Detailed description of electricity.

Electricity and testing questions

I have written these notes and questions to assist you to quickly learn the basics of appliance testing.

It is important that you read and understand the basic electricity and safety information that follows.

This will give you a much better understanding of the appliance testing procedures.

Until you have a good understanding of electrical safety and first aid for accident victims, do not attempt any electrical work.

After you read each section, please answer the section questions on the answer form to assist your understanding.

This information will be continuously updated to improve our understanding of electricity and appliance testing.

Links to other sites and pages for more information are included in the text.

There is a lot of information here, and you will need to refer back to it on a regular bases.

Any questions, suggestions, and corrections are appreciated by email bruce@3760.co.nz or urgent matters by phone

0800 66 99 99. I will respond ASAP.

Overview

Electricity is the movement of electrons through a conductor. As electrons move, they cause heat and magnetism. We use these properties in appliances for our benefit.

The electrons are controlled in appliances by insulation (to keep the electrons in) and resistance (to limit the flow through the appliance). Remember that air is also an insulator.

If the electrons get out of the appliance, they could give a person a shock or cause a fire.

If the flow is too great, too much heat will be produced causing a fire danger.

Inspecting and testing appliances is all about keeping the electrons in, and turning the flow off, if they get out.

To help understand electricity you can think of it as water in a hose.

The flow or current is the movement of water. The pressure to move the water is the voltage. The hose is the insulation to keep the water (electrons) in. A hole in the hose will let the water out (leakage). A tray under the hose will catch the leak - (earthing).

Questions to help you improve your knowledge.

I suggest spending one hour at a time, studying and answering the questions until finished.

Please use this answer form and email it to me with your answers as you do them.

I will add comments to help with your understanding and email it back.

1 . What is Electricity?

2. What are the two main dangers?

3. Why do we test?

Warning

We use electricity in many ways, every day without a thought of electrical safety.

Electricity cannot normally be seen, heard, smelt, or sensed without the use of instruments to indicate its presents. This invisible property is what makes it so dangerous. The two effects that are most often dangerous are electric shock and fire.

Other dangers include burns from hot electrical equipment, radiation in various forms, mechanical damage from motors or other electrical equipment, tripping over leads etc.

When working in the electrical industry you are exposed to these dangers more often than other people. This requires you to have a good understanding of how to avoid accidents with electricity. It injures and kills people every day. To understand and avoid the dangers requires a good understanding of electricity.

We will cover electrical safety in depth after a brief lesson in basic electricity.

4. Why is electricity so dangerous?

5. List the dangers of electricity.

Basic Electricity

This is an abbreviated and modified version of the full text which can be seen at www. Lessons in electric circuits

Conductors, insulators, and the flow of electrons

All materials are made up of tiny "building blocks" known as atoms.

All atoms contain particles called electrons, protons, and neutrons.

Electrons can be dislodged from atoms much easier than protons or neutrons.

The electrons of different types of atoms have different degrees of freedom to move around. With some types of materials, such as metals, the outermost electrons in the atoms are so loosely bound that they are free to move between different atoms. In other types of materials such as glass, the atoms' electrons have very little freedom to move around. They do not move between atoms within that material very easily.

This relative mobility of electrons within a material is known as electric conductivity.

Materials with many free electrons, are called conductors, while materials with few or no free electrons, are called insulators.

Here are some examples of common

Conductors Insulators:

Gold Glass

Silver Mica

Copper Rubber

Aluminium Bakelite

Iron Cotton An atom

Steel Paper

Brass Plastic

Bronze Wood

Lead Air

Tin Oil

Mercury Fibreglass

(Dirty) Water (pure) Water

While the normal motion of "free" electrons in a conductor is random, with no particular direction or speed, electrons can be influenced to move in a coordinated fashion through a conductive material. This uniform motion of electrons is what we call electricity, or electric current. Just like water flowing through the emptiness of a pipe, electrons are able to move within the empty space within and between the atoms of a conductor. The conductor may appear to be solid to our eyes, but any material composed of atoms is mostly empty space! As each electron moves uniformly through a conductor, it pushes on the one ahead of it, such that all the electrons move together like links in a chain. The starting and stopping of electron flow through the length of a conductive path is virtually instantaneous from one end of a conductor to the other, even though the motion of each electron may be very slow. A good analogy is that of a tube filled end-to-end with marbles:

The tube is full of marbles, just as a conductor is full of electrons that are ready to be moved along by an outside influence. If a single marble is suddenly inserted into this full tube on the left-hand side, another marble will suddenly exit the tube on the right. Even though each marble only traveled a short distance, the transfer of motion through the tube is virtually instantaneous from the left end to the right end, no matter how long the tube is. With electricity, the overall effect from one end of a conductor to the other happens at the speed of light: a swift 186,000 miles per second!!!

