UNDERGROUND WIRING
In large cities, the best method of running wires for all varieties of
electrical power transmission is to place them underground. Many city
authorities have made this method of wiring compulsory by law, because
of the difficulty in approaching a burning building, the danger from
crossed and falling wires, and the disfigurement of the streets where
there is a network of overhead wires.
The expense of installing an underground system is very great in
comparison with that of overhead construction, but the cost of
maintenance is much less and the liability of interruption of service
greatly reduced.
=Underground Systems.=—An underground system of electrical conductors
is composed of three essential elements:
1. The conductor itself, which is almost invariably of copper;
2. The insulation, which is either in the form of a complete covering
of insulating material, or simply insulated supporting points;
3. The tube or conduit, which constitutes the mechanical protection
against the effects of the severe shocks, weather conditions, etc., to
which the system is naturally exposed.
The various underground systems may be divided into three classes:
1. Lead encased cables laid directly in the ground;
2. Solid or built in systems;
3. Drawing in systems.
=Ques. What may be said of the first mentioned construction?=
Ans. Where cables are laid directly in the ground, the metallic
covering, consisting usually of a lead tube, which is placed over the
insulation is depended upon for mechanical protection. Such cables are
largely used for short private lines and the first cost is less than
that of the others, but in case of repairs it has to be dug up.
=Ques. Describe the drawing in system.=
Ans. In this construction the cables are drawn in after the conduits
are built. The conduit of the drawing in system may consist of various
forms of pipe or troughs of iron, earthenware, concrete, wood or fibre,
while those of the solid or built in systems are composed of either
iron tubes or concrete trenches.
=Conduits.=—The principal qualifications of a good conduit are freedom
from disintegration by the action of fire, water, acids, alkalies, or
electrolysis; second, a smooth interior surface so as to permit of the
easy drawing in of the cables; and third, a design which will permit
of its economical installation in crowded streets. There are numerous
kinds of conduit of which may be mentioned:
1. Vitrified clay pipe conduits;
2. Vitrified clay or earthenware trough conduits;
3. Concrete duct conduits;
4. Wooden duct conduits;
5. Wooden built in conduits;
6. Wrought iron or steel pipe conduits;
7. Cast iron pipe and trough conduits;
8. Fibre conduits.
[Illustration: FIG. 975.—A few forms of vitrified clay pipe conduits;
view showing single and multiplex types. The dimensions of each duct
are about 3½ × 3½. The lengths vary from two to three feet.]
=Vitrified Clay Pipe Conduit.=—Various forms of vitrified clay
conduit appear to possess the qualifications, desirable in underground
construction, to a higher degree than any other type. They are made in
both single and multiple duct, as shown in fig. 975, the single type
being about 3½ inches in diameter, or 3½ inches square, and 18
inches long. Multiple conduit is made in two, three, four, six and more
sections, ranging from 2 to 3 feet in length.
=Ques. For what conditions is the single conduit especially adapted?=
Ans. It is most suitable for use where the sub-surface conditions are
characterized by a great crowding of gas, water, and other pipes, as
the conduits can be divided into several layers so as to cross over or
under such pipes, and many other sub-surface obstructions which are
present in the streets of large cities and towns.
=Ques. What are the features of the multiple duct conduit?=
Ans. It can be laid somewhat cheaper than the single duct type,
especially in lines of about two to four ducts; it is, therefore,
most suitable for use in outlying communities where the streets are
comparatively free from many sub-surface obstructions.
=Ques. How is the conduit laid?=
Ans. In laying conduit, a trench is dug, usually sufficiently wide
to allow the placing of three inches of concrete on each side of the
ducts, and sufficiently deep to hold at least thirty inches of concrete
on top of the upper layer of concrete forming the conduit, and to
allow for three inches of concrete in the bottom. The trench is graded
from some point near the middle of the block to the manhole at each
intersection, or from one manhole to the next manhole, at a gradient
not less than 2 inches to 100 feet.
=Ques. How are single duct conduits laid?=
Ans. The tiles of the several ducts are placed close together, and
the joints plastered and filled with cement mortar consisting of one
part of Portland cement to one part of sand. When the conduit is being
laid, a wooden mandrel about four or five feet long, three inches in
diameter, and carrying a leather or rubber washer from three to eight
inches larger at one end is drawn through each duct so as to draw out
any particles of foreign matter or cement which may have become lodged
in the joints, and also to insure good alignment of the tiles, as shown
in fig. 977.
