Notes on Electric Lighting

Progress, Volume XII, Issue 2, 1 October 1916, Page 747

Notes on Electric Lighting

From a paper given at the monthly meeting of the Otago District Branch on 25rd. Aug., 1916, by F. R. Sheppard, M.A.I.E.E.

It may be said that one of the essentials of modern civilization is artificial lighting, and, as it is used to-day, it is a serious menace to the eyesight of the community. There can be no doubt that the lighting in many of our public places and oft times in our home is dangerous to health on account of its ruinous effect upon the eyes. Since the days of the candle the source-brightness of our illuminants has steadily increased and where the brilliancy of the candle was equal to three or four candle poAver per square inch, Ave now have the Tungsten lamp with a brilliancy of from one thousand to fifteen hundred candle power per square inch. This brilliancy must have a serious effect on the eye which is perhaps the most sensitive organ in the human body, and is undoubtedly responsible for a large percentage of head-aches which in many cases lead to melancholia, neurasthenia and other nervous disorders.

a Dr. Ellice Alger, a noted opthalmologist states, The general engineering expression seems to be that the room which is best lighted is most lighted. This is a great mistake. Too intense light decomposes the visual purple in the retina faster than it can be replaced, and leaves a condition of retinal exhaustion. Likewise it compels a constant, extreme muscular contraction of the pupil in the effort to exclude the light, which is both fatiguing and painful Most of our buildings are glaring examples of extravagant and visual, inefficient lighting, extravagant because of the waste of light, and inefficient because they are not even comfortable to sit in.'' The problem of good lighting is to provide an illummant that will enable the eye to discharge its functions Avith the greatest possible degree of efficiency without unnecessary strain and the question of cost which is often the only consideration is really of minor importance. The light Avhich best promotes and preserves vision with the least possible strain on the eye and nerves, is cheap at any price. J We cannot overestimate the value of good lighting. In factories and Avorkshops, workers cannot produce the full quantity or quality of Avork that they are capable of producing, if the light is bad

and the surroundings depressing. Many foremen tell us that good lighting means a gain of from half to one hour or more daily and of course the same principle is applicable to offices and dwellings.

The difference between the cost of bad and good lighting is a sound investment and in every case the best is really the cheapest. In the ease of retail shops and show rooms, the value of good lighting is more evident. Intending customers pass a shop that is harshly or insufficiently lighted whereas they are attracted to a shop that is well lighted; they can view the goods displayed with comfort and the articles on view are seen to the best advantage.

A short time ago a Committee was appointed by the Home Office to enquire and report as to the conditions necessary for the adequate and suitable lighting of factories and workshops. The importance of the subject scarcely needs emphasising and yet it is quite clear that there are many factories whose owners do not realise that it is worth while to have anything more than an inferior standard of illumination. In the evidence given before the Committee a case was cited in which the output was diminished 12 to 20 per cent, during the hours of artificial light, and in another factory the earnings of the workers increased 11.4 per cent, after the installation of a better system. There can really be no question that, from the commercial point of view, good illumination in factories is worth paying for.

We often hear it stated that electric light is bad for the eyes. It is so when electric light is misused, but that is equally true of gas, acetylene or the kerosene lamp. It is injurious to look at the sun, but no one would suggest that daylight is injurious. The same principle applies to an artificial light, viz., that the rays from the light source should not be permitted to strike the eye direct. No hard and fast rale can be laid down to govern all lighting installations, and the lights with their globes or shades must be selected, and the arrangement for fixing so designed as to best suit the particular conditions that have to be met. Because a lamp is glaring and brilliant it does not follow that the lighting is good. A well shaded table lamp may look dim because it is well shaded, but may be giving a first class light for working purposes. In ordinary cases general illumination is the best way of lighting an interior unless some of the work carried out, as sewing dark goods or reading fine print, demands specially strong lighting in some parts of the room. In this case local lights may be added, but should not be used without good general illumination because of the strong contrast between the bright spots and dark shadows. The commonest

sort of localised lighting is that furnished by a table lamp. Such a lamp should always be shaded to keep the direct light out of the eyes, and may be best accomplished by the use of a translucent shade which will add something to the general illumination. 'i .' '•''. ;' j

There are four methods whereby electric light may be disturbed, viz. Concealed, Indirect, Semiindirect and Direct. Concealed illumination is ♦

where the light source is entirely hidden from view. This system is used where it is desired to throw the light directly upon the object to be illuminated and is successfully used in shop windows and show cases.

