Publication: The Electrician & Electrical Engineer
New York, NY, United States
THE ECONOMY AND EFFICIENCY OF UNDERGROUND
ELECTRICAL CONDUCTORS IN CITIES.
BY DAVID BROOKS.
WHEN Mr. W. H. Preece, the chief electrician of the English post-office department, was in this country two years since, he stated that whenever the government could get as many as 15 electrical conductors into one route or channel, they found it both advantageous and economical to place them underground. Mr. Preece's experience with underground electrical conductors, as compared with overhead conductors, was almost entirely with the underground conductors insulated with gutta-percha.
The cost of a No. 18 British-gauge wire, insulated with gutta-percha, according to the government standard, is about $75.00 per mile. These conductors arc drawn into cast-iron pipes, usually buried beneath the sidewalks at an average depth of two feet below the surface. This is the cost of the conductors themselves; but the expense of the trenching, the iron pipes, the drawing-in and splicing, brings the cost of these wires when laid to $100.00 or over per mile. Gutta-percha insulation is expensive.
By the use of oil, and a textile covering for the wire, instead of the gutta-percha insulation, we can lay three times as many of those conductors in this country for the same amount of money. In Europe we can lay at least five times as many conductors of the same gauge for the same expense.
The next question for consideration is the comparative durability of the different systems. Gutta-percha, or India rubber, like all vegetable or organic matter, undergoes disintegration and decay, but much depends upon the condition in which it is used. As it is used in England, it lasts from seven to ten years. It is exposed to the influence of damp and impure air. The French use the same kind of insulation, but they draw from five to seven conductors into a lead pipe, which is afterwards pressed tightly around the group of conductors, so as to exclude the air; and by that means they have made the insulation last thirty years; so I was informed in 1881 by the French electrical engineers; and for all I know those conductors are working yet. Within the last few years they have laid a large amount of those lead-covered cables, drawn into cast-iron pipes of larger diameter, and have wires working now underground from Paris to Marseilles and other cities of the Republic. Gutta-percha, when buried in the sea, and away from the influence of the air and the light, is said to be practically imperishable; but when suspended in the Paris sewers, and unprotected by the lead covering, it lasts but a short time.
The life of an insulator is dependent upon its durability. As a gutta-percha insulating material goes into decay, it does not become a conductor of itself, but it cracks and then admits moisture, and the moisture in the cracks becomes a conductor. Conductors insulated with India rubber made from old shoes have every appearance of presenting as high an insulation as those insulated with fresh Para rubber; but the life of the former is comparatively short as an insulator, because the material itself has lost its vitality.
There are different methods of preserving materials of all descriptions, but as a general thing almost any organic matter and the metals can be preserved if we protect them from the effects of light, heat, moisture and the air. As a rule, heat is not a factor in the decay of electrical insulating materials, for, as commonly used, they are not subjected to the influence of abnormally high temperatures.
I insulate underground wires with oil and a textile covering. All traces of moisture, air and light are excluded. There is nothing that can in any way affect the contents of the pipe. The textile covering is preserved by a hydrocarbon, the most stable of all materials, the same that has preserved the linen covering of the Egyptian mummies for thousands of years. The hydro-carbon preserves the inner surface of the pipe, the copper conductors and their covering. No change can take place in the condition of those materials, and, as a consequence, no change can take place in their insulating properties, as the insulation is entirely dependent upon the durability of the insulating materials; or, in other words, the durability is more a question in physics than an electrical question. If I were asked how long a glass insulator would last, I would say just as long as the glass remains whole and entire. I am often asked if the electrical current does not affect the oil. I say, no more than it affects the glass insulator on the poles. Such non-conductors as glass or oil, or any other equally good insulators, are unaffected by a current of electricity.
This much relates to the inner surface of the pipe and its contents. The outer surface of the iron pipe is protected from rust by being enclosed in a wooden box of sufficient dimensions to contain the iron pipe and a concrete covering composed of pitch and sand, which effectually prevents oxidation of the outside surface of the iron pipe, under these conditions the system can be said to be indestructible except by mechanical means. So much for durability.
THE EASE AND FACILITY WITH WHICH THE SYSTEM IS LAID.
As many as 2,000 feet of trench have been dug, and the lengths of iron pipe screwed together, and the trench filled and repaved in one day. The inside of the pipe having been scoured and polished perfectly smooth, 2,000 feet of cable can easily be drawn in. The oil acts as a lubricator, enabling the cable to slip along easily and without friction. Nor is it necessary that the pipe be laid in a straight line. We go either above or below gas or water pipes, or to one side or the other of any obstruction we meet.
Three three-inch diameter pipes, with carrying capacity of 1,530 conductors, are laid in Calhoun place, Chicago. The wooden box containing these pipes has less than one square foot of section. A mile of cable could be drawn in, in one length, if it were desirable.
