Electrical Module Section 1.

ELECTRICITY, ITS CHARACTERISTICS, AND some basic physics.

 

The Greeks were the first to discover electricity about 2500 years ago. They noticed that when an amber was rubbed with other materials it became charged with an unknown force that had the power to attract objects such as dried leaves, feathers, bits of cloth, or other lightweight materials. The Greeks called amber electron. The word electric was derived from it and meant "to be like amber," or to have the ability to attract other objects.

This mysterious force remained little more than a curious phenomenon until about 2000 years later, when other people began to conduct experiments, including Benjamin Franklin, one of the founders of the United States.

 

The atom is the basic building block of the universe. All matter is made from a combination of atoms. Matter is any substance has mass and occupies space. Matter can exist in any of the three states: solid, liquid, or gas. Water, for example, can exist in the form of ice, as a liquid, or as a gas in the form of steam. An element is a substance that cannot be chemically divided into a simpler substance. An atom is the smallest part of an element. The three principle parts of an atom are the electron, neutron, and the proton. It is theorized that protons and neutrons are actually made of the smaller particles quarks.

The proton has a positive charge, the electron has a negative charge, and the neutron has no charge. The Neutron and proton combine to form the nucleus of the atom. Since the neutron has no charge, the nucleus will have a net positive charge.

 

The law of charges states that opposite charges attract and like charges repel. For example, two objects that contain opposite charge are attracted to each other. The two positively charged objects and two negatively charged units repel each other. The reason for this is that lines of force can never cross each other. The outward-going lines of force of a positively charged object combine with the inward-going lines of force of a negatively charged object. This combining produces an attraction between the two objects. If the two objects with like charges come close to each other, the lines of force repel. Since the nucleus has a net positive charge and the electron has a negative charge, the electron is attracted to the nucleus.

 

The law of centrifugal force is the second law of physics. It states that a spinning object will pull away from its center point and that the faster it spins, the greater the centrifugal force becomes. An example of this would be to tie an object to a string and spin it around, it will try to pull away from you. The faster the object spins, the greater the force that tries to pull the object away. Centrifugal force prevents the electron from falling into the nucleus of the atom. The faster an electron spins, the farther away from the nucleus it will be.

 

The outer shell of an atom is known as the valence shell. Any electrons located in the outer shell of an atom are known as valence electrons. The valence shell of an atom cannot hold more than eight electrons. It is the valence electrons that are primary concern in the study of electricity, because it is these that explain much of electrical theory. A conductor for instance, is generally made from a material that contains one or two valence electrons. Atoms with one or two valence electrons are unstable and can be made to give up these electrons with little effort. Conductors are materials that permit electrons to flow through them easily. When an atom has only one or two valence electrons, these electrons are loosely held by the atom and are easily given up for the current flow. Silver, copper, gold, and aluminum all contain one valence electron and are excellent conductors of electricity. Silver is the best natural conductor of electricity, followed by copper, gold, and aluminum.

 

Electrical current is the flow of electrons. It is produced when an electron from one atom knocks electrons of another atom out of orbit. When an atom contains only one valence electron, that electron is easily given up when struck by another electron. The striking electron gives its energy to the electron being struck. The striking electron settles into orbit around the atom, and the electron that was struck moves off to strike another electron. This same effect is found in the game if the moving cue ball strikes a stationary ball. The stationary ball then moves off with the most of the cue ball's energy, and the cue ball stops moving. The stationary ball did not move off with all the energy of the cue ball. It moved off with most of the energy of the cue ball. Some of the cue ball's energy was lost to heat when it struck the stationary ball. Some energy is also lost when one electron strikes another. That is why a wire heats when current flows through it. If too much current flows through a wire, overheating will damage the wire and possibly become a fire hazard.

 

Materials containing seven or eight valence electrons are known as insulators. Insulators are materials that resist the flow of electricity. When the valence shell of an atom is full, the electrons are held tightly and are not given up easily. Some good examples of insulator materials are rubber, plastic, glass, and wood. The energy of the moving electron is divided so many times that it has little effect on the atom. Any atom that has seven or eight valence electrons is extremely stable and does not easily give up an electron.

