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The Power System In Your Home

You can identify its type by the number of wires entering the main switch box
 
Most people take the power system of their homes for granted. They know that all they have to do is flip a switch, and a mysterious, magical force provides light when it's dark, heat when it's cold, and cold when it's hot. This speaks well for the reliability of modern electrical appliances and of the "juice" that makes them work. Inevitably, however, there comes a time when flipping a switch produces only a small noise and nothing else, and that's when you wish you knew something about the wiring and everything connected to it.

That knowledge is fairly easy to acquire. Although there's a lot of wire snaking all through a house, the basic systems are simple and understandable. Let's start where the electricity starts ... at the generating station or "power house" . . . and follow through to the attic light.

Practically all commercially produced electricity in the United States is now "alternating current." This is usually abbreviated to AC as a matter of convenience in both oral and written references. The early electric generating stations made direct current, or DC, but this suffered from the disadvantage that it could not be transmitted satisfactorily more than several miles from the point of origin. Lamps located near the power house would burn brightly, but those near the end of the line would be much dimmer because of loss of pressure or "voltage" in the wires. The great feature of AC is that it can be converted with high efficiency from any voltage to any other voltage, either up or down, over an enormous range of ratios. This is done by devices called "transformers," which have no moving parts, require only-very minor maintenance, and last virtually forever in normal service.

Direct current is so called because it flows smoothly, evenly and without interruption, like water from a faucet. DC is what you get out of all batteries, regardless of size or type. The original Edison "dynamos," rotating machines driven by steam engines, delivered DC. Their modern counterparts, identical in electrical design but smaller in construction, are the charging generators in automobiles.

A never-ending job at a power generating station— the taking on of coal. Here a tug nudges a loaded barge toward a waiting scoop at one of the Brooklyn (New York) stations of the Consolidated Edison Company. Many large power stations are located on waterways because coal can be transported more cheaply on the latter.

AC power is produced by rotating machines called "alternators," to distinguish them from DC "generators." In most large generating stations the actual turning power is provided by high-pressure, highspeed steam turbines. Steam is the vapor of boiling water, and requires a lot of heat. The usual fuels such as coal, oil and gas are used to keep huge boilers cooking round the clock.

The water wheel, a power device dating back to Biblical times, turns the alternators of the biggest generating stations in the world. The water comes from natural configurations in Nature, like Niagara Falls in New York, or from man-made dams which control vast rivers in various parts of the country.

In relatively smaller power houses alternators are driven directly by Diesel engines. In still smaller installations, found on isolated farms or in military service, they are driven by conventional gasoline engines.

The AC "Wave Form"

Figure 1 is the nearest possible graphic representation of the AC "wave form," or the way the current flows in a circuit. Let's follow the action of an alternator in terms of time and generated voltage. For measuring time we'll use an imaginary stop watch that reads 1/60 of a second from start to stop; for showing voltage, a zero center meter whose needle moves to the right when current flows in one direction and to the left when it flows in the opposite direction.

Below, one of the largest power generating plants in the world—the Astoria (Queens) plant of the Consolidated Edison Company of New York. The two large machines are steam-turbine driven alternators.

With the alternator at rest, nothing, of course, happens. Let's click the stop watch the instant the machine starts to turn, and watch the voltmeter. With the first slight movement of the alternator, electrons in its wires are agitated and the meter needle starts to move, let us say to the right. As the rotation continues, the voltage builds up proportionately. At 1/240 of a second after the starting time the voltage reaches its peak, value, and then starts to drop. It falls back to zero after another 1/240 of a second, or a total elapsed time of 1/120 second.

As the machine turns, another section of wires comes into play, and a new voltage is created just as the first one dies to nothing. It builds up in value exactly as its predecessor did, but it flows in the opposite direction, as a left-hand deflection of the voltmeter indicates. At 1/80 second after the starting time this second voltage reaches its peak value, which is identical with that of the first voltage, and then it, too, starts to decay. It drops to its zero 1/60 second after the starting time.
 
