Зображення сторінки
PDF
ePub
[graphic]

RESIDENCE OF ERNEST RECKITT AT EVANSTON, ILLINOIS Ernest Mayo, Architect, Chicago

UNDERWRITERS' REQUIRE

MENTS

PART II

INSTALLATION OF WIRES IN BUILDINGS

The proper choice of wires and their safe installation constitutes the most important part of all electric equipment from the viewpoint of the fire hazard. It has already been stated that in all electric work, conductors, however well insulated, should always be treated as bare, and from one point of view, it may be said that the value of conductors as regards safety lies in their insulation rather than in the copper, for if we assume that a wire of adequate carrying capacity is chosen for a given purpose, there remains only the choice of a suitable covering or insulation on the wire and a reliable and workmanlike method of placing it.

No one material has yet been produced which has every desirable property as a covering and insulation of electric wires and cables. Among the desirable properties of a wire covering are elasticity, flexibility, waterproofness, good insulating quality and resistance to voltage strains, resistance to effects of changing temperature, acids, vapors, etc., and permanence. All of these prop

erties are possessed by rubber in greater or less degree and, all told, to a greater degree than any other material. The properties of rubber-covered wires will be treated at greater length in another place, the foregoing statement being made here to emphasize the reason why rubber-covered wires are used almost exclusively in all inside wiring.

"Slow burning" wire is a copper conductor covered with three closely woven cotton braids saturated with a fire-resisting compound. Its use is limited to places where rubber is liable to be rapidly injured by high temperatures. Its insulating value is slight and it is not capable of resisting moisture.

"Weatherproof" wire consists of a copper conductor covered with three braids saturated with a moisture-proof compound. Its insulating value when new is low (much less than rubber) and it is very inflammable. Its use is practically confined to outdoors.

In the following sections it may be assumed that all references to wire mean "rubber-covered" wire.

Classification and General Principles. There are two classes of wiring which may be named for convenience: enclosed wiring and non-enclosed wiring.

Wires run on insulators such as cleats and knobs exposed on walls and ceilings or on knobs and through tubes concealed in floors and walls are the chief types of non-enclosed wiring. It will be observed that the distinction consists in the presence or absence of special wire-ways or channels for the wires. Wires must not be laid in plaster, cement, or similar finish, because such materials may contain either alkalies or acids which will injure the insulation and corrode the copper. Wires must never under any circumstances be fastened with staples because of the probable injury to the wire coverings, the insecure fastening obtained, and the possibility of such staples affording a path between wires in case twin conductors are used.

Twin wires must never be used except in conduit or where flexible conductors are necessary. The nearness of the two wires, on the opposite sides of the circuit, renders twin wire of any description somewhat more liable to failure and an injury to one wire generally involves an injury to both with resultant certainty of a short-circuit. The added safety of keeping the wires of a circuit separated is lost in twin wires.

In any scheme of wiring it is essential that all electric wires be installed so that they cannot come into contact at any point with any materials other than those expressly intended for their enclosure or support. This means that they should be kept absolutely free from contact with gas, water, or other piping, and from all metal work of any description unless it be that of piping, boxing, or molding provided as wire enclosures. Wires should also be installed so as not to touch woodwork or other combustible material, even if such material is not a good conductor. This general principle finds an exception in the case of wood molding.

Contact of wires with metallic substances may, in case of a failure of the insulation, permit dangerous arcs, short-circuits, or grounds, while contact with wood or combustible material is objectionable on account of setting fire to it in case of overheated wires or from leakage due to the presence of moisture on materials which when dry would be good insulators. These are, therefore, the general principles of wiring under the established rules and will be illustrated in the discussion of the various classes of wiring which follow.

Open Work in Dry Places. rubber-covered, slow-burning, or

Wires in open work may be either

special and now little used wire

having a weatherproof braid covered by a slow-burning braid-but as a matter of fact only rubber-covered wire is used to any extent

[graphic]

Fig. 84. Large Feeder Wires Exposed on Insulators

and it is much to be preferred except in exceptionally hot places as over steam boilers, where rubber insulation will deteriorate very rapidly. The rubber-covered wire used for open work has a single braid over the rubber. The chief advantages of open work are its cheapness and its accessibility. The latter may be of great advantage in cases where frequent changes and additions are likely to be required or where renewals of wire are frequent because of peculiarly unfavorable conditions such as exist in packing houses,

Open work finds its chief use in mills and factories and for large conductors which it is especially difficult and expensive to enclose in

Fig. 85. Porcelain Two-Wire Cleat

conduit. Fig. 84 shows an example of a set of large feeders run exposed on insulators. It should be noted that such a large group of heavy cables covered with the inflammable braid and rubber insulation fur

nishes a very considerable amount of fuel for fire and the necessity for excellent spacing and reliable fastening is obvious. The heavy porcelain blocks carried in metal frames as shown in the illustration are of an approved type. In all open work, wires or cables must be rigidly supported on non-combustible, non-absorptive insulators. Formerly wood cleats were used but these are now obsolete and have been replaced by porcelain. Where the voltage is less than 300 volts, wires must be separated from each other at least 2 inches and from the surface wired over at least inch in dry places in damp places at least 1 inch. For voltages from 301 to 550 volts, the limit for "low-potential systems," the wires must be kept 4 inches apart and 1 inch from the surface wired over.

Split knobs or cleats must always be used for the support of conductors which are smaller than No. 8 B. & S. gauge except when they are found at the end of runs.

Fig. 85

Fig. 86. Porcelain One-Wire Cleat

Fig. 87. Porcelain One-Wire Cleat

shows a typical form of a two-wire cleat and Figs. 86 and 87 show good

« НазадПродовжити »