Indirect illumination is where the light source is hidden from view and the light itself thrown on to the ceiling and walls, and reflected by them throughout the room. This system is successfully used in theatres, large halls and sometimes in the draughting room. It must be remembered however, that the reflecting surfaces should be of light colour since dark colours are poor reflectors. This system is the least efficient.

Semi-indirect illumination is where the light source is contained in a glass bowl, and diffused through the glass. This system, as its name implies, is a compromise between the indirect and direct, and though not so economical as the direct, lends itself to decorative work as well as usefulness.

Direct illumination is that form in most common use, that is, the lamp is in sight and covered more or less by a shade for diffusing purposes. There are very few situations where direct lighting should not be employed, but much depends on the type of shade used to secure efficiency. With the advent of the Tungsten lamp, and the brilliant filament of the "half-watt" lamp, it became necessary to give more attention to the type of shade in order to protect the eye from the glare of the light source. As an example of the application of the above methods we will consider the case of a jewellery shop where the requirements for correct illumination arc of a somewhat complex character. A very pleasing effect can be obtained by reflection from the bright surfaces of the ware and polished stones. The general illumination should be by diffused light and is best obtained by the Semi-indirect method. A row of suitable sized lamps enclosed in globes or bowls of opal or prismatic glass, may be installed down the centre of the shop. The number required depends, of course, on the size of the shop. Drop pendants, with holophane shades, should be used over the counter show cases which will give the desired effect in displaying cut stones and glassware. In the side show eases concealed lighting should be used and may be effected by the use of "linolites," or lamps carefully concealed behind metal reflectors. There are many establishments, such as drapery, hosiery, jewellery, bookstores, large dining halls etc. where excellent results may be obtained by the use of Semi-indirect lighting for general illumination, and although the initial cost of the fittings may appear rather high, the expenditure is fully justified.

In the selection of fittings, care should be exercised that they are well suited for requirements, not too conspicuous, and in harmony with their surroundings. Too often they are selected on account of their decorative design, and no thought is given as, to their effect on the lighting. They should hold the lamp or lamps at the correct distance from floor, walls and ceiling, and at a proper angle for the best distribution of the light. The shade or globe is a very important part of the

installation, as on it depends the question of efficiency. "We have frequently seen shades with deep red or green tints selected because "they match the carpet." This method of selecting is quite wrong because many designs in plain or etched glass are quite as effective. Glass will either transmit, reflect, or absorb light. There is necessarily more or less absorption in all shades, but the ideal shade should be a combination of transmission or diffusion, and reflection. The four typical forms of glass used in making globes and shades are clear glass, frosted glass, opal or milk glass and prismatic glass. Clear glass simply lets the light through and does not hide the light source in any way. It is used mainly as an outer globe to protect the lamp, generally in an exposed position or in situations where steam is present. Frosted glass diffuses, but absorbs a great deal of light. It also quickly accumulates dust and dirt on its roughed surface, which further obstructs the passage of the light. Opal glass has a high degree of diffusion and reflection, but also so great an absorption as to be wasteful. Prismatic glass is one of the best reflectors known, and some very handsome globes, bowls and reflectors are now on the market. They have a defect, however, inasmuch as they require frequent cleaning to remove the accumulation of dirt and dust due to their construction.