If we let the alternator run, the process keeps repeating itself. One complete variation of current from zero through peak to zero, and again from zero to peak to zero the other way, is called a "cycle;" each half is called an "alternation." The number of cycles per second (c.p.s.) is called the "frequency;" in this case 60, which is universally supplied to homes in the United States.

There is nothing magical about 60 c.p.s. It was probably adopted because there are 60 seconds to a minute and 60 minutes to an hour and the number suggested itself to the early American electrical engineers. For certain industrial and railroad purposes 25 cycles is used (why 25 and not 30, no one knows!) and in some factories 400 cycles is found. This is called "high-cycle" current and is advantageous for special applications calling for small but high-speed motors. In most of the rest of the world the standard is 50 cycles.

Because of an ability of the human eye known as "retention of vision," electric lights operated on 60-cycle AC appear to burn steadily. Although the voltage drops to zero three times in every cycle, it does this too quickly for the lamp filaments to cool off.

One alternation of the AC cycle is sometimes referred to as "positive" and the other as "negative." These are purely mathematical   terms   and   are   somewhat misleading because "negative" conveys the meaning of uselessness or nonexistence. The two alternations are absolutely identical in their ability to do work.

Figure 1: AC in action is shown in diagram form; fluctuations produce magnetic effects by means of which voltages  can be stepped up or down.

Power companies are kept busy installing new distribution cables, with the increased demand lor electric power; photo shows Con-Edison workmen greasing the path of four-inch electric cable.
 
The Distribution Network

Power is generated in modern stations at voltages between 11,000 and 14,000. By means of transformers, this is boosted to values ranging from 23,000 to as high as 275,000 volts, the higher voltages being used for the longest lines. The higher the voltage, the lower the current in amperes for any particular power load, and the smaller the wire required to handle the latter. This is an important consideration, as large diameter wire is heavy, is difficult to handle, accumulates dangerous quantities of ice and is buffeted around by strong winds; all these effects are reduced with thin wire.

As power is needed in various areas, the high voltages are brought to much lower levels by step-down transformers. A primary distribution point or "substation" changes them to between 2500 and 15,000 volts. A secondary distribution point, which may be merely a transformer on a pole or concealed in a vault below street level, brings the power down to the eventual consumer level. In residential areas this is usually either 115 or 230 volts, or both on the same circuit. See Figure 2.

The figure "115" is a flexible one. Depending on the age of the power system, the number of houses fed by one transformer, the time of the day, the size of the actual wiring in the individual home, and the number and type of appliances in use at one time, the voltage may vary from 110 to 125. In older residential districts it will run to the low side; in newer ones steady readings of 120, 121 and 122 volts are normal. For purposes of discussion let's use 115 volts to represent all values between 110 and 125, and 230 volts for voltages from about 220 to 240. The figure of 208 volts appears in some cases, but this is a special value, not evenly related to either 115 or 230. It is taken up later in this section in the discussion of four-wire systems of distribution.

This pole represents the last step in the power distribution network; high-voltage wires on the top of the pole are connected to a step-down transformer (in the large cylindrical case). From this the lower wires run through to home outlets.

Figure 3 shows basic hook-up of two-wire power distribution  generally found in older residences.

Figure  4  shows  basic  three-wire  system,  which makes both 115 and 230 volts available in house.

The Two-Wire House System

The majority of small houses built prior to the television and electric appliance boom of the post-World War II period are fed with a simple, basic two-wire power system. Two wires, running from the nearest secondary distribution transformer, enter the house. They might be suspended aerially from a pole on the curb line, or they might be completely out of sight in buried pipe. With such a two-wire service, the voltage is always "115." See Figure 3. One wire has white or gray colored insulation. It is connected to the nearest water pipe and is called the "ground" wire. The second wire has black insulation and is called the "hot" side of the line only to distinguish it from the other. The grounded wire is by no means "cold" by implication; the two wires can function only together, not separately. Standard practice is to keep the grounded wire a continuous circuit throughout the house, and to insert fuses and switches only in the hot wire.

The main switch and the main fuse are usually in a single steel box. The cover of the latter is linked to the switch handle in such a manner that the fuse is accessible inside only when the switch is thrown to "off." With the switch open or the fuse burned out, the entire electrical system of the house is dead.