If we want electrons to flow in a certain direction to a certain place, we must provide the proper path for them to move, just as a plumber must install piping to get water to flow where he or she wants it to flow. To facilitate this, wires are made of highly conductive metals such as copper or Aluminium in a wide variety of sizes.

Remember that electrons can flow only when they have the opportunity to move in the space between the atoms of a material. This means that there can be electric current only where there exists a continuous path of conductive material providing a conduit for electrons to travel through. In the marble analogy, marbles can flow into the left-hand side of the tube (and, consequently, through the tube) if and only if the tube is open on the right-hand side for marbles to flow out. If the tube was blocked on the right-hand side, the marbles would just "pile up" inside the tube, and marble "flow" could not occur. The same holds true for conductors: the continuous flow of electrons requires there be an unbroken path to permit that flow. Let's look at a diagram to illustrate how this works:

A thin, solid line (as shown above) is the conventional symbol for a continuous piece of wire. Since the wire is made of a conductive material, such as copper, its constituent atoms have many free electrons, which can easily move through the wire. However, there will never be a continuous or uniform flow of electrons within this wire unless they have a place to come from and a place to go. Let's add an hypothetical electron "Source" and "Destination:"

Now, with the Electron Source pushing new electrons into the wire on the left-hand side, electron flow through the wire can occur (as indicated by the arrows pointing from left to right). However, the flow will be interrupted if the conductive path formed by the wire is broken:

Since air is an insulating material, and an air gap separates the two pieces of wire, the once-continuous path has now been broken, and electrons cannot flow from Source to Destination. This is like cutting a water pipe in two and capping off the broken ends of the pipe: water can't flow either direction through the pipes anymore. In electrical terms, we had a condition of electrical continuity when the wire was in one piece, and now that continuity is broken with the wire cut and separated.

If we were to take another piece of wire leading to the Destination and simply make physical contact with the wire leading to the Source, we would once again have a continuous path for electrons to flow. The two dots in the diagram indicate physical (metal-to-metal) contact between the wire pieces:

Now, we have continuity from the Source, to the newly made connection, down, to the right, and up to the Destination. This is analogous to putting a "tee" fitting in one of the capped-off pipes and directing water through a new segment of pipe to its destination. Please take note that the broken segment of wire on the right hand side has no electrons flowing through it, because it is no longer part of a complete path from Source to Destination.

It is interesting to note that no "wear" occurs within wires due to this electric current, unlike water-carrying pipes which are eventually corroded and worn by prolonged flows. Electrons do encounter some degree of friction as they move, however, and this friction can generate heat in a conductor.

6. In conductive materials, are the outer electrons in each atom held tightly or loosely?

7. In insulating materials, are the outer electrons in each atom held tightly or loosely?

8. Are metals electrically conductive or insulating?

9. Electric current is the uniform motion of what through a conductor?

10. For electrons to flow continuously (indefinitely) through a conductor, there must be a continuous, what?

Electric circuits

You might have been wondering how electrons can continuously flow in a uniform direction through wires without the benefit of these hypothetical electron Sources and Destinations. In order for the Source-and-Destination scheme to work, both would have to have an infinite capacity for electrons in order to sustain a continuous flow! Using the marble-and-tube analogy, the marble source and marble destination buckets would have to be infinitely large to contain enough marble capacity for an indefinite "flow" of marbles.

