wires A and B, so that if at c, for instance, a short circuit occurs, that is, if a by-path is formed across the terminals of the lamp, there is less resistance between A and B through this particular series and a larger current will flow. This is likely to burn out the other lamps, c', c', etc., one after the other, with increasing speed and with liability of excessive heating and consequent danger.

A "series-multiple system" is one in which groups of lamps in multiple are connected in series. (Fig. 46.)

C

8

FIG. 46.-Series-multiple System.

The same current must go through each group since all groups are connected in series, and while all is in good order this current will divide properly between the different lamps. If, however, one lamp, say c, burns out and thus interrupts the current going through it, the other lamps, c' and c', will have to take more than their share and are likely to burn out with the same attendant dangers as with the multiple-series system.

Gas-pipes are always connected with the ground and if series lamps are attached to the fixtures, the high

potential causes great danger of leakage to the pipes, however carefully the insulation is looked after.

The use of incandescent lamps on series circuits is becoming infrequent on account of the dangers and the inconvenience. The system is not suitable to the high degree of subdivision that is the great advantage of the incandescent lamp, for the greater precaution necessary to make the series circuit safe, makes the devices clumsy and rigid, and the portability that adds so much to the usefulness of the incandescent lamp on multiple systems, is thus not possible.

CLASS C.

LOW-POTENTIAL SYSTEMS. -300 VOLTS OR LESS.

Outside Conductors.

15. Outside Overhead Conductors:

a. Must be erected in accordance with the rules for high-potential conductors.

b. Must be separated not less than twelve inches, and be provided with an approved fusible cut-out that will cut off the entire current as near as possible to the entrance to the building and inside the walls.

[Section b. An approved fusible cut-out must comply with the sections of Rules 23 and 24 describing fuses and cut-outs. The cut-out required by this section must be placed so as to protect the switch required by Rule 17.]

The conditions to which outside wires are exposed are such that no difference can be made between the construction used for high-potential circuits and that used for low-potential circuits. Though there is less liability of leakage where the electrical pressure is low, there are still all the bad effects when leakage does occur and it is not advisable to attempt less rigid requirements. Sections under Rule 10 accordingly apply also to low-potential circuits.

The fusible cut-out required just inside the walls is similar to that required for motors, shown in connection with Rule 8 (a). As with motor-circuits, the cut-out is to be double-pole, and it is to be placed as near as possible to the point where the wires enter the building, so that the whole of the interior wiring may be protected. If it were some distance away from the entrance, cross-connections between the wires at any point lying between the cut-out and the wall would not melt the fuse in the cut-out, because the excessive current would not pass through the fuse.

16. Underground Conductors:

a. Must be protected against moisture and mechanical injury, and be removed at least two feet from combustible material when brought into a building, but not connected with the interior conductors.

b. Must have a switch and a cut-out for each wire

between the underground conductors and the interior wiring when the two parts of the wiring are connected.

These switches and fuses must be placed as near as possible to the end of the underground conduit and connected therewith by specially insulated conductors, kept apart not less than two and one-half inches.

[Section b. The cut-out required by this section must be placed so as to protect the switch.]

c. Must not be so arranged as to shunt the current through a building around any catch-box.

Underground conductors are usually brought into the basement of a building, where it is likely to be damp and where often there is not the best of order, so that it is necessary to take precautions against moisture, and against the abrasion of the wires that would be caused by piling objects round them. There is apt to be greater carelessness when the wires are brought in simply for future use, so there is the additional requirement in this case that they be two feet from combustible material, as this might be set on fire were there to be leakage between the wires.

When the underground conductors are connected with interior wires a cut-out is needed for the reason stated under Rule 15 (6). The switch makes it pos

sible to disconnect the interior wires from the main wires whenever necessary. The cut-out in order to protect the switch must be between the switch and

the wall, so that if the switch become defective and allow an excessive current to leak between the wires, this current will pass through the fuse and melt it.

A "catch-box" is a box containing a fuse and is in reality only another name for a fuse-box. The term is applied usually only to the fuse receptables in the underground street service. In making the street connections the wires are sometimes improperly connected so that they are not protected by the fuse in the catch-box, but form a by-path, or "shunt," round it through the building.

In Figure 47, for instance, the wiring in the building may first have been connected only with the street mains A, no connection being made with the mains D. If the mains A were supplying all the current and trouble were to occur so that the mains would become connected at B, the excessive current that would flow in consequence of the short circuit would melt the fuse in the catch-box C. Suppose, however, that more lamps have been put in the building and that the current required is greater than the mains A were designed for. In this case there is sometimes another connection made at a point D, so that all the current supplying the building will not have to be taken through the mains A. Now, if a short circuit occur at B, the fuse in the catch-box, C, even though it melt, will not prevent the wires from overheating,