wire is
inserted between jaws and is held fast when tension is applied to the
ring.]
[Illustration: FIG. 956.—An improved form of "come along" or wire
stretcher. The jaws which grip the wire are smooth and remain parallel
in closing, thus the wire is not scratched or indented, as with
circular jaws having teeth.]
=Ques. How much tension must be put upon the wires?=
Ans. In applying tension to the wires as they are strung on the poles,
it is the rule to allow some sag. The amount of sag to be allowed
varies with different line hangers.
A typical case quoted by one or two authorities gives a sag of
four inches at the center of each 130 foot span for a given
size of wire, at a given temperature. A more general rule is to
make the tension on a wire as it is drawn up between each pair
of poles equal to one-third of its breaking weight. Thus No. 10
B.& S. gauge, would be drawn to about 163 pounds, and No. 12
to about 102 pounds. The temperature at the time of stringing
and the distance between the poles are, however, important
considerations in applying tension and allowing for sag. Thus,
one construction company specifies a dip of 10 inches in summer
and 8 inches in winter for spans of 130 feet, or 40 poles to
the mile. Several authorities specify figures about as given in
the above table for No. 14 iron or copper wire.
[Illustration: FIG. 957.—Wireman's "come along" with hook and tackle.]
SAG TABLE
+——————+————————————————————————————-+
| | Temperature Fahr. |
| Span +————————-+————————-+————————-+
| in | 30° | 60° | 80° |
| Feet +————————-+————————-+————————-+
| | Sag in Inches |
+——————+————————-+————————-+————————-+
| 75 | 1¾ | 2½ | 3⅛ |
+——————+————————-+————————-+————————-+
| 100 | 3 | 4¼ | 5⅜ |
+——————+————————-+————————-+————————-+
| 130 | 5⅛ | 7 | 8⅝ |
+——————+————————-+————————-+————————-+
| 150 | 6¾ | 9 | 11¼ |
+——————+————————-+————————-+————————-+
=Ques. How is the wire drawn out?=
Ans. In drawing out the wire, it is customary to use a wire clamp, or
"come along." This tool is attached to a block and tackle, or drawn in
by hand, and, as soon as the proper force has been applied, the wire is
held, while the lineman secures it to the insulator.
[Illustration: FIG. 958.—Lineman's block and fall with "come alongs"
for stretching wire and holding same when making splices.]
[Illustration: FIGS. 959 and 960.—Approved method of attaching wire to
an insulator; elevation and plan of insulator and tie. The line wire is
first laid in the groove of the insulator, after which a short piece of
the same size of wire is passed entirely around to hold it in place,
then it is twisted to the line at either side with pliers.]
Another contrivance for this purpose is the pole ratchet, by
which the wire is drawn tight and held until attached to the
pole.
=Ques. How are the wires attached to the insulators?=
Ans. An approved method is shown in figs. 959 and 960. Standard rules
specify that all wires shall be tied to the side of the insulators
toward the pole, except on the insulators next to the pole, where they
are to be attached on the opposite side. On curves, however, it is
required that all wires shall be arranged so that the strain shall be
against the insulator and not on the wire.
[Illustration: FIG. 961.—American wire joint. This is a simple method
of connecting the ends of the sections of wire by tightly twisting the
ends around each other for a few turns; it is _the standard Western
Union_ wire joint.]
[Illustration: FIGS. 962 and 963.—McIntire sleeve and sleeve joint.
An approved method of making the joints of telephone lines is by the
use of some form of sleeve, such as is shown in fig. 962. This consists
of two copper tubes of the required length, and of sufficient inside
diameter, to admit the ends of the wires to be joined, fitting tightly.
The tubes are then gripped with a tool, shown in fig. 964, and twisted
around one another, so that the wires are securely joined and locked,
as shown in fig. 963.]
=Ques. How are the wires spliced?=
Ans. There are several methods of splicing wires. Fig. 961 shows the
American wire joint, and fig. 963 the McIntire sleeve joint. In making
a joint, the two ends are gripped by come alongs and drawn up to the
proper tension with tackle as shown in fig. 958. The joint is then made
as shown in the illustrations.
=Transpositions.=—In some classes of circuit, as for instance
telephone lines, the current is often seriously affected by
electrostatic induction from other lines, and also from power circuits,
owing to the fact that the surfaces of the wires form, as it were, so
many charging plates of an electrical condenser, with the intervening
air as the insulating layer or dielectric.
[Illustration: FIG. 964.—McIntyre's twisting clamp for wires 00 to 16
B. & S. gauge.]
[Illustration: FIG. 965—Method of making a "transposition." This is
usually done by means of _transposition insulators_, which are either
double insulators, one being screwed to the pin above the other, or
else such caps as are shown in fig. 967. Such insulators are intended
to act as circuit breakers, the particular wire to be transposed being
cut and "dead ended," or tied around, on both the upper and lower
grooves of the cap. The free end of each length is then passed back and
around the insulator and twisted, or sleeve jointed to the other limb
of its own circuit.]