The answer to this paradox is found in the concept of a circuit: a never-ending looped pathway for electrons. If we take a wire, or many wires joined end-to-end, and loop it around so that it forms a continuous pathway, we have the means to support a uniform flow of electrons without having to resort to infinite Sources and Destinations:

Each electron advancing clockwise in this circuit of conducting wire pushes on the one in front of it, which pushes on the one in front of it, and so on, and so on, just like a hula-hoop filled with marbles. Now, we have the capability of supporting a continuous flow of electrons indefinitely without the need for infinite electron supplies and dumps. All we need to maintain this flow is a continuous means of motivation for those electrons.

It must be realized that continuity is just as important in a circuit as it is in a straight piece of wire. Just as in the example with the straight piece of wire between the electron Source and Destination, any break in this circuit will prevent electrons from flowing through it:

It doesn't matter where the break occurs: any discontinuity in the circuit will prevent electron flow throughout the entire circuit. Unless there is a continuous, unbroken loop of conductive material for electrons to flow through, a sustained flow simply cannot be maintained.

11. What is needed to allow electrons to flow continuously through a conductor?

12. If a circuit is "broken”, can electrons continue to flow?

Voltage and Current

As was previously mentioned, we need more than just a continuous path for electrons to flow before we can actually have a continuous flow: we also need some means to push these electrons around the circuit. Just like marbles in a tube or water in a pipe, it takes some kind of influencing force to initiate flow. With electrons, this force or energy can come from a battery or generator.

This energy is not unlike the energy stored in a high reservoir of water that has been pumped from a lower-level pond:

The influence of gravity on the water in the reservoir creates a force that attempts to move the water down to the lower level again. If a suitable pipe is run from the reservoir back to the pond, water will flow under the influence of gravity down from the reservoir, through the pipe:

It takes energy to pump that water from the low-level pond to the high-level reservoir, and the movement of water through the piping back down to its original level constitutes a releasing of that stored energy.

If the water is pumped to an even higher level, it will take even more energy to do so, thus more energy will be stored, and more energy released if the water is allowed to flow through a pipe back down again:

Electrons are not much different. Just as the pumping of water to a higher level results in energy being stored, "pumping" electrons to create an electric charge imbalance results in a certain amount of energy being stored in that imbalance. And, just as providing a way for water to flow back down from the heights of the reservoir results in a release of that stored energy, providing a way for electrons to flow back to their original "levels" results in a release of stored energy.

This potential energy, (pressure) stored in the form of an electric charge, can be expressed as a term called voltage. In the context of electrical power sources, voltage is the amount of potential energy (pressure difference between 2 points) available to move electrons through a conductor.

Voltage can be generated by chemical reactions, (battery) and the influence of magnetism on conductors (generator).

The flow of electrons through a conductor is called current and is measured in amps.

If we break the circuit's continuity at any point, the electric current will cease in the entire loop, and the full voltage produced by the battery will be manifested across the break, between the wire ends that used to be connected:

13. Electrons can be motivated to flow through a conductor by what?

14. Voltage is the measure of what available to move electrons?

15. When a voltage source is connected to a circuit, the voltage will cause a uniform flow of electrons through that circuit what is this flow called?

16. In a single (one loop) circuit, is the amount of current at any point, the same as the amount of current at any other point?

Practical circuits

The battery circuit above, is not a very practical one. In fact, it can be quite dangerous to build (directly connecting the poles of a voltage source together with a single piece of wire). The reason it is dangerous is that the magnitude of electric current may be very large in such a short circuit, and the release of energy very dramatic usually in the form of heat with an explosion of molten conductor. Even a low voltage ( from 1V up) can cause a large explosion if the circuit can deliver a high current e.g. a large 12V battery. Usually, electric circuits are constructed in such a way as to make practical use of that released energy, in as safe a manner as possible.

One practical and popular use of electric current is for the operation of electric lighting. The simplest form of electric lamp is a tiny metal "filament" inside of a clear glass bulb, which glows white-hot ("incandesces") with heat energy when sufficient electric current passes through it. Like the battery, it has two conductive connection points, one for electrons to enter and the other for electrons to exit. A photograph of a small lamp is shown here:

Connected to a source of voltage, an electric lamp circuit looks something like this:

As the electrons work their way through the thin metal filament of the lamp, they encounter more opposition to motion than they typically would in a thick piece of wire. This opposition to electric current depends on the type of material, its cross-sectional area, and its temperature. It is technically known as resistance. (It can be said that conductors have low resistance and insulators have very high resistance.) This resistance serves to limit the amount of current through the circuit with a given amount of voltage supplied by the battery, as compared with the "short circuit" where we had nothing but a wire joining one end of the voltage source (battery) to the other.

