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Electricity 101: Basic Fundamentals

Electricity 101: Basic Fundamentals

The purpose of this Info-Tec is to help you understand the fundamentals of electrical systems.  Many problems encountered in service work are electrical problems or related to electrical problems.

There are two types of electrical current in common use today:  alternating current (AC), and direct current (DC).

AC current is produced by all electric utilities.  AC is very "flexible".  Its voltage can be easily stepped up or down by transformers.  AC can be converted to DC for final use through rectifiers or solid-state devices.

DC current flows always in the same direction.  AC current flows first in one direction, then reverses and flows in the opposite direction.  The current flow "alternates", hence alternating current.  When AC changes direction, it does not jump from full value in one direction to full value in the other direction.  It builds up gradually to maximum, drops off gradually to zero, then repeats this in the opposite direction.  See Figure 1.

Figure 1.

One of the periods of buildup and back to zero in one direction of flow is an alternation.  Two alternations, one in one direction and one in the opposite direction is a cycle.  AC current in this country is generated at 60 cycles per second.  Older equipment will be identified in cycles (CY) and newer equipment in Hertz (HZ).  CY and HZ mean the same thing.  (The CY was changed to HZ to honor a German physicist, Heinrich Hertz, who unraveled the mystery of cycles in AC current.)

Electricity has two characteristics:

1.         Voltage.  Also called "potential" or electro-motive force (EMF).  It is the "pressure" of electricity.  Electricity does not have to be flowing to have voltage.  If a voltmeter is connected to a "live" circuit, it will show voltage whether the circuit is connected to a load or not.  This can be compared to water in a pipe and a pressure gauge.  Voltage is the measure of electrical "pressure" or potential.

2.         Amperage.  This is the "rate of flow" of the current.  It's the "gallons per minute".  If water is flowing in a pipe, there is resistance to its flow.  There will be a pressure drop from one end of the pipe to the other, depending on the size of the pipe, length of the pipe, and the flow rate.  The same situation applies to electricity.  Wire offers resistance.  The smaller the wire, the longer it is, the amount of electricity (AMPS) it is carrying all determine the pressure drop, or in electrical terms, voltage drop.

Another German Physicist, G.S. Ohms, developed a formula known as "Ohms Law".  He found that voltage is the product of amperes and ohms (resistance measure he named after himself) in a resistive circuit.  Symbols were established to describe these values.  E for voltage, I for current, and R for resistance.  Therefore, voltage equals amperes times ohms:  E = I x R.  This is a cornerstone formula on which knowledge of electricity is built.  All the values in Ohms law can be calculated in relation to remaining values.  As an example, amps equals voltage divided by resistance, or:

                                                    I  =  E

                                                           R

An easy way to remember the different mathematical forms in which Ohm's Law is expressed is in Figure 2.  If any one of the three symbols is covered, the two left uncovered are in the proper form.  For example;

E, if covered, equals       I x R

I, if covered, equals         E

                                        R

R, if covered, equals        E

                                         I

Figure 2.

Alternating current circuits are grouped into two main classes:  Single-phase (SF) and three-phase (3F).  There is two phase, but its use is so minimal it will not be discussed.

If there are only two wires in a circuit, it must be single-phase (not counting DC which we are not considering at this time).  If there are three wires in a circuit, it may be three-phase or single-phase!  It's easy to identify three wire single-phase from three wire three-phase.  In single-phase service, two are hot wires and one is a neutral.  In the usual residential or light commercial building that has three wire 115/230 volt service, the two hot wires enter the service cabinet and are fused, or use resetable overloads, but the neutral passes through the cabinet without any switch or fuse.  This type of entrance is single-phase even though there are three wires.  It is really a two wire, 230-volt, single-phase circuit with a neutral.  Two or more 115 volt circuits can be taken from it and one or more 230-volt circuits.  If a voltmeter is put between either of the hot wires and the neutral, it will read 115 volts.  Between the two hot wires, it will read 230 volts.  See Figure 3.

