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September, 1951

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At the present time we are in the midst of one of the periodic revolutionary stages in the field of mine lighting. Progress has been characterised, over the past 150 years, by a steady improvement in the design of mine lamps, and by a rather less steady advance in the application of them in the mine. From time to time, legislation has brought about the general adoption of the best types of lamp available by setting minimum standards of illuminating power for lamps in use, and by requiring the adoption of approved types. The present revolution has a twofold impetus. First, the Coal Mines Lighting Regulations of 1947, which is designed to raise the illuminating power of portable mine lamps and to maintain their efficiency, as well as making general lighting by means other than portable lamps more widely adopted, and secondly to the common acceptance within the industry itself, that good lighting is a necessary pre-requisite of efficient working methods.

The application of portable lamps

The probable future pattern to be taken in the lighting of mines was forecast in the Reid Report which suggested the use of mains lighting on roadways, and power-fed lamps, supplemented by portable lamps, preferably cap lamps, at the face. This may well be so, but there is no doubt that, no matter what developments take place in the field of general illumination of underground workings, it will always be necessary to have a secondary means of illumination for use in the event of breakdown of the mains supply in order to ensure that persons can find their way to the shafts at any time. This means, in effect, that each man employed underground must always have with him some form of portable lamp.

Since portable lamps are therefore necessary, good economics demands that they should give the best possible light during the whole of the time that they are underground, and that this light should be distributed in the most effective manner, at a minimum cost. It would be manifestly wasteful to provide mains lighting generally throughout the mine, and to ignore the fact that each man possesses a portable lamp of his own. Power-fed general lighting, and the light provided by portable lamps should be complementary in a mine-lighting system.

Levels of illumination in mines

Desirable levels of illumination in the various parts of the mine are given in Table I.*

There are many parts of the mine which general lighting can serve only with great difficulty. At the coal face, for example, restrictions exist as to height, positioning and spacing of fixed lamps, and there are often many obstructions such as machinery or debris, in addition to the roof supports, which are always present. There is also the necessity to illuminate different parts of the face at various times in the cycle of operations. Fixed lighting, in these circumstances, can perhaps serve best to produce a background illumination of relatively low intensity, its main object being to reduce the diversity of illumination in the field of vision of the person wearing a cap lamp, that is, to give added illumination at and towards the boundary of his field, and to render visible objects not illuminated by the direct beam from his lamp.

When considering standards of underground lighting we must take note of the Coal Mines Lighting Regulations of 1947, relative to the necessary provision of general lighting at key points underground. The statutory requirement is that the general lighting must be of such a character as to render possible the safe performance of all work at that locality without the aid of portable safety lamps. In view of the fact that all underground workers must possess such a lamp this requirement is illogical. It would be better to lay down statutory limits for an average level of general illumination at some arbitrary horizon, say at floor level, with limitations on the maximum permitted diversity of illumination over the area concerned, and then also to prescribe by regulation a minimum level of illumination on the tasks performed in that locality. This would involve, in many cases, combined methods of general or background lighting assisted by directional lamps positioned according to the requirements of the tasks involved. In the case of a worker whose task involves movement, it should be permissible to make use of his portable lamp in order to ensure that his task will always be illuminated to the requisite level, no matter what position he adopts relative to the mains lighting units.

Types of portable lamps

There is no doubt that the type of portable lamp which will be generally adopted in the future is the cap lamp, and the use of the hand lamp for general purposes is rapidly declining. There are, however, many who feel that the cap lamp does not quite, in its present form, serve so well as a hand lamp for certain types of work, and there may yet be a place for portable mine lamps of a type not worn about the person.

Virtually no improvement in the lighting performance of miners' hand lamps has been made during the past fifteen years, and practically all the new features associated with such lamps are concerned with their maintenance and charging in the lamp room. Self service now being widely adopted for cap lamps, similar arrangements have been made available for hand lamps. In some cases this has involved no change in the lamp itself, but in others, notably in one form of lead-acid lamp, a one piece construction has been adopted. This is the Oldham Automatic Hand Lamp, Type W.S.A.