The telephonic current changes the pressure of its own charging
surface as frequently as it alternates, and this fact in itself is
amply sufficient to account for a vast weakening of the current
before it reaches its destination. The only practicable method of
overcoming this annoyance in pole lines is by the arrangement known as
"transposition," which is, briefly, _the practice of regularly shifting
the relative position of the two limbs of each circuit as regards other
wires in the same pole system_, as shown in fig. 965.
For short lines and pole systems with only a few wires it is
not necessary to transpose very frequently. On longer lines
it has been found amply sufficient to transpose once every
quarter mile; that is to say to change the relative position
of the wires of the different circuits at posts situated about
that distance apart. This does not mean, however, that each
pair of wires is transposed so often, but that on ordinary
sized systems, the transposition of some one circuit is amply
sufficient to secure balanced relations and effectually
counteract the effects of cross induction. It is a matter which
must be carefully calculated and planned in each particular
instance in order to secure the best advantages.
[Illustration: FIG. 966.—Telegraph and telephone line glass insulator.]
[Illustration: FIG. 967.—Type of insulator used in making a
transposition.]
=Insulators.=—Glass and porcelain are employed almost universally
for supporting overhead wires. Insulators made of these materials are
superior to those made of other material such as hard rubber, or
various compounds of vegetable or mineral matter, with the exception
perhaps of mica insulators used on the feeders of electric railway
lines.
[Illustration: FIG. 968.—Tree insulator. This type of insulator is
especially useful in connection with temporary or repair work, or where
the wires pass through trees having numerous branches. The illustration
shows a Cutler tree insulator lashed to the trunk of a tree. It is
made of a single piece of glass, and is provided with a slot which the
wire cannot leave accidentally. The back of the device is concave and
provided with ribs which prevent sliding. It can be readily slipped
over wires already in place, is available for electric light circuit,
and will take wires up to ½ inch, in diameter.]
[Illustration: FIGS. 969 and 970.—Overhead cable construction. In some
cases, particularly on short lines exposed to inductive disturbances
from power and other electrical circuits, it is usual to string the
cables on poles such as usually carry the bare conducting wires. It is
not necessary, however, to insulate the cable in any way; consequently
it is merely hung to a supporting wire rope or cable, called the
"messenger wire," being attached either with some form of hanger,
such as is shown in figs. 969 and 970, or by loops of tarred marline.
The marline is sometimes wound over the cable and messenger wire from
a bobbin, but frequently it is merely wound on by hand. Cables used
in such overhead construction consist of bundles of wires, the pairs
twisted together. The size most often used is No. 19, B. & S., which is
about .03589 inch in diameter, weighs 20.7 pounds, and has a specific
resistance of about 8 ohms to the mile.]
Glass insulators are generally used on low tension lines, and porcelain
insulators on high tension lines, the latter type being usually
stronger and less brittle. Porcelain is more expensive than glass, and
its opacity prevents the detection of internal defects which would be
readily observed through glass.
[Illustration: FIG. 971.—Clark's "antihum;" a device designed to
prevent the humming of telegraph wires.]
=Ques. What is a petticoat insulator?=
Ans. An insulator which has one, two or three deep flanges or
"petticoats" around the base for the purpose of increasing the leakage
path from the line to the pin.
Both glass and porcelain insulators may be the double or triple
petticoat type which may be cast or moulded solid, or made in
two or more parts which are subsequently cemented together.
=Service Connections and Loops.=—Whenever it is necessary to tap an
overhead conductor for service connection, the method of connection
will depend upon the character of the circuit. In the case of a
parallel circuit, an extra insulator must be placed on the cross arm
so as to prevent the service main putting a side strain on the main
line conductor. In the case of a series circuit the main line conductor
is usually dead ended at the nearest pole and a loop taken to the point
of service, as shown in fig. 972.
[Illustration: FIG. 972.—Method of making a series "loop" service
connection.]
[Illustration: FIG. 973.—Parallel service connection. Service wires
tapped to the main wires, are run to insulators on an auxiliary cross
arm, thence to insulators on the side of the building, and through the
drain tube to the service switch.]
[Illustration: FIG. 974.—Joint pole crossing, showing wires of two
lines crossing each other. Four guard wires (shown heavier than the
others) extend for one span either side of the joint pole parallel to
the wires of the lower circuits and protect them from contact in case
of a break in the wires of the upper circuits. These guard wires are
insulated. The minimum distance between high and low tension wires
should be three feet. Five is better. The end guards, which prevent
wires slipping off ends of cross arms and dropping on the lower wires,
should extend about six inches above the level of transmission line.]
=Ques. What are service wires?=
Ans. Wires which enter a building.