With electric lighting, the overheating of ignitable material is the one danger to be apprehended. Such heating may result from various causes. The amount of heat generated by the transmission of electrical energy depends, in general terms, upon the resistance encountered in the conducting medium, the quantity of current flowing, and the rapidity of heat radiation. For this reason a poorly const racted joint in wire laid in wooden moulding may, by interposing a high resistance, develop a local heat sufficient to char the woodwork, which eventually bursts into flame; or, a circuit designed for the supply of 16-candle power lamps, may be called upon to carry the same number of lamps of 32-candle power, thus doubling the flow of current and causing an increased heating effect throughout the entire length of wire; or, a wire designed for carrying a certain current when strung on insulators and exposed to the air, may be placed in wooden moulding, at the whim of the tenant, who cannot understand why a larger wire is required for the latter method,because he does not stop to consider the decrease in the rate of heat radiation which results from the change. So, also, the presence of weak or defective insulation may cause a series of infinitesimal leaks from wire to wire, which, added together, assume finite proportions, and may result in adding to a circuit a burden entirely ignored in calculations of wire sizes.


To guard against this overheating the insertion of a “fuse” in the line is usually resorted to. This is a strip of metal of such material and cross-section as to insure its melting—and thus opening the circuit—upon the passage of a current sufficient barely to overheat the wire. The fuse, how ever, is no protection against the heat developed at bad joints or at points where leakage from one wire to another (or to some other conducting medium) is localized. It prevents an excessive flow merely, not a flow along unauthorized paths or over improper obstructions. And yet heavy copper or iron wire is not infrequently used to replace these fuses, thus defeating the object of the device; and, indeed, cases are on record where the brass-capped fuse plugs were filled solid with lead, in order, as the engineer expressed it, “ to do away with the only weak part of the system!”

When a fuse “ blows,” it invariably scatters molten metal, which may communicate a dangerous degree of heat to the material upon which it falls; and yet very many people fail to understand why the inspector requires that the fusing appliance shall be inclosed in a non-combustible case.

The only protection against poor joints lies in the prevention of them. The best protection against local leakage consists in the adoption of a good insulating covering for the wires and of such a method of construction as would permit of the operation of the circuits without leakage if the wires were entirely uncovered, in which case the insulation becomes an efficient factor of safety.—Hubert S. Wynkoop, in Cas sier’s Magazine for J annary.

No posts to display