Street Lighting

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The 'Introduction' states:

[...] In the past two decades, notable progress has also been made in the adoption of modern street lighting materials and methods in many areas throughout the country. However, much remains to be done to replace vast numbers of obsolete luminaires and lighting systems, particularly on secondary traffic streets and in rural and residential areas. In addition, highway lighting is almost nonexistent and presents a challenge as well as an opportunity for public good yet to be improved. [...] The EEI Street Lighting Manual aims to provide basic information needed by those persons interested in or responsible for roadway lighting design, sales, installation, and maintenance. [...] Conceived primarily as a training aid addressed to utility sales, engineering, and operating personnel, the Manual has found wide acceptance as a general reference on street lighting theory and practice, no work of comparable scope having been published since 1930. [...]

The following information is excerpted from this book.

Historical Background

[...] In the annals of early times, street lighting is almost invariably ascribed to an effort to combat robberies and other crimes. Before streets were lighted by regulation, cities and towns were patrolled by watchmen carrying lanterns or torches. Citizens who ventured forth after dark did so at considerable risk, even though they were accompanied by a 'link-boy', or torch bearer, hired as a measure of safety and protection.

One of the earliest instances of a modern city to provide street lighting was in 1558, in Paris. At the time, the city was infested with robbers, as a result of which an ordinance was passed requiring all citizens to keep lights burning in windows that fronted the streets. Because the houses were close together, there was a semblance of street lighting. Records indicate that the robbers were thwarted to an appreciable degree. [...]

The attractiveness of street lighting was demonstrated, unintentionally perhaps, as early as 1880, when "four giant arc lights were turned on atop the tall courthouse" in Wabash, Ind. Wabash claims to be the first electrically lighted town in the country. The idea of outdoor lighting on such a scale was considered fantastic and extravagant by 19th century standards; however, it was such a novelty that two railroads ran excursion trains to bring spectators. [...]

The seriousness of the accident hazard on city streets became evident in the early 1900s, coincident with traffic congestion, at first horse-drawn, later complicated by the introduction of and ultimate replacement by the 'horeseless carriage'. [...] Small wonder that a prime objective in street lighting today is that of traffic safety. [...]

Table 1-1 lists electric street lamps in chronological order from the earliest to the most recent.

Fig. 1-5 - Early electric arc street lamp. Table 1-1 - Historical Development of Electric Lamps for Street Lighting

Fig. 1-6 - Incandescent lamp arches, Flint, Mich.

[...] The first oil, gas, and gasoline street lighting luminaires were usually mounted on short posts as low as 10 feet above the street for easy access. These were the prototypes of 'ornamental' or 'boulevard' lighting systems. Electric street lighting systems were wired overhead on poles usually placed on the sidewalk near the curb line. Luminaires were suspended at the end of a bracket attached to the wooden line pole or from cables stretched across the street, particularly at intersections. In the early days, some means of lowering the lamps for maintenance purposes was usually provided. Arc lamps, for example, required frequent electrode replacement. Also, they were very bright, and therefore were generally mounted much higher than oil or gas lamps, thereby making them less accessible.

The illuminating power of the electric arc was such that a number of early installations employed exceptionally tall poles or towers. In Cleveland, in 1881, three years following the installation of the Brush arcs, four steel masts, 256 feet in height, were provided with "crowns of dazzling arc lights". By 1893, one tower which remained was cut down to 100 feet, it having been agreed that higher mounting was "inefficient". Many other cities installed carbon arc lamps on towers as high as 90 to 165 feet although, except for the city of Austin, Tex., all have long since disappeared. The towers in Austin, 250 feet in height, are now equipped with 16,000-lumen mercury lamps. See Fig. 1-13. Modern practice, however, is keyed to the problem of installing and maintaining street lights which, of economic necessity, are attached to the existing line poles in most cases. Mounting heights, therefore, are more apt to be 25 to 30 feet and these heights satisfy most requirements today. Wide limited-access highways coming into use may require mounting heights up to 50 feet. [...]

Fig. 1-13 - Texas tower, modernized with mercury luminaires.

Physics of Light

[...] There is a gradual transition of colors as wave length changes through the spectrum; however, the principal color sensations occupy the following approximate ranges or wave lengths:

See Fig. 2-1.

Fig. 2-1 - Electromagnetic spectrum. The chart shows (pictorially rather than to scale) the complete electromagnetic spectrum. Only a very small portion of the entire spectrum is known as light.

