Зображення сторінки
PDF
ePub

be the intensity of the beam. To multiply this value by the ratio area of beam

, gives for any possible values of the numerator and area of crater' 87 denominator a figure much greater than 10,000, the true candlepower of the arc. A more rational method would be the product of the light intensity of the beam in candle-feet, by its area.

The movements of the beam of light are produced independently of the arc mechanism, by hand or by distant motor control. In the latter instance the projector may be provided with a vertical wheel and chain actuated by a motor, which turns the barrel around a horizontal axis, as well as with a motor-driven revolving base, to swing the whole lamp in either direction. These motors may be operated by a distant controller, and the lamp suitably fitted with a two-joint receptable for the lighting cables and usually a five-point socket for the motor cables

CHAPTER XVI.

INTERIOR WIRING.

The laying of electrical wires does not appear to be as important from the engineering point of view as the construction of overhead and underground conductors; nevertheless, an additional and most important consideration is involved, this being the fire hazard. When electric lighting was first introduced this difficulty was so great, being naturally magnified by prejudice against the new method of illumination, that insurance and municipal fire department authorities were often strongly opposed to the introduction of electrical conductors into buildings. But improvements in methods of construction have gradually reduced the risk, until now insurance companies and fire departments consider electric lighting less dangerous than any other form of artificial illumination. This is undoubtedly a fact; but electrical wires are still the cause of many fires, the consequences of which are often very serious. Hence, it behooves those who are responsible for the installation of electrical apparatus and conductors inside of buildings, to exercise the greatest possible care. This is the more necessary, in view of the conditions under which electric light wiring must be installed to meet the varied requirements. In a large class of installations no small amount of judgment, ability, and ingenuity is often required to overcome the difficulties met with, to adapt the material at hand to the purposes, or to devise new methods to secure unusual results. Slaughter houses, dye houses, chemical works, bleacheries, and breweries offer many peculiar difficulties to proper wiring.

Before the actual interior wiring can be of use it must be connected with the service wires, and this necessitates in most cases that at some point the wires enter the building. In order that the moisture may not travel along the wires from outside to the interior installation, there is at the service entrance a drip loop outside ; and the hole through which the conductors must pass is bushed with a drip tube, which must slant up towards the inside. (See Fig. 312.) The wire entering these tubes should have solid

rubber insulation, at least 4 of an inch thick, and covered with a substantial braid. The space between the wires as they enter the building

should be at least one foot, and arFig. 312. Drip Tube and Loop.

ranged so that no cross' connection

can be made by water. The wires should never come in contact with anything but their insulators. Running them along the face of the building should be avoided, and they must be fastened to the wall near the entrance tubes by insulators mounted on special brackets having two coats of waterproof paint to prevent the absorption of moisture.

Automatic cut-outs such as circuit breakers or fuses should be placed on each of all service wires as near as possible to the point where they enter the building, on inside of the walls, and arranged to cut off the entire current from the building. The wires then run to the service switch, which should be capable of opening the circuit when carrying the entire current of the building. This is a knife switch, and should be installed so that the handle will be up when the circuit is closed, so that gravity will tend to open the switch, rather than accidentally to close it.

With alternating systems the best place for the transformer is on the pole away from the building. The transformer, when placed on the outside wall of the building, must be hung on wellinsulated supports, the construction being as shown in Fig. 313. Where it is impossible to exclude it from the building, the proper place is a vault or room with brick walls containing nothing else but transformers. As a last resort it may be put in a part of the cellar where it is well ventilated and dry, being carefully insulated from the walls and the ground.

The next piece of apparatus in the building is the switch board or in small installations the distributing panel board. This will carry the meters, the knife switches and the fuses for the feeders. If electric power is to be used besides the lighting the separation of the two kinds of circuits should be rnade at this point.

The principal methods which have been, or now are, used to carry the wires from the entrance devices to the lamps are as follows:

(1) Wires inclosed in molding. (2) Wires carried by wooden cleats. (Obsolete.) (3) Wires carried by porcelain cleats or knobs in open work. (4) Wires carried by porcelain knobs and tubes concealed. (5) Wires concealed in plaster. (Obsolete.) (6) Wires concealed in tubes, interior conduits.

(7) Wires laid on some cornice, wainscoting, or other architectural feature adapted to the purpose.

(8) Fished wires. (Not desirable.)

[merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

Three of these — i.e., the second, fifth, and eighth — are no longer considered good practice, in fact they are forbidden by the National Electric Code. In order, however, to fully appreciate the difficulties in this important branch of electrical engineering, it will be well to consider all of the above methods in the order given.

Wooden Molding. — The advantages of this construction are simplicity, cheapness, and accessibility. It is particularly applicable to buildings in which no provision has previously been made for electrical conductors, the wires being laid after the building is completed. At first this was the general condition, and a very large proportion of the wiring laid during the early history of electric lighting was installed in this way. At present the use of electric light is decided upon, or at least contemplated, before the

[graphic][graphic][ocr errors][subsumed][subsumed][ocr errors][merged small][merged small]

building is erected, and the plans provide specially for it. In such cases, particularly where expense is not a prime consideration, the use of the so-called “ interior conduit,” laid in the walls, is the standard practice for low tension (below 450 volts circuit). For high-tension wires, the only approved plan is to carry them upon porcelain knobs or cleats. These two methods will be considered later, in their proper order.

Wooden molding usually has the cross-section represented in Fig. 315, consisting of a strip or “backing,” in which are cut grooves corresponding to the number of wires to be laid, only one conductor being placed in each groove. The backing is fastened to the wall by thin wire nails or brads, being made continuous as far as possible. Angles and branches are formed by fitting pieces together, as indicated in Fig. 317. The wires are then laid in the grooves, being also preferably continuous, although joints are

« НазадПродовжити »