safely be used on distributing mains between two feeding points, unless

there be a balancer on the section,

for, if the outer distributors be opened, they cut off the connection with the balancers, and the feeder attached to the portion cut off then feeds the lamps on the two sides in series (see fig. 86).

So far it has been assumed that the distributing mains are low pressure, and can be safely handled while alive. It is not worth while to go into much detail concerning high pressure distributing mains, for their great disadvantages are causing them to rapidly become obsolete. Joints are sometimes made on them while alive, but this is a most reprehensible practice and should never be allowed. If, then, it is granted that the pressure must be off the main when a connection to it is being made, it is obvious that the inconvenience to consumers already being supplied must be diminished as much as possible, and, with a view to this, switches should be provided at various points along the main, so that small sections only may be cut out, and every main should be supplied at both ends so that the isolation of one section will not interfere with consumers having installations attached to either side.

High-pressure distributing mains, being nearly always fed with alternating current, were usually made concentric, the most popular system being an

FIG. 86.-Effect of opening outer distributors and feeding section without balancers.

armoured cable laid directly in the ground. The excessive inconvenience of this kind of cable, however, has led to the employment, in a good many cases, of a triple cable formed of three single conductors laid up together, and made circular with yarn, the whole being then armoured like a concentric cable. In a special form of this cable made by the British Insulated Wire Co., the stranded conductors are hammered into the shape of a sector of a circle with rounded corners, as in the cable illustrated in fig. 67. The advantage of this is that the overall diameter of the cable is slightly diminished. In this case, the conductors being insulated with impregnated paper, they are surrounded with lead before armouring.

Again, the fashion of laying the armoured cables directly in the ground is going out, and they are now usually laid solid.

CHAPTER XXII.

SERVICE MAINS AND FEEDERS.

Service Mains. The special conditions obtaining in the case of these are that, in addition to having to stand being laid underground, they must be capable of being safely handled inside the building into which they are led, and must be protected from mechanical damage when laid therein. Owing to their small size, and the consequent thinness of the dielectric, they are specially liable to damage and decay, and consequently especial care should be taken in connection with them. It may be observed that usually service lines are very short, and hence a fairly expensive main may be put down without unduly increasing the cost per consumer; and it is worth while to do this, to avoid breakdown and consequent interruption in supply.

insulated, no system of bare Owing to the ease with which very suitable as a dielectric.

Service mains are always continuously copper service lines having yet been devised. it will bear handling, vulcanised rubber is This must be mechanically protected within the building, and the most convenient way is to have the cable armoured. This armouring is a great protection underground as well, and preserves the cable if it be roughly treated during laying, as small cables are apt to be.

The cables should be laid on the solid system. The complete main used by the Author, and considered by him to be the best, comprises cables insulated with 3000 megohm rubber, armoured with two galvanised steel tapes, the outer covering the interstices of the other, and the whole covered with braiding treated with water resisting compound. These cables are laid solid with Trinidad bitumen in wooden troughs provided with cast iron covers having sides long enough to cover the sides of the trough. A section is shown in fig. 87.

FIG. 87.-Service main.

Another kind of service main largely used comprises vulcanised rubber covered cables drawn into wrought iron pipes. There are, however, several objections to this system. It is extremely awkward, if there are many

bends. The pipe is liable to destruction in certain soils. There is danger of the cables being cut by the edge of the pipe, and the establishment of connection between the consumer's premises and the main pipe or culvert will allow bad smells to enter the house. The decay of the pipe can be avoided if a straight run can be obtained by substituting cast iron or earthenware pipes for the wrought iron. If this be done, however, the cables must be armoured in order to protect them within the houses; this will, at the same time, obviate the danger of cutting the insulating material. If pipes be used, some means must be adopted for blocking the pipe, so as to exclude gas and smells. The utmost care must be used to keep cement away from the cables. Armoured lead-covered cables laid directly in the ground are largely used; in this case, no additional protection is needed within the house, but exceptional care must be taken to protect the ends.

The cost of service mains will, of course, depend to some extent on the system adopted, but there are a number of items of cost that are necessarily incurred in any case. These are the cost of getting the tackle for doing the work on to and away from the job; of opening, and making good the surface of, the ground; of making the joints on to the distributing mains. It may be well to point out that undertakers are under no obligation to cut through the wall of a consumer's premises, but, on the contrary, have no right to do so, and care should be taken, if the work be carried out, that the work is done at the consumer's risk.

The following table will give a fair idea of the cost per yard run of two important systems of service mains of various sizes, excluding the cost of jointing to the mains:

The determination of the size of service main is a simple matter compared with that of distributing mains, but, in this case also, judgment is necessary, for the service should be large enough to deal with any probable future extensions. As with distributors, standard sizes should be selected. The smallest size which will be found convenient for low-pressure work is 7 No. 17 S. W.G. for 200 volts and 7 No. 14 S.W.G. for 100 volts; the

section may then be conveniently doubled, a suitable series being 19 No. 15 S.W.G., 37 No. 15 S.W.G. These sizes will deal with most consumers, those requiring larger cables constituting special cases. The same current density as for distributors will be found suitable.

The jointing on of service cables to the distributing mains calls for great care, and the ease and safety with which it can be done varies greatly with the different classes of main, as also does the method. Taking the

service connection.

FIG. 89.—Insulator to separate cable from strip.

principal sytems of main, the method of making service connections is as follows:

Bare Copper Strip.-The service cables are sweated into gun-metal lugs, of the form shown in fig. 88, which are placed in contact with the strip. On the other side of the strip a gun-metal plate of corresponding form is placed on each cable, and is clamped to the lug by means of two phosphor bronze bolts. In order to avoid any danger from the insulated cable crossing the strip, an insulator, of the form shown in fig. 89, is provided for the cable to rest on where it crosses the strip.

The practice usually followed is to make these connections at the boxes placed over FIG. 88.-Clamps for copper strip the intermediate insulators or in a straining box, and at no other place; but when the intermediate insulators are flagged over, no provision is made for taking off service connections at any particular point, the most convenient point for the service to enter the building being selected, and a flag on the top of the culvert lifted at this point. A hole is cut into the side of the culvert, to admit the pipe or trough containing the service cables. The side of the culvert is then made good round this, and, the cables having been run and clamped on, the flag is replaced.