Fig. 2-2 - Spectral energy distribution: (a) daylight, (b) incandescent lamp, (c) deluxe warm white flourescent lamp, (d) deluxe cool white flourescent lamp.


Principles of Light Control

[...] Every surface absorbs some of the light flux it receives. The overall percent reflected, or coefficient of reflection, varies greatly with different materials. The total reflectances or reflection coefficients of some common materials are shown in Table 3-1.

Table 3-1 - Total Reflection Coefficients of Various Surfaces


Light and Sight

[...] Color of light is not a problem with most street lighting systems. Research shows that we see about equally well under any of the light sources which are commonly used, assuming that equal footcandles are provided. It is conceivable that, with special problems involving colored objects, detail would become more visible under a particular color of light. For example, a colored surface would appear brighter under light of the same color, or it would appear dark under light of a complimentary color. [...]

Objects illuminated by light of one color will appear in sharper focus than when illuminated by light of several colors. [...]

Theory and Principles of Street Lighting Design

[...] Economic considerations likewise place a limit on the total amount of light - that is, footcandles - which can be provided in modern practice. With street lighting, one is concerned with low levels of one footcandle, plus or minus. Relatively high lighting levels in downtown business streets may be in the order of three to five footcandles. Exceptionally high-level installations of 15 to 20 footcandles are usually beyond the means of the average city or town government to provide as a public service; such installations are usually made possible by special contributions from the local merchants.

Table 6-1 - Typical Footcandle Levels

[...] By preventable glare is meant the effect of that luminous flux which the eye receives directly from the light source of the luminaire itself. This is light which contributes negatively to visibility of objects, and which can be eliminated of minimized by careful luminaire design and placement. Unpreventable glare results from light reflected from the object itself directing light flux to the eye as an essential element in the seeing process. The exess light, or brightness contrast, from the object and the surround tends to lower visibility, because of the inability of the eye to adapt itself to verying brightnesses instantaneously. The eye sees most efficiently in a field of uniform brightness.

Fig. 6-16 - Glare vs. mounting height. Relative blinding effect of glare from street lights at different mounting heights. Candlepower assumed constant.


Mountings for Street Lighting Luminaires

In many instances, there is an ordinary standard line pole at locations where roadway lighting luminaires are to be located. Although these poles carry distribution lines, municipal wires, and telephone cables, they can also be used to support luminaires. Interference between the several uses of the pole can be overcome with the proper bracket or mast arm. Care must be taken to provide the proper mounting height and to properly locate the luminaire. See Fig. 11-1.

Fig. 11-1 - Typical line pole installation.

Where standard line poles are not at luminaire locations, it will be necessary to provide special non-line poles. Such poles will be required when distribution circuits are underground, along the rear property lines, or not properly located. In some cases, the roadway light supply wires are run from the line pole, at the rear, to each nonline pole supporting a luminaire, thereby eliminating wires running parallel to the streets. nonline poles for roadway lighting should be furnished in accordance with the accepted standards for dimension, size, and, if wood, for preservation treatment. See Fig. 11-2.

Fig. 11-2 - Non-line pole installation.

There is a general opposition to poles of any type. It is, therefore, of much importance to provide the best appearance. Among the choice of wood poles, consideration should be given to:

  1. Specially selected straight poles.
  2. Certain species of wood with superior decay resistance without preservative treatment.
  3. Special preservative treatment to provide cleanliness and/or ready adherence of paint.
  4. The appearance of certain types of poles may be improved by shaving.

As a means of maintaining the appearance of old line poles, some companies have found it economical to return them to the pole yeard, where they are rotary-shaved, leaving only the hard wood center. This provides a usable pole at little cost.

Class of poles is determined by the load to be carried. The poles are classified by butt and top diameter.

Natural wood poles are used primarily in the interest of economy, in lieu of more expensive poles made of other materials. The trend is moving to the use of manufactured poles, to conform to political beautification emphasis. Manufactured poles include the use of steel, fiberglass, concrete, aluminum, plastics, and laminated wood.

The selection of the pole, bracket, luminaire, and wiring make up the esthetics of the overall system. The most important single element affecting appearance of a system is the actual location and alignment of the luminaire with other units and surrounding guide points.

Some roadway lighting installations may not require additional investment, as present line poles or future line poles can be used to support the roadway luminaires. See Fig. 11-1. Pole space is often conserved or made available with the use of a Triplex cable, which can be straddled with a mast arm installation.