 

Semiconductors are the materials that are neither good conductors nor good insulators. They contain four valence electrons and are characterized by the fact that as they are heated, their resistance decreases. Heat has the opposite effect on conductors, whose resistance increases with an increase of temperature. Semiconductors have become extremely important in the electrical industry since the invention of the transistor. All solid-state devices such as diodes, transistors, and integrated circuits are made from combinations of semiconductors materials. The two most common materials used in the production of electronic components are silicon and germanium. Of the two, silicon is used more often because of its ability to withstand heat. Before and pure semiconductor can be used to construct electronic device, it must be mixed or "doped" with an impurity.

 

Voltage (volts) is the force that moves electrons, forcing a current.  Voltage can be compared to a tank of water elevated at a certain height (potential).   If the tank is placed low (low voltage), water will not flow very quickly (low current).  If the tank is raised to a higher location (higher voltage), the water will flow rapidly (high current).

Current (Amperes) is, in simple terms a measurement of how many electrons flow through a device.  In the water tank analogy, current would be water flow rate.

Resistance (Ohms) slows down current flow.  The higher the resistance of a circuit, the lower the current will be.  Resistance would be equivalent to pipe size.  If you have the water tank at a high level, but the pipe is very small in diameter (high resistance), not much water will flow.  If you use a big pipe (low resistance) then the water flow rate will be larger.  Knowing the relationships between voltage, current and resistance brings us to ohm's law:  "Current is proportional to Voltage divided by Resistance".  This equation can be manipulated to obtain any value knowing the other two.   

 

On a DC circuit, current flows in one direction only.   Voltage can remain at a level or change, but it always has the same polarity.   A car's battery produces DC voltage.

AC circuits are a bit more complicated to understand.  The voltage supply reverses its polarity switching from positive to negative.  The current produced goes in one direction while the voltage is positive and then flows in the opposite direction when voltage is reversed.  AC circuits have a frequency associated with them.  The frequency (Hertz or Hz) is how many times per second (cycles) the current (and voltage) switch from positive to negative and back.  The higher the frequency, the faster the circuit will switch polarity. 

 

Electrons flow in a circuit from the negative side of the battery to the positive side of the battery (that is why physicists will argue with the direction of the current in the circuit).  Engineers represent current in the opposite direction of electron flow, as in the diagram.  It does not matter what convention you follow for current direction.  The important thing to keep in mind is how much current flows through the circuit. For a circuit to have current there has to be a path (i.e. wire) and an electrical source.  A circuit also has a resistance, which slows down flow of electrons. If the path is broken, current cannot flow.  Each accessory, or device added to the circuit has a resistance.  As more accessories are added, the resistance drops, and more current flows through the circuit. 

 

While electricity and water are never a good combination, one can use the similarity to the flow of water to help illustrate some basic principles of electricity. There are few things you can do with electricity you can’t do with water. Water is a tangible that we see and feel; so if you understand how water flows, you can also understand how electricity flows through wires.

A common misstatement is that electricity is energy. This is not correct. Water-flow is energy, but water is water, not energy. Whenever something moves or exerts force...that is energy. The substance flowing is not energy, the flow is.

Electricity is a flow of electrons. While you can’t actually see them, electrons are solid particles possessing volume and weight. They flow through wires much the same way water flows through pipes. While I use the simile of water, there is one important difference between water and electron flow. You can’t compress water in a space; you can compress electrons.

There are three primary measurements of water energy: pressure or force (measured in pounds per square inch – p.s.i.), volume or intensity of flow (measured in gallons per minute – gpm), and the resistance to that flow.

Voltage is the measurement of electrical pressure or force. As p.s.i. is a measurement of water pressure, voltage (or volts) is a measurement of Electromotive Force, or EMF.

You may have heard the old saying: Volts won’t kill you, amperage will. This is true because volume of electrical flow is what you feel. Static electricity generated in your body by walking across a carpet can build to several thousand volts, yet it doesn’t knock you dead when you touch a doorknob. This is similar to the water stream from a squirt gun. There is high pressure but little substance. But, a high voltage has the ability to push more electrons through a given resistance.

 

How amperage kills:

 

 

1 mA                           Threshold of perception.

5 mA                           Maximum “harmless” current.

10-20 mA                   Maximum “let-go” current.