The watt-hour meter registers the power consumed in the house. Following it, there are usually several individual "branch" circuits, each with a fuse. These feed power to various parts of the house. If the builder was conscientious, he arranged the branch circuits so that the ceiling lights and the wall outlets in the rooms are on different branches. Thus, if an appliance plugged into a -wall outlet blows a fuse, the room lights still work. It is also sensible to provide individual lines for outlets that require a lot of current: one in the kitchen, for instance, for an iron or toaster, and another in the basement for a washing machine.

The Three-Wire System

In many areas power is brought into a building by a three-wire, dual-voltage system, as shown in Figure 4. The center wire is called the "neutral," has white or gray insulation, and is grounded. Between this neutral and either outside black wire is the normal 115 volts. The various 115-volt branch circuits are distributed so that each half of the system carries about the same power load.

In most cases the three-wire system is wired only to feed standard 115-volt lamps and appliances. It is a simple matter, however, to obtain a circuit from the two outside black wires alone to give 230 volts, for the operation of an air conditioner, a large freezer, etc. The advantage of using the higher voltage is that the current in amperes is reduced, and this minimizes heat losses in the line wires and the possibility of overloading the main fuses. A specific example is a standard ½ horsepower motor that works equally well on either 115 or 230 volts through a slight shifting of its internal connections. It develops the same 1/3 h.p. in either case and registers the same power on the watt-hour meter. However, while it draws 6 amperes on 115 volts, it takes only 3 on 230 volts.

The importance of minimizing line current is taken up in detail in the chapter entitled "Is Your Wiring Adequate?"

The presence of both 115 and 230 volts in the same house means that special precautions must be taken to prevent 115-volt appliances from being plugged into 230-volt outlets. There is no harm in making the opposite mistake; for instance, a 230-volt air conditioner just wouldn't start on the lower voltage.

The usual safety measure takes the form of power receptacles and matching plugs having oddly spaced connectors, quite different from the ones used with ordinary 115-volt lamps, irons, vacuum cleaners, etc.  These fittings are  generally of the "crow-foot" or "tandem-blade" type, and are treated in detail in the section of this book entitled "The Third Wire Is a Life-saver."

To distinguish further between 115- and 230-volt outlets, it is the practice in some areas to paint the latter bright red.

Three-wire installations are standard in homes having all-electric kitchens. There is usually a separate heavy-duty 230-volt line from the meter directly to the range, which can easily take as much as 50 or 60 amperes even at this higher voltage. The range alone thus represents three or four times as much power as required for the entire rest of the house.

The Four-Wire System

The wave form shown in Figure 1 is a picture of AC as it would be generated in a simple, basic alternator. This is called "single phase" power because only one build up of voltage in each direction takes place during the 1/60 second of a complete cycle. In actual practice the large stations do not generate single phase power, but what is called "polyphase" or "three-phase" power. From it single phase circuits are derived as needed.

Until recently polyphase power was used exclusively for industrial purposes where motors of one horsepower or more constituted the major part of the load. With the increasing use of central air conditioning in the home, the power companies are putting polyphase circuits into many new residential districts. These conditioners run to 2, 3, 5 and even more horsepower, and represent little "industrial" installations all by themselves.

The voltage step-down transformers required in new residential areas are too heavy to be hung on poles. Instead, it is generally found necessary to place these bulky elements in nearby underground vaults.

Figure  5:  Three-phase  power generated  by most large generating stations. Curves A. B, C are of three separate voltages which flow in same circuit 1/180-second apart. This type is for large motor operation.
If you have a somewhat older house and are considering the idea of modernizing its wiring, installing a big air conditioner to work through existing heat ducts, etc., by all means consult your local utility. The company's engineers know exactly what kind and size of power lines are available in the neighborhood, and will tell you whether a three-wire or a four-wire system will best serve your needs. This advice is free, as the company is glad to have you increase your use of electricity!

