Varied Topics Related To Street Lighting

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Topics Covered:

    How the Ballast Works
    Gas Streetlights
    Manufactured Gas
    Ohm's Law and Voltage Drop

How the Ballast Works

(Quick explanation) The ballast limits the current flowing through the lamp so as to prevent overloading, overheating, and self destruction.

As an example, a mercury or sodium lamp has two(1) wire ends (called electrodes) with a space filled with mercury vapor or sodium vapor between, sealed inside a small bulb (also called the arc tube). As there is no filament to serve as a conducting path, the electricity has to be of a high enough voltage to jump the gap (you can think of a high enough pressure to burst out and across).

When the power is first turned on, the space between the electrodes is at a cool temperature, say 70 degrees F. The resistance of the vapor is quite high so only a small amount of current flows. Very soon, the vapor starts to heat up and its resistance starts to drop(2). Then the current flow starts to increase. Since the resistance continues to drop as the temperature rises, the current would increase to such large levels that the components of the lamp overheat and self destruct, were it not for the ballast.

I will have to introduce some mathematical terms, or at least say that the current goes from point A to point B to point C, etc. Let's say point A is where the live wire is connected to one side of the ballast. Point B is where the other side of the ballast is connected to one side of the lamp. Point C is where the other side of the lamp is connected to the neutral wire by which the electricity completes it scircuit back to the power plant.

If much of the voltage is spent in the ballast (electrical engineers say "there is a significant voltage drop across the ballast") then a large number of volts will show on a voltmeter when you try to measure the voltage between points A and B. But if we have a 120 volt circuit, the maximum voltage you can see between points A and C (with point B in between) is 120. So if a significant voltage is measured between A and B (the ballast), then what is measured between B and C (the lamp) has to be much less than 120 volts.

The voltage drop is a consequence of Ohm's Law. The ballast has a constant resistance so if the current flowing through it increases, the voltage drop across it (between points A and B) increases. Ohm's law also applies to what goes on inside the lamp (between points B and C). The lamp's resistance varies inversely with temperature and also the higher the current the greater the temperature which is why the current flow would get excessivly large if it were allowed to flow unchecked.

The voltage drop caused by the ballast is intentional. As the lamp draws more current, the voltage drop across the ballast gets larger so there are fewer volts left to feed the lamp. Lowering the voltage across the lamp causes the current flow to be reduced even while the lamp's own resistance is relatively constant. Actually a happy medium is usually reached very quickly, the 120 volts are shared between the ballast and the lamp, and the current flow in the lamp stabilizes at a level that does not cause overheating. The happy medium actually changes gradually as the lamp gradually brightens and warms up to operating temperature.

The ballast must be matched to the lamp's wattage. Otherwise the current flow does not reach a happy medium. If the ballast's resistance is too small, it will not produce enough of a voltage drop and the lamp will suffer excessive current and self destruct. If the ballast's resistance is too large, the voltage drop across the ballast will get too large, there won't be enough volts to jump the gap between the lamp's electrodes, and the lamp will fizzle out.

There comes a time when the ballast that used to serve a lamp faithfully is no longer a good match because the lamp characteristics changed due to aging during usage. Some of the sodium or mercury may have chemically combined with the glass bulb so that the vapor no longer has the correct resistance. The electrodes may have worn away due to evaporation given the heat within the bulb and the gap is wider than it should be. Whatever the reason, the voltage and current the lamp gets is no longer what the lamp needs for correct operation. In turn, further aging and deterioration of the lamp accelerate. It is not practical and not cost effective for the town or electric utility to re-match lamps with ballasts, swapping ballasts as needed to keep an aging lamp operating. Instead the lamp, which usually shows visible signs of age such as dimming or blinking or complete extinguishing, should be promptly replaced.

Well aged high pressure sodium lamps are very likely to get into a condition where they fizzle out, cool down, and relight themselves over and over again(2). This is called cycling. This subjects the ballast to undesirable levels and changes in current often causing the ballast's characteristics to change. Then when a new lamp is installed, it is forced into early failure because the ballast is now producing voltage and current regulation that no longer matches a good lamp's needs.

The resistance of the ballast is vastly oversimplified in this discussion. Through the miracle of the "power factor" (not discussed here), very little of the power consumption implied by the voltage drop across the ballast shows up as consumption on the electric meter, although the power expended in the lamp, of course, does.

The ballast unit for a sodium lamp often contains additional circuitry called an "igniter" used to get the arc started.

(1)  In mercury lamps there are three electrodes, two close together at one end of the small bulb, the third at the other end. When the power is first turned on, the resistance of the mercury vapor is too high for current to jump to the far end but an arc forms between the two electrodes close together put there for the purpose of heating up the vapor. This is a small arc, not enough to give a lot of light. A small resistor inside the lamp acts as a ballast and keeps this arc small. After a few seconds the vapor inside the arc tube warms up, its resistance drops, and current jumps to the third, far away, electrode to produce lots of light.