When electrons move against the opposition of resistance, "friction" is generated. Just like mechanical friction, the friction produced by electrons flowing against a resistance generates heat energy. The concentrated resistance of a lamp's filament results in a relatively large amount of heat energy being manifested at that filament. This heat energy is enough to cause the filament to glow white-hot, producing light, whereas the wires connecting the lamp to the battery, (which have much lower resistance) hardly even get warm while conducting the same amount of current.

As in the case of the short circuit, if the continuity of the circuit is broken at any point, electron flow stops throughout the entire circuit. With a lamp in place, this means that it will stop glowing:

As before, with no flow of electrons, the entire potential (voltage) of the battery is available across the break, waiting for the opportunity of a connection to bridge across that break and permit electron flow again. This condition is known as an open circuit, where a break in the continuity of the circuit prevents current throughout. All it takes is a single break in continuity to "open" a circuit. Once any breaks have been connected, the continuity of the circuit re-established, it is known as a closed circuit.

What we see here is the basis for switching lamps on and off with remote switches. Because any break in a circuit's continuity results in current stopping throughout the entire circuit, we can use a device designed to intentionally break that continuity (called a switch), mounted at any convenient location that we can run wires to, to control the flow of electrons in the circuit:

This is how a switch mounted on the wall of a house can control a lamp that is mounted down a long hallway, or even in another room, far away from the switch. The switch itself is constructed of a pair of conductive contacts (usually made of some kind of metal) forced together by a mechanical lever actuator or pushbutton. When the contacts touch each other, electrons are able to flow from one to the other and the circuit's continuity is established; when the contacts are separated, electron flow from one to the other is prevented by the insulation of the air between, and the circuit's continuity is broken.

Perhaps the best kind of switch to show for illustration of the basic principle is the "knife" switch.

A knife switch is nothing more than a conductive lever, free to pivot on a hinge, coming into physical contact with one or more stationary contact points which are also conductive. The switch shown above is constructed on a porcelain base (an excellent insulating material), using copper (an excellent conductor) for the "blade" and contact points. The handle is plastic to insulate the operator's hand from the conductive blade of the switch when opening or closing it.

Knife switches are great for illustrating the basic principle of how a switch works, but they present distinct safety problems when used in high-power electric circuits. The exposed conductors in a knife switch make accidental contact with the circuit a distinct possibility, and any sparking that may occur between the moving blade and the stationary contact is free to ignite any nearby flammable materials. Most modern switch designs have their moving conductors and contact points sealed inside an insulating case of some kind in order to mitigate these hazards.

17. What is the measure of opposition to electric current called?

18. A circuit offering little or no resistance to the flow of electrons is called what?

19. Short circuits are dangerous with high Voltage or current sources because the high currents encountered can cause large amounts what?

20. Explain an open circuit?

21. Explain a closed circuit?

22. A device designed to open or close a circuit under controlled conditions is called a what?

23. The terms "open" and "closed" refer to entire circuits as well as what.

Safety

When using or working with electricity, the topic of safety is of paramount importance.

This section comes after basic electricity on purpose, as a fundamental understanding of the basics is required to fully understand the safety implications of using electricity.

Have you ever wondered why birds don't get shocked while resting on power lines? Read on and find out!

Physiological effects of electricity

Most of us have experienced some form of electric "shock”, where electricity causes our body to experience pain or trauma. If we are fortunate, the extent of that experience is limited to tingles or jolts of pain from static electricity build-up discharging through our bodies. When we are working around electric circuits capable of delivering high power to loads, electric shock becomes a much more serious issue, and pain is the least significant result of shock.

As electric current is conducted through a material, any opposition (electrical resistance) produces a dissipation of energy, usually in the form of heat. This is the most basic and easy-to-understand effect of electricity on living tissue: current makes it heat up. If the amount of heat generated is sufficient, the tissue may be burnt. The effect is physiologically the same as damage caused by an open flame or other high-temperature source of heat, except that electricity has the ability to burn tissue well beneath the skin of a victim, even burning internal organs.