By having a short piece of the pipe bent so as to form a ninety-degree segment of a circle, the radius of the circle being two feet, street corners can be turned with ease, and cables have been drawn around two of such corners without producing a sensible or material increase of friction. By the use of the splice-boxes, or hand-holes, as many as 500 splices have been made by one person in a day. In New York, hand-holes were inserted every 45 feet, making it easy, to bring out conductors and lead them into adjacent buildings; and they are so introduced, every portion of the wire from the building to the exchange being underground. Of those 800 conductors now in use for two years, not one has failed or cost one penny for maintenance or repairs. Another method is to bring out under the sidewalk, through the cellar to the rear of the building, as many conductors as will be needed in that block. They can be distributed from a short pole easily to any or every building in that block. A property owner would sooner grant this privilege than have poles standing in front of his building, as such poles must necessarily be high, and the snarl of wires radiating from that structure presents a very unsightly appearance.
FOR TELEPHONIC LINES.
The system is peculiarly adapted for telephonic purposes, being free from inductive disturbances and overhearing or eavesdropping. Those having interest in telephonic exchanges should consider the advisability of applying a portion of their earnings to the placing of the conductors underground. They should take into consideration the expense they are now under of having so large a force employed to hunt up and remove "trouble," and the vexatious annoyances occasioned by the frequent interruptions of the overhead service. The outlay incurred in maintaining the force simply to remove "trouble" would pay an interest on a very large amount of capital. They should take into consideration that these overhead conductors are getting weak and brittle from exposure to the atmosphere of cities. The main telephone patent will expire in a little over six years. During this time, with judicious management their conductors can be buried, and the expense of this work paid out of the earnings of the company. A stockholder would part with his stock while it is paying 12 per cent, at a less price than he would were it paying six per cent., if he knew that when the time arrives for the patent to expire the conductors would be underground, and the expense of putting them there paid. Would a competing company easily raise the funds to put down a plant to compete against a company whose plant is already down and paid for? It is evident that the present companies would be as well protected under these circumstances from competition as they are now by the patent itself.
It is also well to bear in mind that since electric lighting came into use many of the telephonic exchanges and telegraph offices have been either destroyed or seriously damaged by fires occasioned by electric light conductors coming in contact with the telephone or telegraph conductors, and this danger will continue so long as these different electric conductors remain overhead. In New York, Brooklyn, Philadelphia, and other places, people have been killed by coming in contact with electric light conductors. Chicago is the only large city that has not suffered in this respect, and the reason is that all the electric light conductors are underground. Chicago is in advance of all other American cities in this underground business. Chicago has one-third more telephone subscribers than Philadelphia. The Bell Telephone Co. of Philadelphia employ 20 more men than the Chicago Telephone Co. to remove interruptions to their circuits. The Philadelphia Co. have no wires underground.
FOR TELEGRAPHIC LINES.
For telegraphic purposes few are aware of the advantages of having the wires placed underground in cities. The glass insulators in cities soon become coated with soot or carbon, and thousands of insulators are in use that will not show an insulating resistance of one megohm. There is as much loss of current over one insulator so situated, in rain and dampness, as there is in twenty miles of the line in the country, where the insulators are clean and affected by moisture only, which is comparatively free from impurities. When lines are worked upon the same set of poles with positive and negative currents, the interference from wire to wire by these currents short-circuiting into each other, commonly called cross-currents, is so great that some of the circuits have to be opened in order to enable the others to be worked. The telegraph officials in Chicago all speak of the great advantage derived in this respect from having their wires placed underground in that city.
FOR ELECTRIC LIGHTING WIRES.
For electric lighting, this system is peculiarly well adapted, both for arc and incandescent. By the use of the splice-boxes and hand-holes the conductors can be brought into the buildings for house-to-house purposes, or into the ordinary street lamp posts. The conductors for such purposes can be drawn in singly or a number of them in a group, as circumstances may require. Oil is the strongest insulation known, or least liable to be affected by currents of high potential and quantity combined.
Underground conduits first began to be invented in England about the year 1850, and from that time until 1860 many patents were taken out for different forms and materials. But a great many patents have been taken out in this country within the past ten years for that class of inventions. Neither in England nor in this country have the patentees been known as having much or any electrical experience. These various patented conduits are supposed to facilitate repairs by easy drawing in and out. From the nature of the case, conductors placed in these receptacles are continually in need of repairs. The atmosphere of the conduit is full of gases that will soon affect and destroy anything that is put in them. Some of the gases are heavier than the air, and their tendency is to collect and stay in the conduit. They act upon the metals, lead and iron like stagnant water, and no insulating covering of any kind can long withstand the effect. They are always damp, and the inner surface so wet that nothing is gained by having the material of the conduit itself made of a non-conducting substance.