Single duct conduits are usually laid by bricklayers. This fact
accounts for the somewhat greater cost of the single over the
multiple conduit which is usually laid by ordinary laborers.
One good brick-layer and helper, however, will lay from 200 to
300 feet of single duct conduit per hour.
Practically the same standard of construction is maintained
on all conduit lines from two ducts up to twenty-five ducts,
as many of the smaller lines may extend for miles into the
outlying districts, and contain transmission lines of the
maximum working voltage.
[Illustration: FIG. 976.—Vitrified clay or earthenware trough conduit;
this type of conduit consists of troughs either simple or with
partitions, the latter type being shown in the figure.]
=Vitrified Clay or Earthenware Trough Conduit.=—It consists of troughs
either simple or with partitions as shown in fig. 976. They are usually
made in tiles 3 or 4 inches square for each compartment, with wall
about one inch thick. The length of the tiles ranges from two to four
feet. Each of the two foot form duct troughs weighs about 85 pounds.
When laid complete, the top trough is covered with a sheet of mild
steel, about No. 22 gauge, made to fit over the sides so as to hold it
in position, and then covered over with concrete.
=Joints in Multiple-duct Vitrified Clay Conduit.=—In laying multiple
duct earthenware conduit, the ducts or sections are centered by means
of dowel pins inserted in the holes at each joint, which is then
wrapped with a six inch strip of asphalted burlap, or damp cheese
cloth, and coated with cement mortar as shown in fig. 978. Economy of
space and labor constitutes the principal advantages derived from the
use of multiple duct conduit.
[Illustration: FIG. 977.—Method of laying single duct vitrified clay
conduit. The tiles of the several ducts are placed close together as
shown in the figure, and the joints plastered and filled with cement
mortar consisting of one part Portland cement and one part sand.]
=Concrete Duct Conduits.=—These are usually constructed by placing
collapsible mandrels of wood or metal in a trench where the ducts
are desired and then filling the trench with concrete. After the
concrete has solidified, the mandrels are taken out in pieces, leaving
continuous longitudinal holes which serve as ducts. Some builders
produce a similar result by placing tubes of sheet iron or zinc in the
concrete as it is being filled into the trench. These tubes have just
enough strength to withstand the pressure to which they are subjected,
and are, therefore, very thin and liable to be quickly destroyed by
corrosion, but the ducts formed by them will always remain unimpaired
in the hardened mass of concrete.
=Wooden Duct Conduits.=—In this type of conduit, the ducts are formed
of wooden pipe, troughing, or boxes, and constitute the simplest and
cheapest form of conduit. A pipe conduit consists of pieces of wood
about 4½ inches square, and three to six feet long, with a round
hole about three inches in diameter bored through them longitudinally.
As shown by fig. 979 a cylindrical projection is turned on one end of
each section, which, when the conduit is laid fits into a corresponding
recess in one end of the next section. The sections are usually laid in
tiers, those of one tier breaking joint with those of the tiers above
or below.
[Illustration: FIG. 978.—Method of laying multiple duct vitrified clay
conduit. The sections are centered by the dowel pins shown in the cut.]
The trough conduit consists of ducts about 3 inches square made of
horizontal boards and vertical partitions, usually of yellow pine about
one inch in thickness. This form of conduit can be laid in convenient
lengths of 10 or 12 feet, or it can be built along continuously.
=Ques. What is the objection to the use of wood for conduits?=
Ans. The decay of the wood tends to form acid which corrodes the lead
sheath of the cable.
[Illustration: FIG. 979.—Wooden pipe type of conduit. It consists of
pieces of wood about 4½ inches square, and three to six feet long,
with a wide hole about three inches in diameter, bored through them
longitudinally.]
=Ques. How can this be prevented?=
Ans. The decay of the wood can be prevented to a certain extent by the
application of sterilizing processes, thereby preserving it in fairly
good condition for about ten to fifteen years.
=Ques. For what service is wooden conduit best adapted?=
Ans. For temporary installations which will be discontinued before the
wood decays.
=Wooden Built-in Conduits.=—Within recent years several forms of
wooden built-in conduit have been designed and successfully used for
permanent work. They possess several advantages over any of the duct
systems, the chief of which are high insulating quality, the capability
of using bare wire and rods for underground conductors, and reduced
cost. An approved form of wooden built-in conduit is shown in fig. 980.
[Illustration: FIG. 980.—Perspective view of wooden built-in conduit.
It consists of an outer rectangular casing of wood which is lined
inside with impregnated felt.]
=Ques. How are wooden built-in conduits installed?=
Ans. A wooden trough is laid in a trench about 18 inches deep.