Another effect of electric current on the body, perhaps the most significant in terms of hazard, is what it does to the body's nervous system. By "nervous system" I mean the network of special cells in the body called "nerve cells" or "neurons" which process and conduct the multitude of signals responsible for regulation of many body functions. The brain, spinal cord, and sensory/motor organs in the body function together to allow it to sense, move, respond, think, and remember.

Nerve cells communicate to each other by acting as "transducers:" creating electrical signals (very small voltages and currents) in response to the input of certain chemical compounds called "neurotransmitters”, and releasing neurotransmitters when stimulated by electrical signals. If electric current of sufficient magnitude is conducted through a living creature (human or otherwise), its effect will be to override the tiny electrical impulses normally generated by the neurons, overloading the nervous system and preventing both reflex and volitional signals from being able to actuate muscles. Muscles triggered by an external (shock) current will involuntarily contract, and there's nothing the victim can do about it.

This problem is especially dangerous if the victim contacts an energized conductor with his or her hands. The forearm muscles responsible for bending fingers tend to be better developed than those muscles responsible for extending fingers, and so if both sets of muscles try to contract, the "bending" muscles will win, clenching the fingers into a fist. If the conductor delivering current to the victim faces the palm of his or her hand, this clenching action will make the hand, grab the wire firmly, thus worsening the situation by securing excellent contact with the wire. The victim will be unable to let go of the wire.

Medically, this condition of involuntary muscle contraction is called tetanus. Electricians familiar with this effect of electric shock often refer to an immobilized victim of electric shock as being "froze on the circuit”. Shock-induced tetanus can only be interrupted by stopping the current through the victim.

Even when the current is stopped, the victim may not regain voluntary control over their muscles for a while, as the neurotransmitter chemistry has been thrown into overload. This principle has been applied in "stun gun" devices such as Tasers, which momentarily shock a victim with a high-voltage pulse delivered between two electrodes for the purpose of temporary immobilization.

Electric current is able to affect more than just skeletal muscles in a shock victim, however. The diaphragm muscle controlling the lungs, and the heart -- which is a muscle in itself -- can also be "frozen" in a state of tetanus by electric current. Even currents too low to induce tetanus are often able to scramble nerve cell signals enough that the heart cannot beat properly, sending the heart into a condition known as "fibrillation." A fibrillating heart flutters rather than beats, and is ineffective at pumping blood to vital organs in the body. In any case, death from asphyxiation and/or cardiac arrest will surely result from a strong enough electric current through the body. Ironically, medical personnel use a strong jolt of electric current applied across the chest of a victim to "jump start" a fibrillating heart into a normal beating pattern.

That last detail leads us into another hazard of electric shock, this one peculiar to public power systems. Though our initial study of electric circuits will focus almost exclusively on DC (Direct Current, or electricity that moves in a continuous direction in a circuit), modern power systems utilize alternating current, or AC. The technical reasons for this preference of AC over DC in power systems are irrelevant to this discussion, but the special hazards of each kind of electrical power are not.

Direct current (DC), because it moves with continuous motion through a conductor, has the tendency to induce muscular tetanus quite readily. Alternating current (AC), because it alternately reverses direction of motion, provides brief moments of opportunity for an afflicted muscle to relax between alternations. Thus, from the concern of becoming "froze on the circuit," DC is more dangerous than AC.

However, AC's alternating nature has a greater tendency to throw the heart's pacemaker neurons into a condition of fibrillation, whereas DC tends to just make the heart stand still. Once the shock current is halted, a "frozen" heart has a better chance of regaining a normal beat pattern than a fibrillating heart. This is why "defibrillating" equipment used by emergency medics works: the jolt of current supplied by the defibrillator unit is DC, which halts fibrillation and gives the heart a chance to recover.

In either case, electric currents high enough to cause involuntary muscle action are dangerous and are to be avoided at all costs. In the next section, we'll look at how such currents typically enter and exit the body, and examine precautions against such occurrences.

24. Electric current is capable of producing deep and severe what in the body due to power dissipation across the body's electrical resistance?

25. Tetanus is the condition where muscles involuntarily contract due to the passage of what through the body?

26. When involuntary contraction of muscles controlling the fingers causes a victim to be unable to let go of an energized conductor, the circuit must be what before assisting that person?

