The numerous disastrous storms of the last winter have brought out very
vividly the advantages of having all wires placed underground, and many
inquiries have been addressed to the companies operating underground
circuits as to their success. It is not probable that all of the answers
to these inquiries have been of the most favorable character. To many
central station managers an underground system means frequent
break-downs and interruptions of service, with, perhaps, slow and
expensive repairs, which bring in their turn numerous complaints, loss
of customers, and reduced profits. In many installations burn-outs both
underground and in the station are frequent, with the natural result
that the operating of circuits underground is not there considered an
unqualified success. The writer has in mind two very different
experiences with underground cables. Several miles of cable were bought
by a certain company, carefully laid, and up to to-day not a single
burn-out or interruption of service can be attributed to failure of
cables; at about the same time another company bought about an equal
amount of the same kind of cable, and in a comparatively short time the
current had to be shut off the lines and the whole installation repaired
and parts of it replaced. Both of these experiences have been repeated
many times and will be again, although it is simply a distinction
between a good cable properly laid and a good cable ruined by careless
and incompetent workmanship.
Every failure can be traced to poor work in the original installation or
to the use of a cheap cable, both causes being due, generally, to that
false economy which looks for too quick returns. A poorly insulated line
wire and a poorly insulated cable are two very different things.
However, it is a fact that by the use of a good cable it is not
difficult to construct an underground system for light, power, telegraph
or telephone uses that will be superior to overhead lines in its service
and in cost of maintenance. The ideal underground system must have as a
starting point a system of subways admitting of the easy drawing in and
out of cables and affording means of making subsidiary connections
readily and with the minimum of expense and interruption of service.
This is practically accomplished by a subway consisting of lines of pipe
terminating at convenient intervals, say at street intersections, in
manholes, for convenience in jointing and in running out house
connections. These pipes, or ducts, as they are called, should be for
two kinds of service; the lower or deeper laid lines for the main or
trunk circuits, and a second series of ducts laid nearer the surface,
running into service boxes placed near together for lines to "house to
house" connections. In some cities where it is allowed to run overhead
lines, the plan of running but one service connection in a block is
followed, all customers in the block being supplied from a line run over
the housetops or strung on the rear walls.
This makes unnecessary all subsidiary ducts except a short one from the
manhole to the nearest building in the block, and effects a considerable
saving in pipe, service boxes, cables and labor. The manholes should
have their walls built up of brick, the floors should be of concrete,
and there should be an inside lid which can be fastened down and the
manhole thus made water-tight.
For ducts wood, iron or cement lined pipe may be used. To preserve the
wood it is generally treated with creosote, which, in contact with the
lead cover of the cable, sets up a chemical action, resulting in the
destruction of the lead. Wood offers but little protection for the
cable, as it is too easily damaged and broken through in the frequent
street openings made by companies operating lines of pipe in the
streets, and as one of the main purposes of a subway is that of a
protection to cables, wooden ducts have little to recommend them except
their cheapness.
Iron pipes are either laid in trenches filled in with earth or are laid
in cement. Iron pipe will of course rust out in time, and if absolute
permanence in construction is desired, should be laid in cement, for
after the pipe rusts out, the duct of cement is still left. However, if
we are going to the expense of laying in cement, it would be much
preferable to use cement lined pipe, which is not only cheaper than iron
pipe, but makes the most perfect cable conduit, as it affords a
perfectly smooth surface to draw the cable over and give a good duct
edge.
It is not necessary, however, in small installations of cable,
especially where additional connections will not be of frequent
occurrence, to go to the expense of subways, for cable may be safely
laid in the ground in trenches filled in with earth, or can be inclosed
in a plain wooden box or a wooden box filled with pitch.
There are, of course, many localities where, if the cable is laid in
contact with the earth, a chemical action would take place which might
result in the destruction of the cable.
Underground cables are of the following classes: 1. Rubber insulated
cables, insulated with rubber or other homogeneous material. 2. Fibrous
cables, so called from the conductors being covered with some fibrous
material, as cotton or paper, which is saturated with the insulating
material, paraffine, resin oil, or some special compound. Under this
latter head is also included the dry core paper cables.
The first thing to do is to get the cable drawn into the ducts, and on
the proper accomplishment of this depends to a great extent the success
or failure of the whole installation. Probably the ducts have been wired
when the subway was constructed, but if not a wire must be run through
as a means of pulling in the draw rope. There are several kinds of
apparatus for getting a wire through a duct--rods, flexible tapes,
mechanical "creepers," etc.; but probably the best is the sectional rod.
This simply consists of three or four foot lengths of hard wood rods,
having metal tips that screw into each other. A rod is placed in a duct
at a manhole, one screwed to that, both are pushed forward, another one
added and pushed forward, and so on until they extend the entire length
of the duct. Then the wire is attached and the rods are pulled out and
detached one at a time and with the last rod the wire is through. At
least No. 14 galvanized iron or steel wire should be used, for any
smaller size cannot be used a second time, as a rule. In starting to
pull in the draw rope a wire brush should be attached to the wire and to
this again the rope, and when the brush arrives at the distant end of
the duct it very likely will bring with it a miscellaneous collection of
material which for the good of the cable had better be in the manhole
than in the duct.