Brackets or mast arms used to support the luminaire should have sufficient rise so that the luminaire is located properly over the street for proper distribution of light consistent with safety of personnel working on the pole. See Fig. 11-3. If existing poles are of insufficient height to provide adequate mounting heights, the pole should be replaced.

Fig. 11-3 - Typical upsweep bracket to gain height and clearance.

Local rules and safety practices of utility companies sometimes prohibit the installation of roadway light brackets and luminaires on service poles because of the hazard to maintenance personnel. In some states, there are regulations prohibiting the installation of roadway lights on poles carrying high-voltage distribution feeders. Local and state regulations concerning this matter should be investigated before planning a roadway lighting system. If requirements make it necessary, separate poles offer the solution to roadway lighting.

The old upright ornamental street light post became obsolete in the 1930s, and is not recommended for traffic streets or business areas. However, the post-top luminaire has reappeared in recent years, in traditional lantern types and in contemporary modern design, for use in residential areas where it has special appeal esthetically. Models are available with refractors which conform to ASA distribution patterns.

The mounting height of luminaires will depend on the lamp size, location, and size of unit that is being used. The Illuminating Engineering Society's standards for mounting heights, as approved by the United States of America Standards Institute (USASI), should be rigidly followed. With modern-type luminaires, there is a trend to higher mounting heights. However, it is considered permissible with lamp sizes varying from 3,300 to 7,000 lumens to use a mounting height of 25 feet. With the use of the 11,000-lumen lamp and above, mounting heights of 30 to 50 feet are desirable. Lower mounting heights than 25 feet are not recommended except for special applications, such as residential lighting. [...]

Corrosion effects of atmospheric conditions vary in different locations, and sometimes within a given area. Large industrial areas, seacoastal areas, and tropical area environments, all require special consideration. Some of the elements causing rapid corrosion are fumes from chemical plants, coal gases, and salty atmospheric conditions. Soil conditions have a direct effect on poles directly buried in the ground. All of these factors should be discussed with the manufacturers in selecting the type of pole for use in specific areas. [...]

In general, roadway lighting brackets are designed to meet certain conditions. Their strength must be sufficient to withstand wind loads to be encountered and support the weight of the luminaire to be used. The length is determined by the distance the luminaire is located from the pole and the rise sufficient to give proper elevation above the road. Welded pole plates and tie rods are frequently used to strengthen the bracket and minimize vibration. Fig. 11-7 shows several types of brackets commonly used.

Fig. 11-7 - Typical brackets in common use - uplift-type mast arm.

Brackets used on wood poles are normally for overhead wiring, and it is most important that the raceway be smooth to avoid injury to insulation of interior wiring. Grommets are often provided around the entrance to the raceway, to prevent damage of lead-in wires from weather and wind. [...]

Straight and right angle bend-type brackets are in lengths to 8 feet, and are constructed of galvanized steel or aluminum. They are designed for use on metal, concrete, and wood pole mountings. Braces attached to both sides of the pole or to center only, support the brackets from below or above the design, needed for strength and rigidity.

The straight bracket is more applicable to conditions where there is no interference with other circuits.

Upsweep brackets are pleasing in appearance, and provide increased mounting height from shorter poles while 4- to 12-foot overhang can be obtained. There is the advantage of keeping pole space available for other than roadway lighting equipment; thus, code standard mounting heights can be maintained even though only lower space is available on the pole. Upsweep brackets are used on wood for space need, and often on non-wood poles to give added height without pole replacement to provide for larger sources.

A few of the types of upsweep brackets are shown in Fig. 11-7. The length of thee brackets varies from 4 to 20 feet. The longer sizes and some types require braces for adequate strength.

Illustrations of upsweep brackets are shown in Figs. 11-2 and 11-8. The brackets were originally designed for wood pole mounting. They are now available for poles of all types of materials.

Fig. 11-8 - Typical upsweep bracket. Detail for installation of rigid fixture mounting on upsweep mast arm, 12-foot and 18-foot arms.

Brackets and mast arms are available in extreme lengths to as much as 35 feet. Brackets should be capable of supporting without failure on 250-pound vertical load and a 50-pound side force, as applied at the luminaire end. These extreme lengths are suitable to meet the desires for interstate highways.

At one time, the majority of roadway lighting systems were operated from high-voltage, constant-current, and series, circuits. There are many of these systems in operation, but extremely few new systems employ this method. Improvement of individual luminaire control by the use of photoelectric cells has changed this concept. With a multiple system, the immediate source of electricity is usually the distribution secondaries, which may be either overhead or underground. Such installations normally operate on 120- or 240-volt circuits. Where separate multiple feeds are used for the roadway lighting, it may be economical to use 480-volt circuites.