27. Diaphragm (lung) and heart muscles are similarly affected by what?

28. Even currents too small to induce tetanus can be strong enough to interfere with the heart's pacemaker neurons, causing the heart to flutter instead of strongly beat. This is called what?

Shock path

As we've already learned, electricity requires a complete path (circuit) to continuously flow. This is why the shock received from static electricity is only a momentary jolt: the flow of electrons is brief when static charges are equalized between two objects.

Without two contact points on the body for current to enter and exit, respectively, there is no hazard of shock. This is why birds can safely rest on high-voltage power lines without getting shocked: they make contact with the circuit at only one point:

In order for electrons to flow through a conductor, there must be a voltage present to motivate them. Voltage, as you should recall, is always relative between two points. There is no such thing as voltage "on" or "at" a single point in the circuit, and so the bird contacting a single point in the above circuit has no voltage applied across its body to establish a current through it.

This might lend one to believe that it's impossible to be shocked by electricity by only touching a single wire. Like the birds, if we're sure to touch only one wire at a time, we'll be safe, right? Unfortunately, this is not correct. Unlike birds, people are usually standing on the ground when they contact a "live" wire. Many times, one side of a power system will be intentionally connected to earth ground, and so the person touching a single wire is actually making contact between two points in the circuit (the wire and earth ground):

The ground symbol is that set of three horizontal bars of decreasing width located at the lower-left of the circuit shown, and at the foot of the person being shocked. In real life, the power system ground consists of some kind of metallic conductor buried deep in the ground for making maximum contact with the earth. That conductor is electrically connected to an appropriate connection point on the circuit with thick wire. The victim's ground connection is through their feet, which are touching the earth.

The best protection against shock from a live circuit is resistance, and resistance can be added to the body with insulated tools, gloves, boots, and other gear.

Emergency response

Anyone working around electrical systems should be aware of what needs to be done for a victim of electrical shock.

If you see someone lying unconscious on the circuit, the very first thing that needs to be done is to shut off the power by opening the appropriate switch or circuit breaker and remove the plug if possible. If someone touches another person being shocked, there may be enough voltage dropped across the body of the victim to shock the would-be rescuer, thereby "freezing" two people instead of one. Don't be a hero. Electrons don't respect heroism. Make sure the situation is safe for you to step into, or else you will be the next victim, and nobody will benefit from your efforts.

One problem with this rule is that the source of power may not be known, or easily found in time to save the victim of shock. If electric current paralyses a shock victim's breathing and heartbeat, their survival time is very limited. If the shock current is of sufficient magnitude, their flesh and internal organs may be quickly roasted by the power the current dissipates as it runs through their body.

If the power disconnect switch cannot be located quickly enough, it may be possible to dislodge the victim from the circuit they're frozen on to by prying them or hitting them away with a dry wooden board or piece of non-conductive material. Another item that could be used to safely drag a "frozen" victim away from contact with power is an extension cord. By looping a cord around their torso and tugging hard, their grip with the conductor(s) may be broken.

Once the victim has been safely disconnected from the source of electric power, the immediate medical concerns for the victim should be respiration and circulation (breathing and pulse). If the rescuer is trained in CPR, they should follow the appropriate steps of checking for breathing and pulse, then applying CPR as necessary to keep the victim's body from deoxygenating. The cardinal rule of CPR is to keep going until qualified personnel have relieved you.

If the victim is conscious, it is best to have them lie still until qualified emergency response personnel arrive on the scene. There is the possibility of the victim going into a state of physiological shock (different from electrical shock), and so they should be kept as warm and comfortable as possible. An electrical shock insufficient to cause immediate interruption of the heartbeat may be strong enough to cause heart irregularities or a heart attack up to several hours later, so the victim should pay close attention to their own condition after the incident, ideally under supervision.

29. A person being shocked needs to be disconnected from what?

30. Locate the disconnecting switch/breaker and turn it off. Alternatively, if the disconnecting device cannot be located, the victim can be pried or pulled from the circuit by what?

31. Victims need immediate medical response: check for breathing and pulse, then apply what as necessary to maintain oxygenation?