50 mA                         Pain, possible fainting, mechanical injury.  Heart and respiratory functions continue.

100-300 mA              Ventricular fibrillation will start, respiratory center remains intact

> 300 mA                   Sustained myocardial contraction. Temporary respiratory paralysis.  Burns if current density is high.

 

As can be clearly seen, it does not require huge amounts of current to cause real harm, or even death from electrocution.

 

More about current flow, voltage, ratings, and measurement:

 

The current flow in an electric circuit is measured in amps or amperes and is the same as gallons per minute (gpm). It is the measurement of the volume of electrical flow. As larger diameter pipes are needed to transport more water, larger wires are used to carry more current. For that reason most electrical devices are rated in amperes.

As no water flows before a valve is turned on, there is no current flow until something is switched on. Similarly, only so much water can flow through a pipe, and only so much electricity through a wire. This is one instance where electrical characteristics are more like airflow. Because you can compress it, you can squeeze more air through a duct. But, any time you try to move excess volume through something that resists flow, it will heat from friction.

Ohm is the measurement of electrical resistance. It is the same as pressure drop in a water piping system. Anything water moves through resists its flow. This is also true of electrical conductors.

Resistance converts energy from one form to another. It reduces both voltage and amperage, creating heat, light, sound, magnetism, and several other waves. Efficient use of electricity or any other form of energy amounts to channeling most of the total power spectrum into the attempted work.

A very important basic relationship among these three units has been established and is called Ohm’s Law. This law states that the current flow is proportional to voltage, but is inversely proportional to resistance. Stated as an equation; V = I x R, where V is voltage, I is current and R is resistance.

Power is the rate of doing work or the amount of work electromotive force and current intensity produce together. Watts is a measurement of power or  (P = V x I).

Direct and alternating current are different forces to provide electrical energy. Electrical flow in DC power is in one direction and electrons flow from negative to positive. A battery is common sources of DC power. Since the battery is produced by a chemical reaction, the voltage produced would not change for a given load. The current flow as a result of this voltage would not change its value.

Alternating Current (AC) is energy similar to the waves on a beach. They are continuously moving in and out, doing a lot of work, but don’t really go anywhere. Alternating current drives electrons one way, then stops and drives them back again. Starting from zero, the voltage rises to a positive peak, returns to zero, rises to a negative peak, then returns to zero again, creating what is known as a sine wave. This is called a single cycle. Sixty cycles per second, also known as 60-Hz, repeats this cycle with 60 positive peaks, 60 negative peaks, and 120 points at which the wave crosses the 0-voltage level – per second. The number of times a voltage goes through its cycle of change per unit of time is called the frequency of the system.

As one can see, AC voltage is constantly changing. It isn’t constant like DC. The rated voltage measured as an average of the peaks and valleys. Since AC electricity flows in both directions, going from positive to negative peaks, there is no positive or negative connection.

If you think of electricity in terms of something that can be seen and felt, such as water pressure, it will help you visualize flow through wires and other devices. Remember that as with water, nothing happens until there is a pressure difference; and nothing significant happens unless there is sufficient flow volume.

The key features of electricity are voltage, current, resistance, power, and frequency.     

 

An electrical current is the flow of electricity around an electrical circuit. The flow of electricity follows similar principles to the flow of water in pipes, as we shall see, with the exception that an electrical system must make a complete circuit.

    

In domestic electrical work, current is generally measured in amps. Currents you will encounter in practice range from about 0.5 amps (through a light bulb) to about 40 amps. Technically `amps' is short for `Amperes'. The mathematical symbol for current, as it is written in calculations, is not `C' (for current) or `A' (for amps) but in fact `I'. This is just because the symbols `C' and `An' are reserved for other things. You will occasionally come across currents measured in milliamps (`mA' for short). A milliamp is a thousandth of an amp.

    

To get an electrical current to flow, we need a power source, and some sort of conductor. A conductor is defined as anything that can carry a flow of electricity. In electrical practice, conductors tend to be copper wire or copper bars, usually hidden away inside plastic sleeves. The sleeves are insulators, that is, materials that prevent the flow of electricity. It is the insulator that keeps the electrical current where it belongs - inside the cable.
    