Porcelain carriers as shown in figs. 981 and 982 are placed in the
trough at intervals of 4 to 5 feet, to act as bridgework for supporting
the conductors. This bridgework is placed on and is surrounded by
impregnated felt or similar material, and the spaces between the
carriers, after the conductors have been placed in position on them, is
filled with voltax, which hardens rapidly and forms a solid insulating
material throughout the conduit.
=Wrought Iron or Steel Pipe Conduits.=—These are formed of pipes
similar to gas or steam pipes, with screw or other connections. They
are laid either simply in the earth, or in hydraulic cement, and are
the strongest and one of the most satisfactory forms of underground
conduit. An appropriate standard of this kind of work is shown in fig.
983.
[Illustration: FIGS. 981 and 982.—Porcelain bridgework or carriers for
supporting underground conductors.]
=Ques. What is the ordinary method of construction?=
Ans. A trench, the width of which will depend upon the number of pipes
to be laid, is first dug in the ground, and after its bottom has been
carefully leveled, is braced with side planking and filled to the
depth of two to four inches with a layer of good concrete, consisting
of two parts of Rosendale cement, three parts of sand, and five parts
of broken stone capable of passing through a one and one-half inch
mesh. This concrete is well secured in place and forms the bed for
the lowermost layer or tier of pipes. Ordinary wrought iron pipe is
employed, in 20 foot lengths about three to four inches in diameter,
depending upon the size and number of cables they are intended to
carry. After the last tier of pipes have been put in place, and a layer
of concrete from two to four inches placed over it, a layer of two inch
yellow pine planking is laid over the whole.
[Illustration: FIG. 983.—Cross section of wrought iron pipe conduit
laid in hydraulic cement.]
The pipe connections consist of a taper screw thread coupling
which can be easily made up as the pipes are laid, and which
forms a tight joint.
The pipes in each tier are usually laid from ½ to ¾ of
their diameter apart, and when the first tier is in place, the
spaces between and around the pipes are filled in with concrete
which is carried up over the pipes to a depth of about one-half
a diameter to form the bed for another tier of pipes.
=Ques. What is the principal object of the top covering of planks.=
Ans. To protect the conduit against the tools of workmen making later
excavations.
Practical experience shows that workmen will dig through
concrete without stopping to investigate as to the character
of the obstruction, but under similar circumstances, will
invariably turn away from wood.
=Ques. How are the pipes treated before being laid?=
Ans. They are dipped in tar to protect the outside surface from rust.
=Ques. What is the most satisfactory form of lined iron pipe?=
Ans. Pipe lined with cement. The internal surfaces of these pipes are
usually covered with a lining of pure Rosendale cement about ⅝ inch
thick and containing no sand. The internal surface of the cement lining
does not offer much friction to the introduction or withdrawal of the
conductors.
These pipes are laid in cement or concrete in the same manner
as plain iron pipe, and are given a coating of tar on the
outside to prevent rusting.
=Cast Iron Pipe and Trough Conduit.=—Cast iron pipe for underground
conduits is similar to ordinary wrought iron pipe, except that it is
thicker. The additional thickness is necessary to make the strength
equal to that of wrought iron; it is therefore heavier to handle and
more expensive.
=Ques. Describe a cast iron trough conduit.=
Ans. It consists of shallow troughs of cast iron in six foot lengths,
laid directly in the earth so as to form a system of continuous
troughing in which the conductors are placed and then covered over by
cast iron covers which are bolted to the trough.
=Ques. What advantages does this form of conduit possess over the duct
type?=
Ans. First, the cables can be laid directly in place, thus eliminating
any chance of injury during the process of drawing in, and second, the
cables are easily accessible at any point by simply removing one or
two of the sectional cast iron covers, thus permitting of their being
readily inspected and repaired.
[Illustration: FIG. 984.—Fibre conduit. It consists of pipes made of
wood pulp, having about the same thickness as cast iron pipe. Slip
joint conduit for electrical subways is three inches inside diameter.
The socket joints keep the lengths centered and make it easier to lay
than a mere butt joint. It is laid in cement like iron pipe.]
Branch connections can be made with greater facility than in
the case of any duct system, so that it is especially suitable
for distribution systems were it not for the fact that it is so
expensive as to be practically prohibitive.
=Fibre Conduits.=—This form of conduit consists of pipes made of
wood pulp impregnated with a bituminous preservative and insulating
compound. These pipes are laid in concrete in a manner similar to iron
pipe. Fibre conduits are made in sizes ranging from 1 inch to 4 inches
in diameter and from 2½ to 5 feet in length, with walls ranging from
¼ to ½ inch in thickness.