Economy can be effected by the use of reflectors suitable for a particular purpose. Certain types are designed which may be described as concentrating, semi-concentrating, and distributing. The first acts almost like an automobile headlight, throwing its light downward over a comparatively small area. The second spreads the light over a much wider area, of diameter perhaps as great as the height of the lamp above- the table, while the third is designed to distribute the light evenly over a comparatively large area. In determining the number and size of lamps to be used, many factors have to be taken into consideration, such as nature of work, colour of surroundings, obstructions by girders, cranes, columns, stacks of materials, belts and shaftings, and height of ceilings etc. There is a certain relation between the mounting height depending partly on the reduction of eye fatigue by the use of small lamps for low ceilings, and partly on the shadow effect which varies with the ceiling height. For low ceilings therefore, the lamps must be fairly close together to avoid unsatisfactory shadow effects, and should be of small candle power. For higher ceilings the spacing distance may be greater for the given shadow effect and the lamps correspondingly larger.

No hard and fast rule can be made in regard to the size and height of lamps, but the following may be taken as an approximate rule under average conditions—

Mounting height, 7-12 feet, 60 watts; 12-16 feet, 100 watts; 16-28 feet, 250 watts; 28-40 feet, 500 watts.

A considerable percentage of light is absorbed by the surfaces with which we are commonly surrounded, while the balance is reflected. In all cases far more light is reflected from a light coloured

wall than a dark one, and the average amount of light reflected from differently coloured papers may be taken as follows—

White paper, 80% ; Orange paper, 50% ; Yellow paper, 40%; Light Pink paper, 35%; Light Blue paper, 25%; Emerald Green paper, 18%; Dark Brown paper, 10%.

In the matter of preparing a specification for an electric light installation, a great deal more is required to be given than a schedule of lights with, perhaps, a clause stating that the work is to be carried out in accordance with Fire Underwriters' rules. Certainly the installation should be carried out in accordance with these rules in order to reduce the fire hazard, but there are other factors that demand consideration. The rules are by no means a specification, and do not deal with the matter from an . engineering, or an illuminating point of view, and are intended to cover, in a general way, the different systems which may be adopted, and for any voltage up to 250. In fact, clause lof the rules states "They (the rules) are intended to include "only such requirements and precautions as are "generally necessary, but they are intended neither "to take the place of a detailed specification, nor to "instruct untrained persons." In a modern installation the usual method of protecting conductors is to enclose them in steel

conduit. In earlier days, and before the advent of conduit, wood casing or open wiring were the usual systems; but they quickly gave place to conduit on account of their many disadvantages. To-day, open wiring is not permitted except under special circumstances and by permission of the Fire Underwriters. Casing is still permitted but limits are placed on it. For example, it may not be buried in plaster, or placed under floors, and under no circumstances may it be used in damp situations. This is on account of the fire hazard which may quickly arise due to moisture getting into the casing.

Conduit, on the other hand, may be used in practically any situation, is safe from the fire hazard, and has a long life.

In specifving conduit it should be stated that the conduit and accessories are to be in accordance with the British Standard Specification for "Steel Conduits for Electrical Wiring." Conduit made to this specification are of steel, close-joint brazed, welded or solid drawn, and free from burrs or flaws. They and their accessories are stove-enamelled or galvanized inside and out and are screwed with a certain number of threads to the inch. This "prevents gas or other type of fittings being used in conjunction with it. Conduit should be installed as a complete system first and the conductors drawn through after. This is important because it ensures continuity of the conduit and enables- a wire to be withdrawn at a later period if necessary. Too often do we find a conduit filled with as many wires as it can hold, with no possible chance of withdrawing in the event of one of the wires breaking down. Tn such a case, if the conduit is concealed in the plaster it means either cutting into the wall to get at the conduit, or fixing a length of conduit outside

and rewiring that section, when either alternative might be avoided by the simple method of "drawing in." The advantage of this system is exemplified in the case of a new building where the wiring has to be concealed. The free ends of the wires at all outlet points are subject to the tender mercies of the plasterer who is, oft times, not too gentle, resulting in damaged insulation at the edge of the conduit. Again, there is the action of moisture on the insulation, which cannot be avoided, also the serious effect due to the action of lime on rubber.

