Electrical Power Plant Design: Distribution System

WEEK 12: DISTRIBUTION: STREET LIGHTING

Sections: Characteristics | Circuits | Fixtures

Characteristics The lighting of streets, parkways, and other roadways is about the only service in which the electric utility is often responsible for the utilization equipment. This involves the complete service of installation, maintenance, and operation of lighting systems during the hours of darkness (approximately 4000 hours/year) when they are required.

Factors to consider in planning a street lighting system includes:
(1) How much light is needed.
(2) Types of structures along the road: homes or business.
(3) Design of the road.
(4) How much traffic passes by.
(5) Crime potential.
Studies have proved conclusively that good street lighting considerably reduces traffic accident and crime rates. It also improved retail business, as well as adds considerably to the beauty of a town street or highway.

Lighting Patterns A street-lighting engineer can make light diffuse at any angles, depending on the area to be illuminated, the spacing between the units and the mounting height of the luminary. Although most of the light is directed to the road, there are many situations where the sidewalk must be comparably illuminated. There are five different patterns of street light diffusion, differentiated by the degree of the diffusion angle. Type I consists simply of a light hung at an intersection. Type II is mounted at the side of the street and diffuses light at a 65° angle. Type III is also mounted on the side of the street; but in this case light is diffused at a 45° angle. Type IV, the asymmetric pattern, light is diffused on the both sides at a 90° angle and all the way across the street. Type V, the symmetrical pattern, lights up the whole intersection, roads, and corners alike, using a 360° angle of diffusion.

Sections: Characteristics | Circuits | Fixtures

Street Lighting Circuits

Series Circuits (constant current) circuits, historically a common supply for street lighting, have become obsolete. It is operated by connecting the lamps in succession. The brilliance of light output for the various lamps are different.

Multiple or Parallel Circuits (constant voltage) are normally supplied directly from the local distribution (120/240V) The high-intensity-discharge lamps used have compatible ballasts, of high-power factor or normal-power-factor types, for all common voltages. A multiple street lighting system consists of a group of lamps in multiple connected to the secondary street mains through relays or astronomical time switches, a switch actuated by a relay which opens or closes its contacts in accordance with the hours of sunrise and sunset.

Photoelectric controlled-circuit are frequently used integrally with individual lights but may also be used to switch contactors controlling circuits used for lighting only. Older systems use a phototube, which consists basically of a pair of electrodes contained in an evacuated envelope. The electrodes allow more current to flow in the tube as the intensity of the incident light increases. The phototube circuit is integrated with a control circuit which incorporates a relay. When the current in the phototube circuit reaches a predetermined value the relay in the control circuit functions and causes the light to go out. As the day darkens, the reverse action takes place.

Newer systems use a solid-state photocell instead of a phototube. The operation is essentially the same as that of the phototube system, except that photocell reduces maintenance because the life span of the latter is very much longer than the former.

Sections: Characteristics | Circuits | Fixtures

Street Lighting Fixtures

A street lighting consist of a post (wood, iron, aluminum, concrete), a fixture (upright or pendant), a lamp (bulb or tube) and a photocell, in most newer lighting system.

Poles Street lights may be installed on steel aluminum or concrete standards, usually fed from underground cables. More often they are attached to some fixture or bracket attached to a pole. The shapes and types of fixtures vary widely.

lamps Lamps The usual lamp used in street lighting circuits is of the regular incandescent type. Light is given off from a tungsten filament heated white hot. The amount of light which can be obtained from this lamp is limited by its physical dimensions and manufacturing facilities. The important elements of the incandescent lamp are the tungsten filament, a mandrel on which the filament is wound, and inert gases (usually nitrogen-argon combination) which cool the filament to cut down on evaporation and carry any tungsten which does evaporate to the top of the lamp. Fluorescent lamps, usually one or more tubes four feet in length, provide more illumination and still maintain practical dimensions. These lamps are connected for multiple circuits. A coating of fluorescent material applied to the inside of the glass tube is made to glow and give off light when acted on by rays produced by the flow of electricity. Present street-lighting systems, usually multiple circuits, are designed using high-intensity-discharge sources. The three principal types are clear or phosphor-coated mercury, metal halide, and high-pressure sodium. These lamps are available in several sizes ranging from less than 100 to 1000W. Metal halide lamp is not widely used because of its short life and poor lumen maintenance. Mercury and sodium are the most economical chemicals with respect to supplying the proper temperature, voltage and pressure. In the mercury-vapor lamp. the mercury is vaporized by the current, controlled by a ballast) flowing through the lamp. When the current continues through the gaseous mercury, the gas gives off great amounts of concentrated green-blue light. With sodium lamp, the principle of operation is the same, but the light produced is a highly intense yellow-orange.

Luminaires are sealed and also can be filtered. This minimizes the light loss due to dirt collecting in the luminaire. Street light lamps, without any equipment, in general emit light in all directions. Upward light might be of value in certain business areas where the facades of buildings so illuminated act as secondary sources of light, reduce glare and are generally pleasing in appearance. There are two types of fixtures applied to luminaires: the upright type, usually used for underground-wired systems and the pendant type, applied for street lights that are overhead-wired. The luminaire fixture modify the distribution of light from the lamps and three general types of equipment are (1) reflectors, (2) refractors and (3) diffusing glassware. They may also be equipped with any combination of these three or with all three. A reflector is practically opaque; the light rays hitting its surface are stopped and redirected in the desired direction. A reflector maybe of porcelain, enameled porcelain, metal, silvered glass, and silvered or other plate metals. There are various shapes of reflectors designed to give different types of light distribution. A refractor is transparent; the light rays hitting its surface are bent and pass through in the desired direction. The refractor intercepts a large share of light and redirects it upon the street surface. This builds up the illumination around the street light fixture and utilizes the light more efficiently. Diffusing Glassware, usually rippled to make a large, even light source, prevents glare and comes in may different shapes. It is also used for protection to the lamp and, in many cases, decrease the brilliancy of the light source and make the globe itself act as secondary light source.

There is a large variety of street-lighting equipment. This is required to fit different mounting heights, street widths, and lamp wattage. There are also difference in daytime appearance that are needed to fit the needs of the environment.

Distribution Equipment