=Ques. Name the three types of fibre conduit.=
Ans. The socket joint type, as shown in fig. 984, the sleeve type, fig.
985, and the screw joint type, fig. 986.
=Ques. What is the usual method of laying the socket joint type of
fibre conduit?=
Ans. After the trench has been dug to the required width and depth,
depending upon the number or pipes to be placed in a tier and the
number of tiers, a bed of concrete about 3 inches deep is placed on
the bottom and a line drawn on one side for the alignment of the first
line of pipes. The other lines of pipe or ducts are laid parallel to
the first line, and are separated from it and from each other by means
of ¼ inch or ½ inch wooden or iron pegs. The pipes are well grouted
and covered with a layer of concrete to the depth of ¼ or ½ inch,
and the next tier laid in place in the same manner. When the final tier
of pipes has been installed, it is covered with a layer of concrete
about 2 to 3 inches deep.
[Illustration: FIG. 985.—Sleeve joint type of fibre. Both the socket
type (fig. 984), and the sleeve type here shown are easily aligned
without the use of a mandrel.]
=Ques. What is done when it is necessary to cut a length of pipe to
break joints, or to enter a manhole?=
Ans. The remaining part of the length may be utilized by using a fibre
conduit sleeve having an inside diameter ½ inch greater than the pipe
being used on the system.
These sleeves are furnished by the manufacturers at a nominal
charge per foot. They are about four inches in length and fit
over the ends of the abutting pipes, so that they make tight
joints and give perfect alignment.
Although its employment is not permitted where fireproof
regulations are in force, fibre conduit is now being
extensively used in other places, and is giving satisfactory
service. It is not affected by moist earth and is impervious
to the action of acids, alkalies, and gases. As it is not
subject to expansion and contraction, leakage is practically
eliminated, and since it is a very good insulator, troubles due
to stray currents are reduced to a minimum. It is extremely
light, comparatively non-breakable, and can be accurately laid
at the rate of 12,000 duct feet per day by a gang of common
laborers, consisting of two layers and three helpers.
[Illustration: FIG. 986.—Screw joint type of fibre conduit. This
method of connection will form a tight line and is suitable for running
under the lawns of private houses and parks, under the streets of towns
and villages, and in other places where the cost of building electric
subways is prohibitive.]
=Edison Tube System.=—Of the various built in or solid underground
conduit systems other than those already described under wooden conduit
systems, the most satisfactory are the Edison tube system, the Crompton
naked conductor system, the Kennedy system, which is a modification of
the Crompton and the Callender systems.
=Ques. Describe the Edison tube system.=
Ans. It consists of a series of iron tubes or pipes containing one or
more copper conductors which are placed therein before each complete
section or pipe leaves the factory, so that they only need to be
joined together to form a continuous line of underground conduit with
conductors in place. The arrangement of wires and the details of the
Edison tube system are shown in figs. 987 to 989.
[Illustration: FIG. 987.—Cross section of Edison "feeder" tube.
This runs from the power station to the centers of distribution, and
contains two principal conductors and a smaller conductor to serve as
a neutral wire, and also three insulated cables of seven strands of
No. 19 B. W. G. wire each. These cables form independent circuits and
enable the voltages at the distant end of the feeder to be read at the
central station. For this reason they are commonly called pressure
wires.]
=Underground Cables.=—Electric light and power cables for use in
conduit may be divided into two classes: _moisture proof_, and
_non-moisture proof_, according to the character of the insulator. In
the moisture proof cables, the insulation consists of some form of
rubber, or of bitumen, and a metal sheath or covering, usually of lead,
is provided to protect the cable from mechanical or chemical injuries.
The non-moisture proof cables are insulated with paper impregnated with
oil, wax, or resinous compounds.
[Illustration: FIG. 988.—Cross section of Edison "main" tube. A number
of these tubes, which radiate from the center of distribution and loop
the ends of the feeders together, have three conductors of the same
size. These tubes are placed in the ground so as to bring the positive
and negative conductors on one side of the center of the tube, and the
neutral conductor on the other side. The mains are always laid with
the neutral conductor adjacent to the curb line, and for convenience
this side of the tube is commonly called the _inside_. The feeders are
always laid with the positive conductor on the right hand side, as
shown in fig. 989.]