The lamp case is made of moulded rubber, which permits of a total reduction in the weight of the lamp to 6 lb. 10 oz. As a result of this, it is claimed that the lamp gives 100 per cent more light per unit of weight than any other form of hand lamp. Owing to the one-piece construction, no dismantling is necessary to charge the accumulator. An internal switch, which is operated magnetically, is contained in a sealed plastic moulding, and it comes into action to connect the accumulator to the charging circuit through two external contacts on the lamp. This is achieved automatically when the lamp is placed on its rack.

The charging current is determined by the condition of the accumulator, and reduces to a very small trickle when the accumulator is fully charged. The lamps may therefore be left on charge indefinitely and thus can be conveniently applied as a standby source of illumination in the event of emergency. Removal of the lamp from the rack again brings the internal switch into operation, this time to connect the accumulator to the bulb, so that the lamp lights up. At the same time the external contacts, used for charging the accumulator, become isolated and dead.

The lamp is fitted either with a 4-volt 4-watt bulb of 44 lumens at the rated volts, to give 11 hours burning, or alternatively a 4-volt 3-watt bulb of 30 lumens output at the rated volts to give 14 hours burning. The lamp is scheduled for use in mines, under Schedule A.

Developments in cap-lamp design

Considerable progress in the design of cap lamps has been made in recent years, and to ensure that full advantage can be gained from the equipment which is now available, corresponding progress has been made in methods of lamproom servicing and maintenance.

The headpiece is required to give effective and efficient distribution of light and at the same time to offer the least possible inconvenience to the wearer. This means that its weight must be reduced to a minimum. Since the size and shape of the headpiece largely determine the distribution of light from the lamp, the resultant design is, to some extent, a compromise, popular favour being given to the smallest headpiece dimensions. While an appreciable increase in weight of the headpiece would be very unpopular, it is a matter for consideration whether somewhat larger headpieces are more to he desired than the smaller sizes now in favour.

Simplicity of design is important, the fewer the number of components, and the easier their assembly, the better will be the maintenance of the lamp. In this respect it is interesting to note the use of types fitted with one single filament bulb only, dispensing with double-filament bulbs or auxiliary bulbs. There is much to be said for taking the further step of also eliminating the switch, so as to have the simplest possible headpiece consisting of a single filament bulb positioned so that the filament is placed at the correct position relative to the reflector surface to give the desired distribution of light.

Bulbs. The bulb, being the source of light, is a most important component. The improved lighting performance of modern cap lamps is due, in no small measure, to the introduction of krypton-filled bulbs, which are of higher efficiency than the argon-filled type. Many of the cap lamps used today are fitted with krypton-filled bulbs of 200 hours rated life. This is the nominal life of the bulb when tested under laboratory conditions on alternating current. Under pit conditions, such a bulb should give 400 to 500 hours' life.

The substitution of krypton-filled bulbs for the argon-filled type, and the reduction in the nominal rating from 400 to 200 hours, in order to achieve higher bulb efficiency, has not been attended with complete satisfaction in all respects, and complaints about bulb failures are not infrequent. This matter is receiving the close attention of the manufacturers, and no doubt some improvement in the general performance of bulbs can be looked forward to in the near future. Failing this, it may be that a return to the longer nominal rating of 400 hours may be necessary to give some alleviation of the troubles which some users have experienced with the lamp bulbs now supplied.

Reflectors. It has generally been found desirable to use reflectors of parabolic design, with a single filament bulb positioned so that the filament is at the focus of the reflector. It is then possible to produce any desired variations in light distribution by adopting alternative types of polished surface on the reflector. Figs. 1 and 2 give an example of four such alternatives. For general application types A and B are required, since they give good distribution over a fairly wide angle. The higher beam intensities achieved by the use of polished reflector surfaces such as those of types C and D exist only in a comparatively narrow field, and are obtained at the expense of the illumination over the remainder of the field of view. These latter types are therefore only suited for special work, such as examination or inspection duties when the wearer must remain at some distance from the object being illuminated, or for use in situations such as exist when a dense cloud of smoke or dust must be penetrated by the beam.

There is some difference of opinion as to whether the character of a reflector should be described in terms of the ratio maximum to minimum illumination over the horizontal angle of distribution, or the ratio maximum to average illumination over the same angle. Consideration of Fig. 2 leads to the conclusion that either description is incomplete unless further information is given which would enable the shape of the distribution curve to be more closely described. This could, for example, take the form of a statement as to the ratio of maximum to average beam candle power over say a 20 degree angle in the centre of the field, in addition to the ratio of maximum to minimum illumination over the whole of the effective angle of distribution.