In the US, electricity is distributed around the country in the form of alternating current. This means that the flow of electrical current changes direction, 60 times per second. There are two reasons for this, both historical. First, electrical transformers (which we need to change voltage, see below) only work with alternating currents. Second, we generate electricity by spinning wires around inside magnets (this is a bit of a simplification, of course), and this naturally produces an alternating current. At the points where the current is about to change direction, there will be (for a short time) no current flowing at all. `Alternating current' is usually abbreviated to `AC'.
    

The fact that current is alternating has little practical impact on domestic wiring. If you grab a live conductor you'll get a shock which is just as unpleasant even though, in principle, part of the time no current will be flowing. One area where the alternating nature of the electrical supply is apparent, however, is in the use of fluorescent lights. Incandescent (filament) bulbs generate their light because the filament becomes white-hot. It cannot heat up and cool down as fast as the alternation of the electrical current, so the light is fairly constant. Fluorescent lights, on the other hand, produce a detectable flicker at the speed of the supply alternation. The light from a fluorescent tube will `pulse' about 120 times per second (60 times with the supply current in one direction and 60 in the other). We can't normally see this flicker, but it does tend to make rotating machines look as though they're standing still, or going backwards.   

 

Voltage is a measure of the strength of an electrical supply. A voltage may exist even when no current is flowing. In older textbooks you will find terms like `electrical potential' or `electro-motive force', which gives a better feel for what voltage means. Strictly, a voltage is only defined between two points. When only one point is specified, we tacitly assume that the other point is the earth (which means exactly what it says: the ground beneath our feet). The earth is not a very good conductor of electricity, but there's an awful lot of it, which makes up for this to a certain extent. So when a person says `there's 120 volts at this point', what they really mean is that the voltage difference between this point and earth is 120 volts (it's a bit more complicated than this in practice, as we shall see).
    

Voltage is measured in volts, which is abbreviated to `V'. So `220V' means `220 volts'. The mathematical symbol for voltage is also `V'.

To get an alternating current, we need an alternating voltage. So one leg of an electrical main may cycle from about 120 volts, to zero, to -120 volts, then back to zero, and so on, 60 times per second. 

 

 

We have already mentioned electrical materials, which are conductors (that allow an electrical current to flow easily) and insulators (that don't). In reality nothing is a perfect insulator or a perfect conductor: most materials have a certain degree of resistance, and lie on a scale somewhere between a perfect insulator and a perfect conductor. Materials with high resistance tend to be insulators; those with low resistance tend to be conductors. Even copper electrical cables have a certain amount of resistance. Resistance is measured in ohms. One ohm is a lot of resistance in electrical practice; we normally like our electrical conductors to have resistances much less than an ohm, for reasons that will be explained later.

 

Voltage, current and resistance - are related. It turns out that the voltage can be found by multiplying the current (in amps) by the resistance (in ohms). In symbols this is

V = I R.

This simple formula is, in fact, `Ohm's law', and is probably the most important thing ever discovered in electrical engineering. In domestic wiring, `V' will nearly always be 120 or 240 (volts), so in practice we usually want to work out current (knowing resistance), or vice-versa. We can write Ohm's law in two different ways:

I = V / R and/or R = V / I

 

As an example, a light bulb having a filament with a resistance of 500 ohms at running temperature, using 120 volts, draws approximately ¼ amps. Here’s how it’s calculated: I = V / R and V is 120, and R (resistance) is 500, then I is 120/500, which is 0.24 amps. 

 

The main difference between an electrical system, and the water system, is that electrical current must flow in a circuit. Electricity can't form a puddle in the same way that water can; it has to be confined to conductors. So in some senses a better analogy might be a central heating system, where water flows around a set of pipes and radiators, driven by a pump. In any event, if a circuit is not complete, no current can flow. This is good, because it means we can uses switches to turn things on and off. Traditionally a switch is a mechanical contact: pressing it or moving the lever moves a piece of copper in such a way as to open or close a circuit. It is now possible to get electronic switching devices that have no moving parts.
    

A practical electrical circuit consists of at least the following things: a power source, some conductors, and an electrical appliance.

 

In a domestic system, the `power source' is essentially the wires that bring the electrical supply into the house (and all the power stations, etc., that they're connected to). Since we don't have any control over that, we can usefully think of it as a 120-240 volt power source without worrying too much about it.
    

