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from the regulations enforced by the better class of electric companies, other rules have been suggested and formulated by some of the best-known engineers, and the underwriters now have a corps of electricians trained to judge of construction with regard to safety from fire, and competent to impose restrictions as new conditions are presented. If the rules were unreasonable, the electric-light companies would be the first to protest; but, on the contrary, the National ElectricLight Association has adopted virtually the same rules, and electrical engineers in general accept the insurance code as the best existing guide for safe construction. The complaint that comes from the better class of electrical men is not that the rules are too exacting, but that they are not always rigidly and consistently enforced.
Architects can, of course, do much to insure good electrical construction in their buildings, but they have usually paid little attention to this feature. It is very common to find in the architect's specifications a clause requiring "all work to be done in strict compliance with the underwriters' rules," and then to find, following this, a number of clauses specifying construction that is explicitly prohibited by the rules. With this evidence of ignorance before them, it is not surprising that contractors often feel free to do the work as they please.
The rules are necessarily short, and naturally there are many technical terms used. It has seemed that it would be helpful to have these terms defined, and to have the reasons for the rules briefly stated. Accordingly, the latest rules of the National Board of Fire Underwriters, as amended at New York, January, 1895, are taken up here. The rules, which are printed in small type, must be read continuously to get the full meaning, since the remarks are interpolated. The advantages in taking up a few rules at a time seemed greater than the disadvantages in having sometimes to refer to the last sub-heading in order to find the subject.
CENTRAL STATIONS. — FOR LIGHT OR POWER.
These roles also apply to dynamo rooms in isolated plants, connected with or detached from buildings used for other purposes; also to all varieties of apparatus therein, of both high and low potential.
a. Must be located in a dry place.
b. Must be insulated on floors or base frames, which must be kept filled, to prevent absorption of moisture, and also kept clean and dry.
c. Must never be placed in a room where any hazardous process is carried on, nor in places where they would be exposed to inflammable gases, or flyings of combustible material.
d. Must each be provided with a waterproof covering.
A "generator" receives mechanical energy at the pulley, and gives out electrical energy; a "motor" receives electrical energy and gives out mechanical energy at the pulley. Strictly speaking, both are "dynamos," or dynamo electric machines, but the term "dynamo" is commonly used as synonymous with generator.
"Potential" is used in the rules as synonymous with electrical pressure; voltage; electro-motive force: the
virtue by which an electric current is forced through a wire or other substance. "High" and "low" potential are, of course, relative terms, but in these rules a high potential is that measuring over 300 volts, and a low potential is that measuring 300 volts or less.
The "base frames," on which dynamos are usually set, are simply heavy frames of wood, filled, or impregnated, with oil or other insulating substance that will prevent moisture from penetrating the pores of the wood. If moisture were to penetrate, it would make a semi-conductor of the wood through which currents of electricity might leak. The frames on which the smaller machines are set are usually made in two parts, the top part being adjustable so that the belt may be tightened by moving the machine.
Almost all damp substances are fairly good conductors, and even damp and dirty surfaces will allow considerable leakage. Precautions have to be taken against the beginning of these small leaks. They rapidly become worse, for they are accompanied by heating and sparking which carbonizes surrounding substances, making it possible for still larger currents to flow. If the wires of a dynamo become damp, there is apt to be leakage from one wire to another across the damp insulation, and once started, there will soon be a path of very low resistance, or a "short circuit," that will result in a current heavy enough to melt metal and destroy the usefulness of the machine. Trouble comes in another way, if the dynamo be damp and not well insulated from the ground: though the circuit is supposed to be insulated from the ironwork of the machine, there is liability of a connection, and if the wooden frame under the dynamo be damp and partly conducting, the circuit will be more or less well connected with the earth through the ironwork, wood, masonry, etc.
Suppose, in Figure 10, there is a connection to the iron of the dynamo near B, and a path thence through wet wood to a bolt, and then to the earth at E. If this be all, no current will flow, for there is no path up from the earth to the other wire of the circuit. Suppose, however, that somewhere in a building there is a gas-pipe in electrical connection with the other wire of the circuit, as at A. The gas-pipe runs, of course, to the earth, and the current then has a
path through pipe, earth, bolt, wet wood, and the iron of the dynamo, between the two sides of the circuit, A and B There is thus the full electrical pressure of the dynamo to force the current through this path, and a hole may be burned in the gas-pipe, with easily imagined result.
A dynamo often emits small sparks, and when working badly throws out bits of molten metal, so that it