Cover. Synthetic transparent plastic materials now offer practicable alternatives to the use of glass for covering the front of the headpiece, and these materials possess the advantage of being very much less liable to fracture than is glass. When new they also have a higher light-transmission factor than glass, but this advantage will probably be lost after a very short time in use, unless the material can be surface hardened to resist abrasion. Nevertheless lamps with transparent plastic instead of glass on the headpiece are now in use and their performance will no doubt be observed with great interest by all concerned with mine lighting.

A notable example is the headpiece fitted to the Nife Type 113A lamp in which a combination bezel and lens of transparent "Perspex" is used. This unit, which is shown in Fig. 3, is virtually unbreakable, replaces three parts in the orthodox design of headpiece and at the same time increases the amount of light given by the lamp, and also gives a wider angle of light distribution. Light is thrown behind the headpiece, so that the wearer can see where he is walking. The headpiece is of lightweight construction and of simple design, to reduce the number of parts to a minimum. Silver contacts are embodied, to give good conductivity and absence of flicker. It is claimed that the use of the transparent plastic bezel and lens unit results in an increase of 10 per cent in the mean spherical candle power of the lamp.

Another interesting development in headpiece design can be seen in the alternatives which may be fitted on the Concordia lamp. The alternatives are three in number, type 4 headpiece which is shown in Fig. 4, has main and auxiliary bulbs and a switch, type 5 with main bulb only and a switch, and type 6 with main bulb only and no switch. The switch is operated by rotating the lens ring, the position of which is controlled by stops. Headpiece and lens ring are made of phenolic resin and the headpiece has a magnetic lock.

The Ceag Type A7 cap lamp can be fitted with three alternative patterns of headpiece, constructed of moulded bakelite, stainless steel, or aluminium, respectively. The moulded bakelite headpiece, illustrated in Fig. 5, incorporates a bayonet type bulbholder, aluminium reflector, and a screw-type metal switch, the armoured glass being held in place by a clip-action nickelled-steel bezel ring. The bulb is of the double filament 3.75V. 1A. krypton-filled type and is mounted tangentially in its holder. The stainless-steel headpiece is of smaller dimensions than the bakelite type and is of substantial strength. It is fitted with a 3.75V. 1A. bulb mounted axially in an aluminium reflector and also with an emergency pilot bulb, if desired. The glass is secured by a duralumin bezel ring of screwed type.

The Oldham type G headpiece is made of moulded bakelite and measures a little over 2½ in. diameter. An armoured glass 1/8 in. thick is carried by an external screwed ring. Headpiece fittings include a rotary switch, positive contact studs through which the charging current flows when the lamp is placed on its charging frame, and an internal key switch which permits charging to take place through the cable of the lamp, a key on the charging frame forming the negative contact. The headpiece, which is shown in Fig. 7, contains a main 4 V. 0.8 A. bulb, mounted at the focus of the parabolic reflector, and also a low consumption pilot bulb, mounted tangentially, the pilot bulb being designed to give 40 hours emergency burning, if necessary.

The Youle cap lamp is fitted with a headpiece of aluminium alloy. The glass is held in place by a male threaded ring, which is screwed inside the female screwed bezel. A tumbler type switch is incorporated, and a 4 V. 0.8 A. bulb is held in a bulbholder mounting integral with which is a half-wave selenium rectifier which serves the dual purpose of rectifying an alternating charging current and at the same time preventing any flow of current from the battery in the event of the charging contacts being short circuited. (See Figs. 8 and 9).

The difficulties associated with glare from cap lamps underground are ever present, and much remains to be done before this problem is solved. So far as the headpiece itself is concerned, two methods of approach are available. One is to increase the size of the headpiece and so enlarge the area of the light source, thus reducing its surface brightness. The other is to use an opal type cover instead of the transparent one, and so diffuse the light emitted by the lamp. The former course is possible only to a very limited extent, for reasons which will be obvious, and the latter course necessarily involves a considerable loss of efficiency. The employment of diffusing type headpiece lenses must therefore be deferred until batteries have been designed which will give correspondingly greater output over the shift, if overall M.S.C.P. is not to suffer.

* Standards of Mine Lighting, Iron and Coal Trades Review, September 22, 1950.

(To be continued)

Drawings and Photographs accompanying the article