Traffic Signal and Street Light Information Sheets
Willis Lamm

  Early Lighting Strategies

The need to illuminate streets and alleys was understood centuries ago by the ancient Greeks and Romans. Oil lanterns were lit at night to help pedestrians and equestrians avoid accidents and to help keep robbers at bay.

As technology advanced, streets throughout the world were starting to be lit by coal-gas lamps as well as petroleum and alcohol lamps. At that time, gas lamps generally produced the greatest amount of light per lamp and were preferred where gas supply was available.

However even with the relative brilliance of multiple burning gas mantles, gas lamps still provided limited illumination, requiring many lamp fixtures in order to effectively light a large, busy street.

These early lamps also required a great deal of labor. Someone had to light every lamp each evening and extinguish them all come morning. Alcohol and oil lamps had to be refilled. Wicks, mantles and the lamps' mechanisms had to be regularly serviced. All these expenses were considered as part of the costs of operating a city.

  Arc Lamps

The first practical street light with any real brilliance was the arc lamp. As early as 1801 Sir Humphrey Davey demonstrated that he could produce a luminous arc between two sticks of charcoal powered by a 2,000 cell battery. However due to the difficulty in generating continuous current, Davey's device was for the most part a novelty.

By the mid 19th century, inventors began producing practical generators and dynamos that could produce power all during the night, and in some instances, 24-hours a day.

In 1875 Russian engineer Pavel Yablochkov developed the first practical arc lamp that could be used to light city streets. In 1878, Paris had 80 "Yablochkov" candles installed to illuminate the Grands Magasins de Louvre, fostering Paris' nickname, "The City of Lights."

The Yablochkov Candle produced a brilliant, albeit harsh light. Single lamps positioned at relatively high elevations could illuminate large streets for hundreds of yards distance.

With the success of Yablochkov's lamp, inventors set to work to improve the reliability of arc lamps. The carbon rods used by these lamps were consumed as they operated, so there were at least one hundred ideas advanced to automatically regulate the arc gap by moving one or more of the carbon rod electrodes as they were consumed. A proper gap was critical for maintaining a working arc.

Arc lamps also required a great deal of maintenance as compared with the incandescent lamps that came later. Arc lamps required regular servicing of the carbon rod electrodes and removal of ash. However they had the advantage of many lamps being controlled from one point through series circuits and much fewer lamps were required to provide the same illumination as gas or oil lamps.

  Incandescent Lamps

Thomas Edison designed the first practical incandescent lamp in 1878, however the amount of light produced by his early carbon filament lamps was far less than that produced by arc lamps. Nonetheless Edison lamps did find their way onto many streets, often on relatively short posts in clusters of lamps on narrower residential streets where less lighting was needed and the harsh intensity of arc lamps was less desirable.

Even though incandescent lamps could operate on "house current," conventional line voltage electricity did not travel long distances without suffering significant voltage losses. As a result, multiple lamp systems using conventional line voltage could only supply a few lamps that were located within a small area. To expand the distance that a circuit could be run (all around town rather than be limited to a couple of blocks) high voltage series lighting circuits were predominantly used. These circuits typically ran at levels of 2,000 volts or more, with the lamps arranged in series with each other. With the lamps being wired in series, the actual voltage across each bulb was generally less than 50 volts.

Therefore nearly all circuits consisting of more than a handful of lamps were run in series, allowing many lamps to be controlled from a single source, but with the amperage kept low enough to allow the circuit to be run on relatively small conductors.

Series circuits also produced more light per watt of power used than multiple and single lamp "line voltage" circuits.

(Please see Understanding Series Circuits for a detailed explanation as to how these circuits worked.)

With the advent of the Mazda tungsten filament lamp, the limitation on the amount of light that an incandescent lamp could produce was eliminated and brighter incandescent lamps began to replace the troublesome carbon arc lamps throughout the world.

  Controlling the Circuits

In the 1800s photoelectric controls had not been invented, so strings of lights had to either be turned on manually or by time switches.

Since most large lighting circuits were run in series, many utilities supplied them from dedicated dynamos that were started at dusk and shut down at dawn. However this option required all of the lighting circuits to originate from the utility's "central station" or generating plant.

For circuits distant from a central station, series street lights were generally powered from high tension primary circuits. In those installations, current to the street lights would be supplied by a constant current regulating transformer (generally referred to simply as a "regulator.") The voltage needed to operate a series circuit would vary based on the number of lamps operating in the circuit.

The regulator constantly adjusted the circuit voltage based on the number of lamps that were working. Such circuits could be turned on and off be means of a manual switch, by a time clock or by a control wire that ran to the regulator from a central station. These controls would in turn operate a high voltage relay called an oil switch that switched power on and off to the regulator.

A serious technical issue with series circuits was that since the lamps were daisy-chained together, if one lamp failed the entire string would go dark. To resolve this problem, autotransformers or cutouts were used to automatically bypass failed lamps and keep the circuit flowing.

For a complete discussion on the various shunt transformers and cutouts used on series circuits, please see Understanding Shunts.

For smaller groups of lamps that did not justify the expense of a constant current regulator, lower voltage "multiple" circuit systems were often employed. Since only a few lamps could be powered from a single low voltage source, control circuits were added where large numbers of lamps could be controlled by a single switch, but the individual lamps or small groups of lamps would be powered locally and were operated by means of relays. Early relays were pole mounted. Later designs included relays built into the luminaires themselves.

Multiple circuits could also be daisy chained to spread the load over more transformers. An initial group of lamps fed by one local transformer would be controlled by a switch or clock, or perhaps later by a photocontrol, and a "control lead" would extend from that group to a relay that controlled another group of lamps supplied by another local transformer. A control lead from that group could then extend to yet another group, and so forth.

Ultimately series type circuits were found to be more efficient overall and actually produced more lumens of light per watt consumed, so multiple circuits were far less frequently used.

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