Figure 3.

Figure 3 is what a typical 3-wire 115/230V entrance switch would look like.  (Note:  In some cases of old systems, the switch may be a three-pole switch, and open the neutral.  These are unsafe and should be changed).  Three wire, three-phase systems are usually distributed only to industrial and large commercial areas. As its name indicates, three-phase current has three currents flowing in the three wires.  In ordinary three-phase, there is no neutral; all three wires are hot.  Between any two of the three wires is single-phase current, but never 110, 115, or 120 volts.  In general, the voltage of each phase will be either 208, 220, 230, 440, 550, or higher.

The three-phase currents follow one another a third of a cycle apart.  See Figure 4.

Figure 4.

If a voltmeter is used to test three-phase, full voltage will be found between any two of the three wires.  A little over half voltage will be found from any one wire to ground.

In Milwaukee, WEPCO used a 230-volt only, grounded, three-phase system.  They do not use it anymore, but many of these "grounded B phase" systems were installed (Climatic Control Company is serviced by a three-phase grounded B phase system), and many still exist.  In this three-phase system, one of the phases is grounded (the ground leg).  It is easy to mistake this system for three wire, 115/230 volt, and single-phase since the entrance will have a two-pole switch with two fuses and the third line solid.  Testing with a voltmeter will show the difference.  Even with the grounded phase, the voltage between any two of the three wires will show 230 volts (the grounded B phase system was always 230 volts).  If each wire is checked to ground, two legs will show full voltage and one leg will show 0 volts.  This is the ground leg.  Remember, in the 115/230-volt, single-phase system, there will be 115 volts between neutral and the hot wires and 230 volts between the two hot wires.

Three-phase circuits are not designed for 115 volt single-phase use.  While about 115 volts can be obtained from a hot leg to ground, using this circuitry is forbidden in electrical codes.  It is a dangerous practice.

There are three basic three-phase distribution systems in use today.  Small commercial buildings and some small industrial plants that have about 50 percent of the electrical load as 120 volt single-phase will have 208/120 volt, three-phase, four wire system.  See Figure 5.

Figure 5.

There are three hot lines (A, B, and C) along with a neutral (N), which is grounded.  Single-phase 120-volt loads are fed from line-to-neutral (C to N, A to N, or B to N), and three-phase 208 volt by lines A, B, and C.

Figure 6.

Figure 6 is a diagram for a 480/276-volt, three-phase, four-wire system.  This system serves hotels, shopping centers, etc.  Transformers are used to get 120-volt single-phase circuits.

Figure 7.

Figure 7 shows the system used for large industrial plants where most of the load consists of motors.  It is a 480-volt delta three-phase system.  This system uses transformers to obtain 120/240-volt requirements.

The terms "grounding" or "grounded" and "ground" can be confusing.  "Grounding" is connecting a wire, strap, or other conductor, from the metal enclosure around the piece of electrical equipment to a water pipe, buried plate, rod, or other conducting material in contact with the earth.  This is referred to as having "grounded" the equipment.  This is done for safety and to eliminate or reduce interference (RFI).

The term "ground" has another meaning.  When, for whatever reason, current gets through or around insulation to exposed metal parts that then become hot or "live" is referred to as a "ground".  "Grounds" can be avoided by good equipment design and regular maintenance.  Grounds do happen and can be dangerous.  Equipment should be protected by "grounding" it.

Low voltage is always caused by the wiring or transformer not being large enough to supply as much current as the loads or loads demand.  When current is flowing in a wire, there is always some voltage drop.  It may not be enough to affect operation of equipment, but some voltage drop always exists.  It does no good to test for voltage drop in a circuit unless all the loads in that circuit are turned on.  Splitting the loads or adding more branch circuits can usually correct overloaded circuits causing low voltage.

