LIGHTING IN UNDERGROUND MINES     

 

Don Trotter

 

 

Light Sources in Mining

In 1879 a practical incandescent filament lamp was patented. As a result light no longer depended on a fuel source. Many startling breakthroughs have been made in lighting knowledge since Edison’s discovery, including some with applications in underground mines. Each has inherent advantages and disadvantages. Table 74.3  lists the light source types and compares some parameters.

 

________________________________________________________________________

 

Table 74.3.     Comparison of mine light sources

 

Type of light source

Approximate luminance

 (clear bulb)

Average rated life (h)

DC source

Approximate initial

efficacy lm·

Colour rendition

Tungsten filament

 

750 to 1,000

Yes

5 to 30

Excellent

Incandescent

 

5 to 2,000

Yes

28

Excellent

Fluorescent

 

500 to 30,000

Yes

100

Excellent

Mercury vapour

 

16,000 to 24,000

Yes with limitations

63

Average

Metal halide

 

10,000 to 20,000

Yes with limitations

125

Good

High-pressure sodium

 

12,000 to 24,000

Not advised

140

Fair

Low-pressure sodium

 

10,000 to 18,000

Not advised

183

Poor

 

cd = candela, DC = direct current; lm = lumens.

________________________________________________________________________

 

 

Current to energize the light sources may be either alternating (AC) or direct (DC). Fixed light sources almost always use alternating current whereas portable sources such as cap lamps and underground vehicle headlights use a DC battery. Not all light source types are suitable for direct current.

 

Fixed light sources

Tungsten filament lamps are most common, often with a frosted bulb and a shield to reduce glare. The fluorescent lamp is the second most common light source and is easily distinguishable by its tubular design. Circular and U-shaped designs are compact and have mining applications as mining areas are often in cramped spaces. Tungsten filament and fluorescent sources are used to light such diverse underground openings as shaft stations, conveyors, travelways, lunchrooms, charging stations, fuel bays, repair depots, warehouses, tool rooms and crusher stations.

 

The trend in mine lighting is to use more efficient light sources. These are the four high-intensity discharge (HID) sources called mercury vapour, metal halide, high-pressure sodium and low-pressure sodium. Each requires a few minutes (one to seven) to come up to full light output. Also, if power to the lamp is lost or turned off, the arc tube must be cooled before the arc can be struck and the lamp relit. (However, in the case of low-pressure sodium (Sox) lamps, restrike is almost instantaneous.) Their spectral energy distributions differ from that of natural light. Mercury vapour lamps produce a bluish white light whereas high-pressure sodium lamps produce a yellowish light. If colour differentiation is important in underground work (e.g., for using colour-coded gas bottles for welding, reading colour-coded signs, electrical wiring hook-ups or sorting ore by colour), care must be taken in the colour rendition properties of the source. Objects will have their surface colours distorted when lit by a low-pressure sodium lamp. Table 74.3  gives colour rendition comparisons.

 

Mobile light sources

With working places spread out often both laterally and vertically, and with continual blasting in these working places, permanent installations are often deemed impractical because of the costs of installation and upkeep. In many mines the battery-operated cap lamp is the most important single source of light. Although fluorescent cap lamps are in use, by far the majority of cap lamps use tungsten filament battery-operated cap lamps. Batteries are lead acid or nickel cadmium. A miniature tungsten-halogen lamp bulb is often used for the miner’s cap lamp. The small bulb allows the beam to be easily focused. The halogen gas surrounding the filament prevents the tungsten filament material from boiling off, which keeps lamp walls from blackening. The bulb can also be burned hotter and hence brighter.

 

For mobile vehicle lighting, incandescent lamps are most commonly used. They require no special equipment, are inexpensive and are easy to replace. Parabolic aluminized reflector (PAR) lamps are used as headlights on vehicles.

 

Standards for Mine Lighting

Countries with a well-established underground mining industry are usually quite specific in their requirements regarding what constitutes a safe mine lighting system. This is particularly true for mines which have methane gas given off from the workings, usually coal mines. Methane gas can ignite and cause an underground explosion with devastating results. Consequently any lights must be designed to be either “intrinsically safe” or “explosion proof”. An intrinsically safe light source is one in which the current feeding the light has very little energy so that any short in the circuit would not produce a spark which could ignite the methane gas. For a lamp to be explosion proof, any explosion triggered by the lamp’s electrical activity is contained within the device. In addition, the device itself will not become hot enough to cause an explosion. The lamp is more expensive, heavier, with metal parts usually made of castings. Governments usually have test facilities to certify whether lamps can be classified for use in a gassy mine. A low-pressure sodium lamp could not be so certified as the sodium in the lamp could ignite if the lamp were to break and the sodium came in contact with water.

 

Countries also legislate standards for the amount of light required for various tasks but legislation varies greatly in the amount of light that should be placed in the various working places.

 

Guidelines for mine lighting are also provided by international bodies concerned with lighting, such as the Illumination Engineering Society (IES) and the Commission internationale de l’йclairage (CIE). The CIE stresses that the quality of light being received by the eye is as important as the quantity and provides formulas to ascertain whether glare may be a factor in visual performance.

 

Effects of Lighting on Accidents, Production and Health

One would expect that better lighting would reduce accidents, increase production and reduce health hazards, but it is not easy to substantiate this. The direct effect of lighting on underground efficiency and safety is hard to measure because lighting is only one of many variables that affect production and safety. There is well-documented evidence that shows highway accidents decrease with improved illumination. A similar correlation has been noted in factories. The very nature of mining, however, dictates that the work area is constantly changing, so that very few reports relating mine accidents to lighting can be found in the literature and it remains an area of research that has been largely unexplored. Accident investigations show that poor lighting is rarely the primary cause of underground accidents but is often a contributing factor. While lighting conditions play some role in many mine accidents, they have special significance in accidents involving falls of ground, since poor lighting makes it easy to miss dangerous conditions that could otherwise be corrected.

 

Until the beginning of the twentieth century, miners commonly suffered from the eye disease nystagmus, for which there was no known cure. Nystagmus produced uncontrollable oscillation of the eyeballs, headaches, dizziness and loss of night vision. It was caused by working under very low light levels over long periods of time. Coal miners were particularly susceptible, since very little of the light that strikes the coal is reflected. These miners often had to lie on their sides when working in low coal and this may also have contributed to the disease. With the introduction of the electric cap lamp in mines, miner’s nystagmus has disappeared, eliminating the most important health hazard associated with underground lighting.

 

With recent technological advances in new light sources, the interest in lighting and health has been revived. It is now possible to have lighting levels in mines that would have been extremely difficult to achieve previously. The main concern is glare, but concern has also been expressed about the radiometric energy given off by the lights. Radiometric energy can affect workers either by acting directly on cells on or near the surface of the skin or by triggering certain responses, such as biological rhythms on which physical and mental health depends. An HID light source can still operate even though the glass envelope containing the source is cracked or broken. Workers can then be in danger of receiving doses beyond threshold limit values, particularly since these light sources often cannot be mounted very high.