Look at Figure 5. The wave form or curve marked A is the same as that of Figure 1. It represents the voltage developed by one set of windings on the rotating alternator. As before, the first alternation is completed in 1/120 second, the complete cycle in 1/60 second. In an actual machine, there is not one but three sets of identical windings, separated 120 degrees or ½ of a revolution. As the alternator starts to turn from its theoretical dead starting point, the first winding starts to generate the voltage A and continues to do so as the motion continues. A scant 1/180 second after the starting time, the second winding comes into play and generates the voltage B, which is exactly like A. While voltage A is building up the second alternation of its first cycle, the third winding comes into play at the 1/90 second point, and generates the voltage C. This is a replica of its predecessors A and B. With the alternator turning over steadily, power is delivered THREE times during each cycle of 1/60 second duration, instead of only once. For motor operation, three-phase supply has the same advantage over single-phase supply that multiple-cylinder gasoline engines have over single-cylinder jobs. The torque is smoother, and the motors themselves are simpler in construction and more efficient in operation.

The three windings on the alternator are in effect three separate generators, and can be connected in a number of very complex ways. Of course, the voltages can be stepped up or down in any desired fashion.

As it reaches the home, three-phase power takes the form of a four-wire cable from the street. People with some practical knowledge of electricity and radio are invariably confused by this wiring the first time they see it. They usually know about three-wire, 230-volt service, but they can't figure the four. The arrangement is really quite simple. See Figure 6. One of the wires again has white insulation, and as you might expect by now, is the grounded neu-tral All the other three wires have red covering. Between the ground wire and each of the red wires you pick off 120-volt single-phase power for the usual 115-volt household machines and appliances. Each branch circuit has its own fuse. The big air-conditioner motor, a three-phase unit, is connected by three leads to the three red wires only, through its own set of three fuses. Because of the tricky interweaving of the three phase voltages, a separate return or neutral wire is not needed with a motor load. The voltage between any two red wires of the three-phase circuit is 208 volts when the voltage between ground and any one red wire is adjusted to 120 volts; repeat: 208 volts, NOT 230 or 240. Both the starting and running current of a three-phase motor are relatively low. In an actual installation represented by Figure 6, the air conditioner goes on and off without having the slightest effect on the house lights or anything else connected to the single-phase branch circuits.

Figure 6: Basic four-wire. three-phase power system, with the watt-hour meter omitted for the sake of simplicity. The 115-volt circuits displayed here are  distributed  between   ground  and  red  wires.

Three-Wire. 208-Volt System

In some residential districts of some of the major cities of the United States, the only power distribution system is of the four-wire, three-phase type previously described. The conventional 115/230-volt, three-wire system might not be available at all. A builder of medium-priced homes, which do not have central air-conditioning as original equipment, might not elect to bring the full four-wire service into them. Instead, he will ask for basic "115-volt" service, and let the buyers of the houses worry about the operation of air conditioners, driers, etc., that they might buy later.

Actually, what the power companies then install is three-wire service from the four-wire facilities on their street poles. One wire is the usual common ground; the other two are random pairs of the outside "phase legs." Neighboring houses are on staggered legs, so that the load is distributed over the wires and not concentrated on any one pair. See Figure 7.

Between the ground and either phase leg the voltage is a full 120, and this is led through the house to operate the usual "115-volt" lamps and appliances. Again, the lines are staggered between the common ground and the two phase wires, to distribute the load.

Now comes the joker: The voltage across the two outside wires is not 240 volts, as it would be in an ordinary three-wire system if each half measured 120 volts; IT IS 208 VOLTS, just as it was in the full four-wire system as detailed on the opposite page. Even some professional electricians are fooled; they see three wires and immediately say, "That's 115/230." It takes a voltmeter test and a call to the utility company to convince them that the voltages are 120 and 208.

Are there 208-volt appliances? Certainly. All you have to do is go and ask for them.•

Figure 7: In this three-wire system, derived from four-wire. three-phase supply, single-phase power at 120 and 208 volts is available in an average house.

Triple fuse block feeds three-phase power to large air conditioner in house having hill four-wire service. Three wires on Ie(t go only to conditioner. White wire and dark wires on right go to house circuits, as shown in Figure 6 on opposite page.
 

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