{Contributed by H. Y. Marcary)

Of the many regulations relating to electric lighting, those most immediately important are the rules laid down by the Fire Offices. Next in importance are those of the local Supply Company, or whoever is responsible for the supply of the current.

The Board of Trade rules, based upon the Electric Lighting Acts of 1882 and 1888, rarely affect the consumer or his adviser when the supply is obtained from the street mains: they apply principally to the "undertakers," - that is to say, the Supply Company- and when the rules of the Supply Company and of the Fire Offices have been adhered to it may be considered that there will be no difficulty with the Board of Trade. Board of Trade Regulations. - A copy of these, relating to the Electric Lighting Acts, 1882 and 1888, may be obtained through any bookseller for 2d. Their twofold aim is "for securing the safety of the public" and "for securing a proper and sufficient supply of electric energy." The expression pressure means the difference of electrical potential between any two conductors through which a supply of energy is given, or between any part of either conductor and the earth; and, subject to the variations allowed by No. B3 of these Regulations:-

"(a) Where the conditions of the supply are such that the pressure at any pair of consumer's terminals does not exceed 250 volts, the supply shall be deemed a low-pressure supply ;

"(b) Where the conditions of the supply are such that the pressure exceeds 250 volts but does not exceed 650 volts, the supply shall be deemed a medium pressure ;

"(c) Where the conditions of the supply are such that the pressure exceeds 650 volts but does not exceed 3000 volts, the supply shall be deemed a high-pressure supply; and

"(d) Where the conditions of the supply are such that the pressure exceeds 3000 volts, the supply shall be deemed an extra high-pressure supply."

With reference to the foregoing definitions of various different pressures the first or low-pressure is the one principally to be dealt with in ordinary lighting schemes.

The medium pressure (definition b) will enter into a scheme including power, as, for instance, in factories, theatres, hotels, and such buildings. The high pressures and extra high pressures (definitions c and d) will rarely have to be considered.

It will be sufficient to deal only with the first part of the aim of these rules, namely, "for securing the safety of the public."

It may be noted here that most theatres, hotels, and large buildings are now fitted with power for passenger lifts ; and it is the general practice to supply power at double the voltage of the lighting circuit. Further than this, the use of arc lamps burning in series on power circuits is a practice that rarely obtains beyond street-lighting schemes, which are not within the scope of the present work.

There is an important regulation relating to the three-wire system of distribution which is as follows :- "When the pressure between the outer conductors of a three-wire system exceeds 250 volts, and the three wires of the system or two pairs are brought into a consumer's premises, the supply shall be given to two pairs of terminals, arranged in such a manner that there shall be no danger of any shock, and the wiring from those terminals shall be kept distinct."

Another regulation is important, as indicating the kind of test to which the electrical contractor's work will be subjected:-

"Every low-pressure and medium-pressure main shall be tested for insulation after having been placed in position, and before it is used for the purposes of supply, the testing pressure being the maximum pressure to which it is intended to be subjected in use, and in any case at least 200 volts, and the undertakers shall duly record the results of the test of each main or section of a main."

Another regulation, though formidable at first sight, is considerably modified by the last clause, as practically every system of supply consists of the three-wire system in which the central conductor is earthed everywhere.

"The insulation of every complete circuit used for the supply of energy, including all machinery, apparatus, and devices forming part of, or in connection with, that circuit, shall be so maintained that the leakage current shall not, under any conditions, exceed one-thousandth part of the maximum supply current; and suitable means shall be provided for the indication and localisation of leakage. Every leakage shall be remedied without delay.

"Every such circuit shall be tested for insulation at least once in every week, and the undertakers shall duly record the results of the testings.

"Provided that where the Board of Trade have approved of any part of any electric circuit being connected with earth, the provisions of this regulation shall not apply to that circuit so long as the connection with earth exists."

"Where any electric line crosses, or is in proximity to, an)' metallic substance, special precautions shall be taken by the undertakers against the possibility of any electrical charging of the metallic substance from the line or from any metal conduit pipe, or casing enclosing the line."

Regulations 24 to 31 under the heading "Consumers Premises" are all important, and should be considered towards the end of the work of installing the work on consumers' premises.

"24. The undertakers shall be responsible for all electric lines, fittings, and apparatus belonging to them, or under their control, which may be upon a consumer's premises, being maintained in a safe condition, and in all respects fit for supplying energy.

"25. In delivering the energy to a consumer's terminals the undertakers shall exercise all due precautions so as to avoid risk of causing fire on the premises.

"26. A suitable safety fuse or other automatic circuit-breaker shall be inserted in each service line within a consumer's premises as close as possible to the point of entry, and contained within a suitable locked or sealed receptacle of fireproof construction, except in cases where the service line is protected by fuses in a street box; but no fuse or automatic circuit-breaker shall be inserted in the intermediate conductor of a three-wire system, where the pressure between the adjacent conductors exceeds 125 volts.

"27. All service lines and apparatus placed on a consumer's premises shall be highly insulated and thoroughly protected against injury to the insulation or access of moisture, and any metal forming part of the electric circuit shall not, unless efficiently connected with earth, be exposed so that it can be touched. All electric lines shall be so fixed and protected as to prevent the possibility of electrical discharge to any adjacent metallic substance."

Although these regulations refer principally to the duties of the "undertakers" who are responsible for the supply, it is as well that the consumer's adviser should be acquainted in some degree with their powers to demand alterations, while, at the same time, he may be able to forestall, by careful supervison, any little difficulties otherwise likely to arise.

When a proposed installation reaches a magnitude of, say, one hundred 16 candle-power lamps or the equivalent, say, from 7000 to 8000 watts per hour, it is necessary to consider whether the current should be purchased or generated on the premises.

The first point to consider is the probable consumption of current per annum. This will vary considerably with the purpose for which the premises are used; for instance, it is doubtful whether a mansion or dwelling-house would ever, under ordinary circumstances, be actually using more than one-half the 100 lamps at a time.

It would certainly not be advisable to assume that more than 50 lamps would be in use simultaneously. This would represent a consumption of, say, 3 1/2 units per hour, which is too small for profitable generation on the premises.

For business premises, factories, public institutions, or wherever the whole of the lamps are at work for 3 to 5 hours consecutively, the private plant would result in a considerable saving per annum.

As an instance, a draper's premises where the maximum output is 18 units per hour, the cost for current works out at 3/4d. per unit per hour after paying for gas, oil, waste, repairs, part of porter's time, and 10 per cent, depreciation on the plant.

Modern tendency indicates the private plant where-ever the average consumption reaches 8 or 10 units per hour for 350 hours per annum, but it sometimes occurs that the local Supply Company will make a special concession rather than lose a prospective consumer, and for this reason the authorities should be approached before definitely deciding on the private plant.

For larger outputs it would be better to install generating machinery, as it is unlikely that the supply authorities would be able to make any concession which would pay the consumer.

For generating up to 50 kilowatts (which in 1 hour would be 50 units), town gas may be used with fair economy; but beyond 50 kilowatts a gas production plant will result in the cheapest possible form of lighting.

When taking supply from the street mains it is sometimes possible to choose between "alternating" and "direct" or "continuous" current; and if the charge per unit be the same, it is very difficult to make a choice. If neither arc lamps nor motors are used, each supply is as good as the other; but at present and probably for some years to come motors will work more efficiently on direct or continuous current, while arc lamps specially designed for the purpose will be cheaper to run on the alternating supply.

There is one point, however, which should not be omitted: if either supply should be at a lower pressure it should be chosen, provided the use of motors or arcs do not decide the question.