A good practice, and one well worth specifying, is for short lengths of conduit, say six inches, to be screwed in at all outlet points, and not to be removed until plastering is completed. This will materially assist in keeping the conduits clean. Many "breakdowns" behind switch blocks may be traced to rough handling of the wires during the'proeess of plastering. For ordinary installation work the conductors are rubber insulated, covered with tape and braiding. The Underwriters Rules specify that the insulating material must be of vulcanised rubber of the best quality, impervious to moisture, and only needing mechanical protection. The radial' thickness of the vulcanised rubber is specified, according to the size of the. wire, and certain tests are also given which the conductor must be able to stand. Now these tests cannot be made on the installation, but must be carried out in a properly equipped laboratory, or test room in connection with a supply station if the necessary instruments and measuring meters are available. The ordinary testing instruments carried on to a job by the Inspector is of no use whatever to indicate the class of material used for insulation, and will only record the amount of leakage between any two conductors or between a conductor and the earth.' This test is, of course, a useful and very necessary one, but the point we wish to make is that it gives no indication of what effect age will have on the rubber, or, technically speaking the dielectric.

Not long ago, a sample coil of wire was taken by the writer for testing purposes. On appearances it was of good quality, and stood up well under a stretching and burning test, but under a voltage test we could only obtain an insulation resistance of 5.39 megohms per mile instead of 2000 megohms, and under the "breakdown" test it failed miserably. One of the advantages claimed for the use of electricity is its flexibility and this becomes apparent when switches are used in a judicious manner. The initial cost of installing additional switches is repaid in a short time due to saving of current in lamps not immediately required, with a consequent reduction in the monthly account. Where several lamps are installed in a workroom or factory, the switches should be so arranged that any section or group may be cut off if no work is going on in that particular part. Even in a sitting room, with a three-light pendant, two switches are economical because there are many hours in the course of the year that one light will give sufficient illumination. The location of the switches should also be carefully considered, especially where a number of ' lights are in use. There is a tendency on the part of the contractor

to group all switches in a given spot because the wiring is simplified, but this often leads to inconvenience and loss of time. In the case of a factory, switches for general illumination, should be located in the Foreman's office, and any special lights for machines controlled locally. There should be one switch at the door and if there be a door at each end of the room in general use, two-way switches will probably be an advantage. At present twoway switches do not appear to be used much outside of halls and bedrooms in private dwellings but there are many situations where they would prove very convenient.

In regard to the fire hazard, we are of the opinion that, properly installed, electricity is the safest illuminant. Under other conditions it may be a most serious fire hazard and one of a very subtle nature. In the case of gas, a leak is often discovered through the sense of smell; but as we have no sense which will locate a leakage of current, our first indication of the danger may be a serious outbreak cf fire. Certainly every conduit is, or at least should be, protected with a pair of fuses which are designed to "blow" when the current passing exceeds a predetermined amount, but it must be remembered that it is quite possible for a fire to be started without blowing the fuses, although they be of the regulation size. Generally speaking, the most common causes of trouble are faulty materials, bad workmanship, and moisture. Faulty material would include badly designed apparatus such as motors and their starters, generators, resistances, fuses, switches, lamp-holders, fittings of all descriptions, conduits and shoddy wires. Bad workmanshin may be caused by carelessness, incompetence or deliberate neglect.

Dealing first with materials we would say that, generally speaking those at present on the market are good in quality, design, and finish, but in these days of keen competition, in the case of any commercial product, there is always thedaneer nf unscrupulous manufacturers trying to swell their profits or to undersell their rivals bv producing qoods of inferior quality and finish. The defects, however, are usually apparent on inspection, and we have a chance of rejecting it before trouble can ensue. Our greatest source of worry perhaps, among materials, is the shoddy wire, and that at times is difficult to detect.

Under the heading of bad workmanship we have included the worker who is careless, incompetent, or negligent. The careless worker is slovenly and untidy in his work, tries to solder a joint with a half cold iron or forgets to use rubber tape when insulating a joint, and, as a rule, finishes a job with the expression "That's good enough." Among the incompetent we find bell-hangers, school boys, and the man who wants to add another light to his installation after the contractors have finished. Of the negligent-, we can only say that as a man who deliberately sets fire to a building is charged with arson when caught, so should the negligent in electrical work be dealt with as a criminal. Moisture or dampness must always be. avoided in electrical work and special precautions taken where such situations occur.