This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The following rules and regulations are drawn up by a committee of the Society of Telegraph Engineers and Electricians for the reduction to a minimum, in the case of electric lighting, of those risks of fire which are inherent in every system of artificial illumination, and also for the guidance and instruction of those who have, or who contemplate having, electric lighting apparatus installed in their premises. The difficulties that beset the electrical engineer are chiefly internal and invisible, and they can only be effectually guarded against by " testing," or proving with electric currents. They depend chiefly on leakage, undue resistance in the conductor, and bad joints, which lead to waste of energy and the dangerous production of heat. These defects can only be detected by measuring, by means of special apparatus, the currents that are, either ordinarily or for the purpose of testing, passed through the circuit. Should wires become perceptibly warmed by the ordinary current, it is an indication that they are too small for the work they have to do, and that they should be replaced by larger wires.
Bare or exposed conductors should always be within visual inspection, and as far out of reach as possible, since the accidental falling on to, or the thoughtless placing of other conducting bodies upon, such conductors, would lead to " short circuiting," and the consequent sudden generation of heat due to an increased current in conductors not adapted to carry it with safety.
The necessity cannot be too strongly urged for guarding against the presence of moisture and the use of " earth " as part of the circuit. Moisture leads to loss of current and to the destruction of the conductor by electrolytic corrosion, and the injudicious use of " earth " as a part of the circuit tends to magnify every other source of difficulty and danger. The chief dangers of every new application of electricity arise from ignorance and inexperience on the bart of those who supply and fit up the requisite plant. The greatest element of safety is therefore the employment of skilled and experienced electricians to supervise the work.
(1) The dynamo machine should he fixed in a dry place.
(2) It should not be exposed to dust or flyings.
(3) It should he kept perfectly clean and its bearings well oiled.
(4) The insulation of its coils and conductors should be practically perfect.
(5) All conductors in the dynamo room should he firmly supported, well insulated, conveniently arranged for inspection, and marked or numbered.
(6) Every switch or commutator used for turning the current on or off should be constructed so that when it is moved and left it cannot permit of a permanent are or of heating.
(7) Every part of the circuit should be so determined that the gauge of wire to be used is properly proportioned to the currents it will have to carry, and all junctions with a smaller conductor should be fitted with a suitable safety fuse or protector, so that no portion of the conductor should ever be allowed to attain a temperature exceeding 150° F. (65i° C.).
(8) Under ordinary circumstances, complete metallic circuits should be used; the employment of gas or water pipes as conductors for the purpose of completing the circuit should not in any case be allowed.
(9) Bare wires passing over the tops of houses should never be less than 7 ft. clear of any part of the roof, and all wires crossing thoroughfares should invariably be high enough to allow fire escapes to pass under them.
(11) The position of wires when underground should be clearly indicated, and they should be laid down so as to be easily inspected and repaired.
(18) When these wires pass through roofs, floors, walls, or partitions, or where they cross or are liable to touch metallic masses, like iron girders or pipes, they should be thoroughly protected by suitable additional covering; and where they are liable to abrasion from any cause, or to the depredations of rats or mice, they should be efficiently encased in some hard material.
(14) Where indoor wires are put out of sight, as beneath flooring, they should be thoroughly protected from mechanical injury, and their position should be indicated. N.B. - The value of frequently testing the apparatus and circuits cannot be too strongly urged. The escape of electricity cannot be detected by the sense of smell, as can gas, but it can be detected by apparatus far more certain and delicate. Leakage not only means waste, but in the presence of moisture it means destruction of the conductor and its insulating covering, by electric action.
(15) are lamps should always be guarded by proper lanterns to prevent danger from falling incandescent pieces of carbon, and from ascending sparks. Their globes should be protected with wire netting.
(16) The lanterns, and all parts which are to be handled, should be insulated from the circuit.
(18) To secure persons from danger inside buildings, it is essential so to arrange and protect the conductors and fittings, that no one can be exposed to the shocks of alternating currents of a mean electromotive force exceeding 100 volts, or to continuous currents of 200 volts.
(19) If the difference of potential within any house exceeds 200 volts, the house should be provided with a "switch," so arranged that the supply of electricity can be at once cut off.
With reference to par. (10), Bolas says that the best way to make an electrical joint is, first to thoroughly tin the wires, and then wipe them carefully while they are still hot; any chloride of zinc which may have been used being next removed by a damp cloth. The wires are then bound, and subsequently well grouted with solder, rosin only being used as a flux.
Killingworth Hedges, in a paper recently read before the British Association, alludes to some sources of danger not previously mentioned. Thus, in reference to the development of heat caused by an increased resistance, he recalls Matthiessen's experiment showing that the conducting power of "commercial" copper wire is only 13.6 as against 99.95 for pure copper : hence the wire used must be pure throughout. An absolute essential is a cut-out or fusible plug in the circuit, arranged to melt if the current is more than 10 to 15 per cent. in excess of the working strength.