(2) As the temperature rises in the arc tube, the pressure of the vapor also rises. This by itself causes the resistance of the vapor to rise. Most of the time the resistance drop as the temperature rises is greater than the resistance rise as the temperature rises, so the ballast is still needed to prevent excessive current  flowand lamp failure. In the case of well aged sodium lamps, the pressure rise due to temperature is once in awhile enough to raise the resistance high enough and stop the current jumping through the vapor. When everything cools down enough, the gap is jumped again and the lamp relights.

Gas Lamps

Gas streetlights were used in the late 1800's to about 1920, and a few are still in used today for decorative purposes. Gas lighting was used and different kinds of gas lamps invented as soon as gas could be obtained or manufactured, stored, and piped.

A gas light can be as simple as an open ended tube, or jet, where gas escapes and is lit afire. In a gas streetlight, the jet is aimed at or inside a mantle which is the actual light producing element, since the gas flame by itself produces very little light. Gas streetlights always have a glass enclosure to shield the flame from wind.

In some cities or for some streets in a city, every evening, a lamplighter would come around and light each streetlight. Sometimes s/he would have a ladder, other times the streetlights were low enough to be reached with a flame at the end of a long pole. Then at dawn, or in some cases late at night if there were curfews or expensive utility bills, the lamplighter would come around again to turn off the gas for each light. Usually the valve was within reach of the ground so no ladder was needed.

Spring powered timers that were set to turn on the gas at dusk and off at dawn were used in some cities. They would run about a week between windings. As with most gas appliances, a pilot light fed by a trickle of gas stayed on all the time. Each streetlight needed its own timer which increased the installation cost.

In some places including nowadays, gas streetlights are sometimes left on 24 hours a day to save on the labor of lighting them each evening.

Almost all gas streetlights were non-cutoff fixtures at the tops of vertical poles usually about ten feet high.

When gas streetlights and gas interior lighting was first introduced, the gas was almost always manufactured locally and sometimes referred to as as "illuminating gas". By the time natural gas was discovered in plentiful quantities in the U.S. (1920's) and pipelines built (1940's), most gas streetlights had been replaced with electric streetlights.

Some cities, including San Diego, have a downtown district with a historic atmosphere that is referred to as the "gaslamp district". Sometimes but not always gas streetlights would be installed.

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Manufactured Gas

<<The gas that was used for gas streetlights was called various names: illuminating gas, water gas, manufactured gas, coal gas, city gas, to name a few. It was invented in the late 18th century in England, as I recall and was first used there. [I] don't remember who did it first, but basically it was discovered that if you blew steam across a bed of red hot coal deprived of oxygen, you got a chemical reaction that produced a flammable gas. If executed correctly, the coal would reduce the steam to hydrogen, and would itself be oxidized to carbon monoxide. H2O + C -> H2 (hydrogen occurs naturally as two atom molecules) + CO. Both hydrogen and carbon monoxide are flammable, and it was this mixture of gases that was piped around to the early gas lights in homes and on streets. Since it was manufactured from water, you can see how the names "water" and "manufactured" became applied to it.

Cities that had gas streetlights and gas lighting in homes each had "gas plants" to manufacture the gas. You could even call them generating stations. Incidentally back then each city that had electricity also had its own electric generating plant. Natural gas, which is mostly methane, was not widely used until the 1940's; the large gas reserves were discovered underground in the mid-continent US from Texas on up to Kansas and Nebraska in the 1920's and 1930's and then pipelines had to be built. By this time most gas streetlights had been replaced by electric ones. Natural gas was superior to manufactured gas as it required much less energy to obtain (you didn't have to heat coal or coke and make steam to produce it) and because it was not already partially oxidized as the carbon monoxide component by definition is, natural gas thus had a higher BTU content per volume unit consumed. [I] am not sure of the extent to which manufactured gas was used for heating but it was certainly not cheap due to the process needed to manufacture it. Most people heated with coal stoves and convection furnaces at that time.>>

The above account was furnished by Lee Lowry.

It may be conjectured that the gas industry suffered from the same growing pains as the electric industry as the energy found more and more uses. Electricity was used at first (prior to the 1930's) mostly for light, requiring not too much consumption and not too heavy wiring. In the decades that followed, the use of electricity for motorized appliances, electronics, and heat forced utility companies to upgrade wiring along the streets; homeowners have also had to upgrade wiring in their homes. Similarly, gas mains would have had to be upgraded to deliver the much larger quantities of gas needed for today's primary use, heat.