This circuit will power the appliance (whatever it is) and, because there is not even a switch, it will continue to power it forever, or until the power runs out. As we are dealing with alternating currents, the flow of current around the circuit is constantly changing direction (but this does not cause any problem, as discussed above).
    

Suppose a person wanted to connect two appliances in this circuit (after all, a house with only one light bulb isn't going to be much use). How are they to accomplish this? Parallel circuits, or wiring is the answer. 

    

In nearly all domestic wiring, branch circuits are wired in parallel. Why? Because in a parallel system all appliances get the same voltage across them. In the US this means the 120 volt or 240 volt main feeds. Each appliance will have a particular resistance, and therefore get a current, which is appropriate for its needs.

 

So, think of the main electrical panel as one big parallel circuit, because it is. More on this later.

 

Power is the rate at which an electrical appliance can consume electrical energy, or the rate at which a generator can produce it. In the US we are charged for our electricity in terms of energy: the more energy we use, the more we pay. A high-power appliance uses energy more rapidly than a low-power one, and therefore costs more to run.
    

Power is measured in watts, or in kilowatts. A kilowatt is a thousand watts, and is a more useful figure when dealing with electric fires and heaters. The abbreviations are `W' (for watts) and `kW' (for kilowatts). Note the positions of the capital letters here. It is technically incorrect to abbreviate kilowatts to `KW' (although plenty of people do, including electricity supply companies).
    

The mathematical symbol for power is `P'.
    

If we know the voltage and current in an electrical appliance we can work out its power. It turns out that power (in watts) is equal to the voltage (in volts) multiplied by the current (in amps). In symbols this is:

P = V I

 

So, taking the ligh tbulb case again, its current (as we worked out earlier) was 0.5 amps, the voltage is 120 volts, so the power is 60 watts (0.5 x 120.  The formula can also be written in two other ways:

V = P / I and I = P / V 

 

Electricity, current flow, resistance, and heat buildup: very Important concepts!

 

A light bulb converts electrical energy into light and heat. A filament bulb is very inefficient, in fact, producing about 50 times more heat than light. In fact all electrical equipment gets hot in use, including wires. The amount of energy that goes into heat can always be calculated if we know the voltage and current, but for electrical cables it's easier to do it a different way. Since we know that V = I R (from above) and that P = V I, then a bit of juggling symbols shows that

P = I² R, or in words: power is given by multiplying the square of the current by the resistance. (The square of anything is that number multiplied by itself). Let's take an example.     

Suppose an electrical cable had a resistance of 2 ohms. This cable is carrying a current of 13 amps (which is the maximum allowed for a plug-in appliance). How much power is turned into heat by the cable?     

Power is given by the square of the current times the resistance, so in this case is 13 x 13 x 2, which is 338 watts. That's about the same as three light bulbs. So the electrical cable will get about as hot as three light bulbs. Apart from being a complete waste of energy (which you're paying for), this may be enough heat to melt the cable, which would be a Bad Thing (especially if it's underground). This explains why we need fat cables for high-power appliances and can get away with thin cables for low-power ones. Fat cables have lower resistances, and therefore less energy is wasted as heat, and they don't get hot enough to melt. Is it all right to use fat cables for low-power appliances? Well, it doesn't compromise safety, but it's not very cost-effective. Thick cables are much more expensive than thin ones, and are much harder to work with than thin ones. 

 

Electrical engineers and utility companies measure electrical energy in kilowatt-hours. One kilowatt-hour, which is the same as 1000 watt-hours, is sufficient energy to power a one kilowatt appliance for one hour. Energy of 1 kilowatt-hour may be consumed by an appliance that takes 1000 watts running for 1 hour, or an appliance that takes 1 watt running for 1000 hours, or an appliance that takes 100 watts running for 10 hours, or anything in between so long as the time multiplied by the power comes to 1000.
    

The electricity bill does not distinguish between high-power and low-power appliances, only the total energy. You will normally be charged a certain amount for each kilowatt-hour of energy, plus a certain fixed amount, in each bill. 

   

Three main electrical conductors enter a domestic property, and are distributed throughout it. These conductors are referred to as live, neutral and ground.

