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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.
6. 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 possible 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 O. 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,
because the current can come from D through the wiring of the building to B. There would, of course,
be fuses at the service-entrances of the building, but it is bad practice to depend upon building fuses to prevent street damage; for it allows excessive currents to flow unnecessarily through the interior wiring. With such an arrangement as that shown in Figure
Fia. 47.—Current shunted through a Building round a Catch-box.
47, either all the lamps should be run from one set of mains, or else the wiring in the building should be divided into two distinct parts and one fed from each set of mains.
INSIDE WIKING. — GENERAL KULES.
17. At the entrance of every building there shall be an approved switch placed in the service conductors by which the current may be entirely cut off.
[The switch required by this rule to be approved must be double-pole, must plainly indicate whether the current is "on" or "off," and must comply with Sections a, c, d, and e of Rule 26 relating to switches.]
This "service-switch " is placed at the point where the wires enter the building, for the same reasons that such a switch is used with high-potential circuits. (Page 90.) It is usually, however, placed just inside the building instead of outside. It is to be a double-pole switch, that is, one that will break both sides of the circuit as does that used in connection with the motor. (Fig. 24.) With such a switch, current may at any time be cut out of the building entirely, and if in mercantile buildings a practice can be made of turning it "off" every night, there is no question of electrical troubles at a time when no one is about. It is convenient too, when trouble occurs in any part of the installation, for it is not necessary then to wait for the arrival of some one having a knowledge of electrical matters, but the current may be immediately cut off from the interior wires. The base of the switch must be of waterproof and non-combustible material, such as slate or porcelain, to prevent leakage, and damage from arcing. The form may be like that shown in Figure 24 or it may be a double-pole switch similar to that shown in Figure 36.
a. Must have an approved insulating covering, and must not be of sizes smaller than No. 14 B. & S., No. 16 B. W. G., or No. 4 E. S. G., except that in conduit installed under