If low voltage is experienced at the service entrance, it may be the electric utility's fault.  Demand for electrical services has increased faster than utilities have been able to increase their services.  In some cases, utilities have been known to set up the taps on their transformers to give higher secondary voltage to offset voltage drops during periods of heavy demand.  In these areas, as loads drop off, the voltage at the individual service entrance may increase a great deal above normal.  This results in over voltage problems.  Over voltage causes motors to overheat, capacitors to burn out, and greatly shorten the life of light bulbs, resistance type loads, and will wreak havoc with solid-state devices.

In addition to the usual 120-volt circuits, 230-volt appliance circuits, and three-phase circuits, almost all buildings will employ one or more "low voltage" circuits.  Low voltage circuits are any circuits under 30 volts, usually 24 volts.  24 volt circuits are usually control circuits.  The amount of current in these systems is usually small, under 5 amps.

Since the voltage and amperage is very low, wiring can be much smaller and therefore much cheaper to install than "line" voltage wiring.  Low voltage is also much safer. 

Just as pressure of electricity is measured in volts, and the rate of current flow measured in amps, the power is measured in watts.  One watt is the power produced by one volt at one amp.  Watts is found by multiplying volts times amps.  With constant DC current it is simple to find wattage.  It is volts times amps.  With AC current, however, the voltage and amperage vary in the cycle.  (Remember, first it is 0 then up to maximum in one direction, then back to 0 and maximum in the other direction.  The effective voltage and amperage will be less than the maximums.  Effective values are called "Root - Mean - Square", or RMS.  RMS is equal to .707 times the maximum values.  Therefore, in a 120-volt, 10 amp, AC circuit, the actual maximum values are almost 170 volts and just over 15 amps.  Volts times amps equals watts is true only in DC circuits and pure resistive AC circuits, such as heaters.

Perhaps the outstanding advantage of AC over DC current is that AC can be stepped up or down easily, with little loss, by using transformers.

It is well known that you can't get something for nothing.  The output of a machine will be in some proportion to the input.  More must be put in than is taken out, for some of the input is lost and used up by the machine.  The efficiency is the energy out divided by the energy in.  A transformer is a machine without moving parts.  It is very efficient:  98 to 99 percent efficient.  The 1 or 2 percent lost amps or watts.  If the secondary of a transformer is 24 volts and it is a "40VA" transformer, the current (amps) draw can be 1.667 amps before an overload occurs.

Three-phase transformers are single-phase transformers connected together.  One method of connecting the three coils is known as "delta" and the other is "star " or "Y".  Normally the currents flowing in each of the three wires of the three-phase are equal.  Voltage drop in each phase is the same, and a voltmeter should show the same voltage at a motor's terminals for all three-phases.  See Figure 8.

Figure 8.

If the voltages are unequal, some of the causes may be:

   A single-phase circuit is tapped off one of the three-phases.

   A partial ground or short in the motor windings.

   Pitted or burned contacts in a motor starter contactor.

   Corroded terminals.

   Loose wires.

   Always find the cause and correct three-phase imbalance.

If a fuse blows, breaker trips, or a wire comes loose, anything that causes one phase of a three-phase circuit to "open", only one phase is left--not two!  This is known as "single-phasing".  If this happens with a motor running, it may continue to run if the load is light.  If it is completely unloaded, it may even start.  In any event, if it is not quickly disconnected, it will burn out.  Three-phase motors are usually expensive, and investing in an under voltage, over voltage, phase loss, phase unbalance protector is cheap insurance.  Fuses protect against shorts and grounds and are not designed to protect motors as specific motor overloads.  In the majority of cases of excessive voltage drop, single phasing, unbalanced phases, the case is usually on the user's premises.  The business has grown and many more or larger motors have been added along with other devices, all adding to the load of the original service.  Before calling the utility, make sure the troubles are with the utilities.  It is up to the user to upgrade the service to meet current requirements.