The live and neutral conductors should be considered as the `power supply' to the premises. The voltage between live and neutral will generally be about 120 V AC, per leg. In all normal circumstances, current that enters the premises on the live conductor leaves it on the neutral, and vice-versa. The earth conductor carries negligible current except in fault situations.
    

Although the live and neutral conductors both carry current, only the live conductor is at a voltage that could be harmful. The neutral conductor will normally be at the same voltage as the earth conductor. In fact, at some point the neutral and the earth will be connected together. This situation is shown in the following diagram:     

The origin of `live', `neutral' and `earth' conductors in a domestic premises

 

This diagram really oversimplifies the actual situation; of course, as we don't each have an electrical power station in the backyard, delivering electricity at 120 and 240 volts. In reality, the utility company’s distribution system will be a complex mixture of cables, transformers, sub-stations, and power plants.

 

The utility company’s equipment is connected to earth at one side, and this is what distinguishes `live' from `neutral'. The `neutral' side is connected to earth at the utility’s side. A dwelling will also have an earth connection. In fact, modern installations will most likely have three ground sources, including a conductor brought in from the power company, an attachment made to the water main (usually on both sides of the water meter), and a driven ground rod.

 

From the main cable entering the premises, live, neutral and earth conductors will be distributed to every electrical appliance using a variety of different cable types and sizes.

 

We have seen that electrical current causes a heating effect, and if the current is large, or the electrical resistance high, this effect can be enough to cause damage or a fire. Fire, as a result of overheating, is one of the main risks from sloppy electrical work. However, the tendency of a high current to cause a wire to melt and break is put to good use in the design of the fuse or circuit breaker.
  

A fuse is a simple device that will limit the current flowing in an electrical circuit. In practice a fuse normally consists of a piece of wire of exactly the right length and thickness to overheat and break when the current gets to a particular level. If an excessive current occurs, we hope the fuse will `blow' rather than some other part of the circuit overheating. This is called over-current protection.
    

For now, the important thing to remember is that the fuse must be able to withstand a higher current than the appliances to which it is connected (otherwise it would blow unnecessarily), but a lower current than the cables, which connect them. This ensures that in the event of a fault, the fuse will blow before the cable is damaged.
    

While fuses are still widely used, circuit breakers are now in place in a majority of modern residential installations.
    

On the whole it is moderate overheating that is the problem in electrical wiring, not huge current overloads. If you get a short-circuit between live and neutral in a power outlet, for example, the current that will flow will be immense. Without a fuse it could easily rise to thousands of amps.

Now, although this would be inconvenient, oddly enough it probably wouldn't be all that dangerous, because the cable will simply melt right through in a fraction of a second and break the circuit. There would be an enormous bang and a puff of smoke and that would be the end of the problem. It will be the beginning of your hard work, of course, as you struggle to find which floorboard the burned-out cable is under, but that's a different matter.
    

On the other hand, if you ask a cable that is rated for a maximum of 15 amps to carry a current of 50 amps, and you have a 50-amp circuit breaker behind it, then you get no over-current protection at all. The cable probably won't fail with a huge bang: it will gradually heat up to about 250 degrees Celsius, at which point the copper will melt. However, it may take tens of minutes to do so. In the meantime, you've got something that is hot enough to combust wood clamped to your joists. See the problem?

This is an example of a common problem that home inspectors see. Many times, it is the result of a homeowner increasing the ampacity of a circuit, by upgrading the rating of the circuit breaker. When a homeowner changes a 15 amp breaker, for a 20 amp breaker, he may not be looking to see if the branch conductor is rated for the increase. A 14 gauge copper conductor is rated for 15 amperes, not 20. A 12 gauge copper conductor is rated for 20 amperes. When that 14 gauge copper conductor is connected to the 20 ampere circuit breaker, you have created a situation where overheating can occur. Another common misconception occurs with aluminum branch circuit wiring, when homeowners would see a #12 aluminum conductor. Unlike #12 copper conductors, a #12 aluminum conductor is only rated for use on 15 amp circuits. A #10 aluminum conductor is what is required on 20-ampere circuits.

 

This concludes Section 1. With this basic knowledge, the Inspector has a solid foundation from which to build from.