The reel or drum carrying the cable should be mounted on wheels or jacks
and placed on the same side of the manhole as the duct into which the
cable is to be drawn, and must always be so placed that the cable will
run off the top of the reel.
There are several methods of attaching the draw rope to the cable. As
simple and strong a method as any is to punch two of these holes through
the cable, lead and all, and attach the rope by means of an iron
wire--some of the draw wire will do--run through these holes. Depending
on the length and weight of cable to be pulled it can be drawn either by
hand or by a multiplying winch. The rope should run through a block
fastened in the manhole in such a position that the rope shall have a
good straightaway lead from the mouth of the duct.
The strain on the cable should be perfectly uniform and steady; if the
power is applied by a series of jerks either the lead covering may be
pulled apart or some of the conductors broken. At the reel there must
always be a large enough number of men to turn it and keep the cable
from rubbing on anything, and in the manhole one or more men to see that
the cable feeds into the duct straight and to guide it if necessary. If
the ducts are of iron and are not perfectly smooth at the ends, these
should be made so with a file, and in addition a protector of some sort
should be placed in the mouths of the duct, both above and below the
cable. Six inches of lead pipe, split lengthwise and bent over at one
end to prevent being drawn into the duct with the cable, makes a very
good protector. The cable should be reeled off the drum just fast enough
to prevent any of the power used in pulling the cable through the duct
being utilized in unreeling it. If this latter is allowed to occur the
cable will be bent too short and the lead covering buckled or broken,
and also the cable may be jammed against the upper edge of the duct and
perhaps cut through.
If the reel is allowed to turn faster than the cable is drawn in, the
first three or four turns on the reel will slacken up, and the lead
covering may either be dented or cut through by scraping on the ground.
If the cable end when pulled through up to the block is not long enough
to bend around the hole more than half way, the rope should be
unfastened from its end, a length of rope with a well frayed out end
should be run through the block, and by fastening to the cable close to
the duct, with a series of half hitches, as much slack as necessary can
be pulled in. If this is properly manipulated there need not be a
scratch on the cable, but unless great care is taken the lead may be
pressed up into ridges and the core itself damaged.
Immediately after the cable is drawn in, if the joint is not to be at
once made, the open end or ends should be cut off and the cable soldered
up, as most cables are very susceptible to moisture and readily absorb
water even from the atmosphere. Where practicable it is always a good
plan to pull the cable through as many manholes as possible without
cutting the cable; for the joint is, especially in telephone or
telegraph cables, the weak point. To do this the rope should be pulled
through the proper duct in the next section without unfastening it from
the cable; the winch should be moved to the next manhole, and pulling
through then done as before. There should always be a man in every hole
through which the cable is running to see that it does not bind anywhere
and to keep protectors around the cable.
It is not advisable to pull more than one cable into a duct, and never
advisable to pull a cable into a duct containing another cable, but if
two or more cables have to go into the same duct, they should always be
drawn in together. Lead covered cables and those with no lead on the
outside should never be pulled into the same duct, for if they bind
anywhere the soft cable will suffer where two lead covered cables would
get through all right. Some manufacturers are now putting on their
cables a tape or braid covering, which saves the lead many bad bruises
and cuts, and is a valuable addition to a cable at very little
additional expense.
Practically all electric light and power cables are either single or
double conductors, and the jointing of these is comparatively a simple
matter, although requiring considerable care. The lead is cut back from
each end about four or five inches, and the conductors bared of
insulation for two or three inches. The bare conductors should be
thoroughly tinned by dipping in the metal pot or pouring the melted
solder over them. A sperm candle is better than resin or acid for any
part of the operations where solder is used. A lead sleeve is here
slipped back over the cable, out of the way, and the ends of the
conductors brought together in a copper sleeve which is then sweated to
a firm joint. This part must be as good a piece of work mechanically as
electrically. The bare splice is then wrapped tightly with cotton or
silk tape to a thickness slightly greater than that of the insulation of
the cable, and is thoroughly saturated with the insulating compound
until all moisture previously absorbed by the tape is driven off.
The lead sleeve is then brought over the splice and wiped to the cable.
The joint is then filled with the insulating compound poured through
holes in the top of the sleeve; these holes are then closed and the
joint is complete, and there is no reason why, in light and power
cables, that joint should not be as perfect as any other part of the
cable. When the cable ends are prepared for jointing they should be hung
up in such a position that they are in the same plane, both horizontal
and vertically, and firmly secured there, so that when the lead sleeve
is wiped on the conductor may be in its exact center, and great care
must be taken not to move the cables again until the sleeve is filled
and the insulation sufficiently cooled to hold the conductor in
position.
It is also very important to see that there are no sharp points on the
conductors themselves, on the copper sleeve, on the edges of the lead
covering or on the lead sleeve. All these should be made perfectly
smooth, for points facilitate disruptive discharges. Branch joints had
better be made as T-joints rather than as Y-joints, for they are better
electrically and mechanically, although they occupy more room in the
manholes. They are of course made in the same way as straight joints, a
lead T-sleeve being used, however. For multiple arc circuits copper
T-sleeves and for series circuits copper L-sleeves are used.