I remember reading in Sears Roebuck catalogs in the late 1950's about gas appliances available in a choice of natural gas or artificial gas capability . This would suggest that city gas plants were still in operation at that time. At that time (no longer, I don't recall the date of their removal) the tanks for the gas plant in Cambridge Mass. (where I lived at the time) were on the bank of the Charles River near Kendall Square (MIT). Each tank was a large cylindrical dome that rose and fell within a cylindrical girder frame depending on the amount of gas stored, and, as I was told, the bottom was open and the dome rim was submerged in a narrow but deep moat. Similar tanks were also located in Boston's near suburb of Dorchester and have been a landmark often mentioned by the radio traffic reporters. The latter have since been replaced with pressurized tanks without moveable domes which in turn have been decorated with designs suggested by local artists.

How gas customers were changed over from manufactured gas to natural gas I don't recall. I would suspect that the gas was blended gradually towards 100% natural gas with customers being required to upgrade their appliances to models that could accept either type of gas, and then with periodic adjustments made as the blend changed.

The townhouse I first lived in had a convection furnace with no fan, also referred to as a "gravity hot air" furnace. The ducts, if I recall correctly, were more than a foot in diameter at the furnace and at least eight inches round at the furthest branches.

Nashua, NH, where I live now, has natural gas service but now has no tanks. Thus I believe that the gas pipeline distribution system used nowadays does not require tanks at major usage points. The tank complement originally used by the old time gas plants would have very limited backup capability as it would only hold a few hours' worth of gas at today's citywide winter consumption rate.

Both manufactured gas and natural gas are normally odorless. The smell everyone associates with gas is added by the gas company for safety reasons, to alert the customer if a leak occurs. The chemical odor used nowadays is mercaptan. Canaries are carried into mines to succumb first to asphyxiation or toxic reaction if methane or other gas should be encountered and thus give the miners advance warning whereby they can quickly evacuate.

Ohm's Law and Voltage Drop

Ohm's Law, defined: The relationship between voltage, current, and resistance that is always true; voltage equals current times resistance, or current equals voltage divided by resistance, or resistance equals voltage divided by current. There is no such thing as a perfect conductor. As a simple example to avoid the need to talk about transformers and high tension, just consider the 120 volt wires from the utility pole outside your home to a lamp in your home. Since the wire(s) cumulatively have some resistance, if the voltage at the utility pole is exactly 120 volts, the voltage at the lamp is slightly less, for example 119-1/2 volts. This is because the difference in voltage as might be measured at one end of the wire compared with at the other end (the voltage drop) must obey Ohm's Law. If you turned on lots of lamps and appliances, the current going through the wires is increased and the voltage drop in the wires also increases according to Ohm's Law. Then the voltage measured at a lamp might be 115 volts instead of 119-1/2 volts. Electric companies monitor the current draw at the generating station. When the current draw is so much, the technicians (or nowadays, computerized equipment) estimate what the voltage would be at the far ends (in cities and towns) and jack up or reduce the voltage at the source in order to achieve 110-120 volts at the destination after accounting for Ohm's Law. In times of extreme electrical demand, the generating station is not powerful enough to achieve a high enough voltage and as a consequence the voltage at the destination falls below 105 volts. This is called a brown-out.

The thinner the wire, the more resistance it has. In some rural areas, the wires on the utility poles are too thin for the load because they were installed before there were too many houses. In order for the house at the far end of the street to get, say, 105 volts, the power station (or substation) has to put 130 volts into the wires and the nearest houses during times of peak demand. Putting transformers here and there to give everybody the same voltage is done but is not the perfect solution. If the power usage is less, the difference between the first house and the last house might be only one volt instead of 25 volts according to Ohm's Law. Nowadays automatic voltage regulators are commonplace but they are expensive and not all parts of the country have been upgraded to have them. Therefore 130 volt lamps are still being manufactured because there continue to be homes whose voltage will vary as described here under normal conditions.

The electrical terms that go hand in hand are volt (for pressure), ampere (for volume), ohm (for resistance), and watt (for total power, volts times amperes). A current flow of one ampere at one volt is equal to one watt. A one hundred watt lamp for a 120 volt power source draws 0.83 amperes. For Ohm's Law, if the resistance is one ohm and there is one volt, then exactly one ampere will flow. If the same one volt power supply is used and there are two ohms of resistance, then one half ampere will flow. If per chance the power supply was too weak to provide one half ampere, the voltage will drop until some combination that obeys Ohm's Law is reached, for example you might end up measuring one quarter volt being put out by that "one volt" source providing one eighth of an ampere flowing through that two ohm resistance. There exists an actual number of electrons that stand for one ampere but I don't know that number.

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