=Metal Sheaths on Underground Cables.=—Metal sheaths are used on
rubber covered cables to protect the insulating compounds from the
deteriorating effects of electrolysis and various kinds of acids and
gases which, under present methods of construction, are ever present
in the underground conduits. It is a fact, however, that the lead
sheath on a low tension cable, which is used as one side of a grounded
circuit, has been, in some cases the cause of, instead of, cure for
electrolysis. The proper cure lies in the omission of the sheath
altogether, but as this is not practical except in the case of very
large conductors, the best thing that can be done is to interrupt the
continuity of the sheath by some form of insulating joint.
[Illustration: FIG. 989.—Method of laying Edison underground tube
system. The tubes are laid in trenches about 30 inches deep and 20
inches wide at the bottom, each trench usually containing two lines of
pipes—a _main_ adjacent to the curb and a _feeder_ on the outside. The
copper rods forming the conductors are uniformly 20 feet, 4 inches in
length and project from 2 to 3½ inches from each end of the pipes,
which are connected together by means of coupling boxes. The coupling
box usually employed consists of a two part egg shaped casting into
which the ends of the pipes enter through water tight sleeves at the
opposite ends of the oval. The projecting ends of the copper conductors
are joined by short pieces of flexible cable with sockets on each end,
which are drilled to fit easily over the conductor rods to which they
are thoroughly soldered in order to make a perfect electrical joint.
After the conductors have been thus properly connected the cover is
bolted down on the lower half, and the whole of the interior of the box
is filled with insulating compound through the small hole at the top
of the cover, thereby completely insulating the copper conductors, the
cable connectors, and the ends of the tubes. Finally, the hole in the
cover is closed with a cast iron cap. These coupling boxes are also
made in the form of tees for making branch connections, and in the form
of elbows for turning corners, the ball ends attached to the tubes and
the sockets into which they fit being designed to permit of variation
in direction through an arc of 18 degrees on either side of the central
position. _Services_ or branches to the consumers' premises consist
of short lengths of tube which tap the mains by means of three way or
four way service boxes, the latter readily permitting the taking of two
services from one joint. Services are never taken from the feeders, but
the latter are brought to distributing boxes containing three copper
rings to which the conductors are connected and branched out to one or
more mains which are led out through fuses to supply the districts.]
=Pot Heads.=—The upper end of a lateral cable is equipped with a
discharge bell, which is commonly called a pot head. The purpose of
a pot head is to hermetically seal the end of the cable and bring
the conductors out in such a manner as to permit of their being
conveniently connected to the primary service boxes.
[Illustration: FIG. 990.—Bottom of General Electric manhole junction
box; view from manhole interior. The cables enter the bottom of the box
as shown through composition nozzles to which the lead sheathes are
united by a wiped solder connection, forming a permanent water and gas
tight joint. Stuffing boxes are sometimes substituted, doing away with
the wiped joint, rendering the boxes suitable for use with unleaded or
braided cables. The normal position of the distributing cables is in
the upper ducts so that they may be brought to the junction box without
crossing other lines. The entrance nozzles and seats are so arranged
that all terminals are soldered to cables outside of box and any cable
may be removed without disturbing any soldered joint. The wiped joints
unite electrically the lead sheathes of all cables entering the box and
by connecting a single earth bond to the shell of the box all cable
sheathes are solidly grounded. Incombustible shields prevent the arc
from a blown fuse making a ground connection to the shell or inner
cover.]
=Ques. How are pot heads made?=
Ans. They are usually made in three parts, the base being of cast
brass, having a diameter depending upon the size of the conductors,
with a hole in the lower end threaded within in such a manner as to
make a tight fit on the cable.
=Ques. How is a pot head connected to a cable?=
Ans. After the cable has been bent in to the proper position, the brass
base is slipped down over it with the larger end up, and then screwed
down on the lead sheath. The threads cut down into the lead sheath to a
distance of about ½ inch along the sheath, thus making an air tight
connection without necessitating the making of a wiped joint.
The separate conductors are now bared of their insulation for
a distance of about two inches, and then spliced to heavy
rubber covered braided wire of sufficient length to reach the
primary service boxes. The joints connecting these rubber
covered wires and the cable conductors are spliced in the
same manner as straight splices, the paper sleeves used being
of sufficient diameter to be backed out of the way over the
rubber insulation. When the splice is completed a brass shell
threaded at one end to fit a female thread in the upper end of
the brass base, is slipped over the end of the rubber covered
wire and screwed into the base. A hood of sheet copper having
the form of a quarter section of a ball is slipped over the top
of the frame and its lower edge tracked in position below the
horizontal shelf. This hood makes the pot head water, snow, and
insect proof.