Telephone and telegraph cables are made of any required gauge of wire
and with from 1 to 150 conductors in a cable. In jointing these the
splices are never soldered, the conductors being joined either with a
twist joint or with the so-called Western Union splice. Each splice is
covered with a cotton or silk sleeve or a wrapping of tape, the latter
being preferable, although considerably increasing the time necessary
for making the joint. Great care must be taken that no ends of wire are
left sticking up, for they will surely work their way through the tape
and grounds, and crosses will be the result. The wires should always be
joined layer to layer and each splice very tightly taped in order to get
as much insulating compound around each splice as possible in the
limited space. The splices should be "broken" as much as possible, so as
to avoid having adjoining splices coming over each other. After the
joint has been saturated with insulating compound the wires should have
an outside wrapping of tape to keep them in shape, and then the sleeve
is wiped on and filled. If the insulation resistance of the jointed
telegraph or telephone cable is a quarter of what the cable tested in
the factory, it may be considered that an exceptionally good piece of
work has been done. I have spoken more particularly of fibrous lead
covered cables, as the handling of them includes practically every step
of the work on any other kind of underground cable. In insulating dry
core paper cables a paper sleeve is slipped over the splice, and in
rubber cables the splice is wrapped with rubber tape; all other details
are the same for these as for the fibrous cable.
In the laying of light and power cables every joint, as made, should be
tested for insulation with a Thomson galvanometer, as the insulation
must necessarily be very high, and if one joint or section of cable is
any weaker than another it may be very important in the future to know
it. All tests must be made after the joint has cooled, for while hot its
insulation resistance will be very low.
Tests for copper resistance should also be made to determine if the
splices are electrically perfect; an imperfect splice may cause
considerable trouble. In telegraph and telephone cables the conductors
should be of very soft copper, for in stripping the conductor of
insulation it is very easy to nick the wire, and if of hard drawn copper
open wires will be the result.
All work should be frequently tested for continuity with telephones,
magnetos, or small portable galvanometers. It is only necessary to
ground the conductors at one end and try each wire at the other end. For
this sort of work a telephone receiver used with one cell of some dry
battery is most convenient, and has the additional advantage of
affording a means of communication while testing, and is by far the best
thing for identifying and tagging conductors.
These cables should be frequently tested during the progress of the work
for grounds and crosses with a Thomson instrument, and when the cable is
complete, a careful series of tests of the capacity, insulation
resistance, and copper resistance of each wire should be made and the
exact condition of the cable determined before it is put in service, and
thereafter an intelligent oversight of the condition of the circuits
can thus be more readily maintained.
Where a company has extensive underground service, a regular cable gang
should be in its employ, for quick and safe handling of cables demands
the employment of men accustomed to the work. If the cable has been
properly laid and tests show it to be in good condition before current
is turned on, almost the only trouble to be anticipated will be due to
mechanical injury. Disruptive discharge, puncturing the lead, may occur;
but the small chance of its occurring can be greatly lessened by the use
of some kind of "cable protector," which will provide for the spark an
artificial path of less resistance than the dielectric of the condenser,
which the cable in fact becomes.
If a fault suddenly develops on a circuit, the chances are it will be
found in a manhole, and an inspection of the cable in the manhole will
generally reveal the trouble without resorting to locating with a
Wheatstone bridge. The cable is often cut through at the edge of the
duct, or damaged by something falling on it, or by some one "walking all
over it." To guard against these, the ducts should always be fitted with
protectors both above and below the cable. The cables should never be
left across the manholes, for they then answer the purpose of a ladder,
but should be bent, around the walls of the hole and securely fastened
with lead straps, that they may not be moved and the lead gradually worn
through.
In telegraph cables, when one or two conductors "go," it will probably
be useless to look for trouble except with instruments; but if several
wires are "lost" at once it will probably be found to be caused by
mechanical injury, which can be located by inspection. If it is ever
necessary to loop out conductors, a joint can be readily opened and the
conductors wanted picked out and connected into the branch cable and the
joint again closed without disturbing the working wires. In doing this a
split sleeve must be used, and the only additional precaution to be
taken is in filling the sleeve to have the insulating compound not hot
enough to melt the solder and open up the split in the sleeve. In
cutting in service on light and power cables it is entirely practicable
to do so without interruption of service on multiple arc circuits, even
those of very high voltage; but they require great precaution and
involve considerable risk to the jointer, and where possible the circuit
to which the connection is to be made should previously be cut dead.
Where the voltage is not dangerous to human life, almost any service
connection can be made without interruption of service.
I have only indicated a very few of the operations that may be found
necessary, and the probable causes of troubles that may be encountered
in the operating of underground circuits, believing that the different
problems that arise can, with a little experience, be successfully met
by any one who has a fair knowledge of the original construction of
cable lines.--_Electrical World_.
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