It has been proposed to make these conduits of wood, but such a structure decays very rapidly, as was shown in the early experiments in England. A piece of wood buried in the ground, with the earth packed closely around it on all sides, lasts ten times as long as a wooden box containing damp and impure air. The decay begins on the inner surface of the box, that portion of the box coming in contact with the impure air, and no means have ever been devised to prevent it.
The writer has spent the last four months in Chicago, and has had abundant opportunity to examine the different systems there. That of the Dorsett has fewer conductors and in much shorter lengths than that laid for the Metropolitan Telephone Company, by the writer, in New York, two years ago, which has not cost one cent for repairs nor have any of the wires failed or deteriorated. But in Chicago, although the conductors have been in the ground a shorter length of time than have those I laid in New York, three men are continually employed, in Chicago, in repairing and removing faults or interruptions. The lead covering is often eaten through by rust.
They dare not go near the man-holes with a fire or soldering pot for fear of explosions. I had myself an experience. The Chicago Telephone Company had arranged a man-hole for bringing in the wires, opposite No. 91 Jackson street. When we sent a man into the man-hole with a lantern, to splice the wires, an explosion took place which set fire to the covering of the wires. In order to put out this fire, snow was snoveled on and by that process they got water into the pipe, which delayed our work two days.
A galvanized wire drawn into one of these conduits six months since, when pulled out showed no traces of zinc covering, it having been so affected by the atmosphere of the conduit.
Another objection to these conduits is their cost. I can lay alongside the conduit put down on Sixth avenue, New York, an equal number of conductors that can be got into that conduit, the conductors being of equal insulation and conductivity, for less money than the conduit costs, without wires in it. And I will guarantee the interruptions or failures from every cause to be less in the conductors I lay than those in the conduit, in the proportion of one to one hundred.
LEAD COVERED CABLES.
A lead pipe of sufficient diameter to contain one hundred conductors, costs as much per foot as an iron pipe containing five hundred conductors. If as many as two hundred conductors can be got into one route, it costs no more to place those conductors underground in an iron pipe, than to string them overground in lead pipes, and in the latter position they are liable to injury from fire, as almost every telephone exchange using lead covered cables can testify. If lead covered cables are placed underground they have to be imbedded in asphalt or pitch, to protect the lead covering, and in case of an accident happening to the conductors it is extremely difficult to find and remedy the trouble.
The lead covering is seriously affected, as before stated, by the atmosphere of the conduit, and lead covered cables are extremely unwieldy to draw out of a conduit.
In our system, the cables can be easily drawn out of the iron pipes if occasion demands it. We have only had one such occasion and that was in Chicago, when it became desirable to substitute a 250-wire cable for one which had only 150 conductors in it.
The London Electrician, of May, 1886, contains the following experiments with oil as an insulator, by H. C. Frempt: —
"The oil used to insulate the underground wires running into the telephone exchange, at the corner of Broadway and Spring street, New York, is heavier than water. Take two cotton covered wires which have been boiled in that oil, draw them through glass tubes, twist the ends together, and place them in an ordinary glass battery jar. Fill the jar half full of oil, after which water may be carefully poured into the jar and it will remain above the oil. Attaching the poles of a battery to each of these wires, as shown above the glass tubes, no current will be observed as passing from one wire to the other.
"Take a piece of lead pipe containing a cable of cotton covered wires insulated with the same oil in a fluid condition, and cut a hole in the lead an eighth of an inch in diameter. The oil being viscous will ooze out slowly. Place the cable in a jar, and fill the jar with water, the oil will continue to flow till an equilibrium is established, but no current can be passed from one wire to the other or from either wire to the lead pipe. Now, if instead of oil we use in this experiment paraffine, pitch or rosin, the insulation immediately fails.
"Six years since there was a lead covered cable insulated with oil, armored and laid across the Delaware river. The cable contained 50 conductors, not one of which has failed. The cost of this cable of 50 conductors was no more than that of an ordinary gutta-percha insulated cable of five conductors. The gutta-percha insulated cables across the Delaware during this period have been continually subject to mishaps. Many of the conductors have been injured by lightning, although protected by lightning arresters. When the gutta-percha insulation is punctured by atmospheric electricity, it leaves a hole in the insulation in which water enters, by which the current passes to earth. The cable has to be underrun, the leak found and the hole sealed up, but a hole cannot be made in a liquid."
The splice boxes we use will hold two gallons of oil after the splices are covered with oil. When the splices are covered by the oil we can fill the remaining portion of the box with water without affecting the insulation of the conductors.
|Date completed:||January 23, 2011 by: Bob Stahr;|