Wire may be either single strand or multi-conductor cable. Color coding is frequently employed for convenience in making connections. Modern practice favors dropping the series wire or cable from the crossarm to an insulator mounted on the bracket near the luminaire, and then entering the luminaire head through suitable insulating busings (Fig. 11-9). In other cases, using insulated transformers or gaseous discharge source equipment with separate ballast, the high voltage is eliminated and the lead-in wire is run much the same as for multiple wiring.

Fig. 11-9 - Overhead wiring - series.

With underground systems, the pole wiring is usually placed internally in the pole itself for metal and concrete poles. Connections are usually accessible through hand holes near the bottom of the pole, or in a pole base.

Sometimes it is desirable to feed a special street light pole with overhead conductors. This may occur when the pole is opposite an alley, or when the pole is across the street from a line of distribution poles. In this case, a piece of service cable or single wires may be extended to suitable insulators mounted on the pole top, and wires extended to the luminaire. [...]

Maintenance of Street Lighting

[...] A street lighting system may be maintained in perfect working order mechanically and electrically, yet it may deliver to the surface of the street a third, or even less, of the illumination for which the system was designed. In a typical case, depreciation of light due to dirt and dust alone was 43 percent; depreciation due to aging of lamps was 23 percent. In some cases, additional loss of light can be attributed to operating lamps at less than their rated current or voltage.

One utility reports a case where an enclosed 4,000-lumen light had not been washed for over two years; footcandles on the pavement averaged 0.4 before washing, and 2.8 - or seven times greater - after washing! In another case, the same utility reports that where luminaires were washed regularly, every six months average footcandles before washing were 2.5, and after washing, 3.5. By a regular six months' washing schedule, illumination depreciation, due to dirt alone, was reduced to 28 percent. [...]

A systematic maintenance program, based upon appropriate studies of the particular system, will reduce operational and maintenance costs. Street lighting has been termed the show window of the electric utility, and those systems which are properly operated and maintained will forestall criticism, promote good customer relations, and aid in community development. [...]

Due to the increased acceptance of street lighting, and since the utility is staffed to maintain its electrical supply facilities on a 24-hour basis, the utility is in a position to adequately maintain street lighting in an efficient and safe manner. [...]

Special mobile washing equipment has been designed for street lighting maintenance, which can be rented or purchased outright. Some utilities may prefer to employ equipment of their own design or specification. One particular form of truck of special design is equipped with a unique elevated platform that adjusts from the floor of the truck to the working level, and can be extended in either direction by power at the working position. The truck contains washing tanks, heated drying racks, and ample space for lamps, glassware, and other hardware. An efficent arrangement eliminates wasted motion as much as possible, and has facilitated the cleaning of approximately 60 lights per day per man. See Figs. 13-1 and 13-2.

Fig. 13-1 - Street lighting maintenance truck - exterior. Fig. 13-2 - Street lighting maintenance truck - interior.

[...] Steel posts and brackets should be painted as frequently as required by local conditions. Some utilities may prefer to establish a regular schedule for all such equipment on the system. Dividing a large system into geographical areas and establishing a sequence cycle by area is a most satisfactory method of preparing a schedule. Climatic conditions in a given area will determine the number of months during the year when painting is satisfactory. All rust and scaling paint should be removed and, after priming, a durable, high-grade, exterior paint applied.

Other types of posts such aluminum, concrete, and wood (when fully pressure-impregnated) require very little maintenance. [...]

The Marketing Problem

[...] Any approach to modernization of a lighting system where overhead distribution facilities are present must of necessity adapt itself to the use of the existing wood poles. Such poles may not always conform to the desired spacing nor may adequate mounting height always be available. Yet with some compromises, an adequate lighting system can nearly always be obtained.

With short pole spacing and for all except wider than ordinary streets, one-side lighting with a light on every other pole will provide a good design. Where long pole spacing is found, a light on every pole may fit into a good design. Sometimes it will be found that where the spacing is not adaptable to either of these two cases, and where the street is wider than ordinary, a light on every third pole in the main line with a new pole set midway between on the opposite side of the street will give the best layout. In fact, all sorts of compromise possibilities exist in overhead system layouts. But there is one compromise that never can be permitted, and that is to use lower than prescribed mounting height. [...]


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