32. If a victim is still conscious after having been shocked, they need what?

33. There is danger of physiological shock, what precautions should be taken?

34. Shock victims may suffer what up to several hours after being shocked?

35. When does the danger for the victim of electric shock end?

Common Sources of hazard

Of course there is danger of electrical shock when directly performing manual work on an electrical power system. However, electric shock hazards exist in many other places, thanks to the widespread use of electric power in our lives.

As we saw earlier, skin and body resistance has a lot to do with the relative hazard of electric circuits. The higher the body's resistance, the less likely harmful current will result from any given amount of voltage. Conversely, the lower the body's resistance, the more likely for injury to occur from the application of a voltage.

The easiest way to decrease skin resistance is to get it wet. Therefore, touching electrical devices with wet hands, wet feet, or especially in a sweaty condition, (salt water is a much better conductor of electricity than fresh water) is dangerous. In the household, the bathroom is one of the more likely places where wet people will mix with electrical appliances, and so shock hazard is a definite threat there. Good bathroom design will locate power receptacles away from bathtubs, showers, and sinks to discourage the use of appliances nearby. Telephones that plug into a wall socket are also sources of hazardous voltage (the ringing signal in a telephone is 48 volts AC -- remember that any voltage over 30 is considered potentially dangerous!). Appliances such as telephones and radios should never, ever be used while sitting in a bathtub, unless those devices are battery-powered.

Swimming pools are another source of trouble, since people often operate radios and other powered appliances nearby. The Electrical regulations requires that special shock-detecting receptacles called Residual Current Devices “RCD'S” be installed in wet and outdoor areas to help prevent shock incidents. These special devices have no doubt saved many lives, but they can be no substitute for common sense and diligent precaution. As with firearms, the best "safety" is an informed and conscientious operator.

Extension cords, so commonly used at home and in industry, are also sources of potential hazard. All cords should be regularly inspected for abrasion, cracking of insulation or any thing that could increase risk, and repaired immediately. One sure method of removing a damaged cord from service is to unplug it from the receptacle, then cut off that plug (the "male" plug) with a pair of side-cutting pliers to ensure that no one can use it until it is fixed. This is important on jobsite's, where many people share the same equipment, and not all people there may be aware of the hazards.

Any power tool that shows evidence of electrical problems should be immediately serviced as well. I've heard several horror stories of people who continue to work with hand tools that periodically shock them. Remember, electricity can kill, and the death it brings can be gruesome. Like extension cords, a bad power tool should be removed from service immediately.

Downed power lines are an obvious source of electric shock hazard and should be avoided at all costs. The voltages present between power lines or between a power line and earth ground are typically very high (11,000 volts being one of the lowest voltages used in residential distribution systems). If a power line is broken and the metal conductor falls to the ground, the immediate result will usually be a tremendous amount of arcing (sparks produced), often enough to dislodge chunks of concrete or asphalt from the road surface, and reports rivaling that of a rifle or shotgun. To come into direct contact with a downed power line is almost sure to cause death, but other hazards exist which are not so obvious.

When a line touches the ground, current travels between that downed conductor and the nearest grounding point in the system, thus establishing a circuit:

The earth, being a conductor (if only a poor one), will conduct current between the downed line and the nearest system ground point, which will be some kind of conductor buried in the ground for good contact. Being that the earth is a much poorer conductor of electricity than the metal cables strung along the power poles, there will be substantial voltage dropped between the point of cable contact with the ground and the grounding conductor, and little voltage dropped along the length of the cabling (the following figures are very approximate):

If the distance between the two ground contact points (the downed cable and the system ground) is small, there will be substantial voltage dropped along short distances between the two points. Therefore, a person standing on the ground between those two points will be in danger of receiving an electric shock by picking up a voltage between their two feet!

Again, these voltage figures are very approximate, but they serve to illustrate a potential hazard: that a person can become a victim of electric shock from a downed power line without even coming into contact with that line!

One practical precaution a person could take if they see a power line falling towards the ground is to only contact the ground at one point, either by running away (when you run, only one foot contacts the ground at any given time), or if there's nowhere to run, by standing on one foot. Obviously, if there's somewhere safer to run, running is the best option. By eliminating two points of contact with the ground, there will be no chance of applying deadly voltage across the body through both legs.

36. Wet conditions increase risk of electric shock by lowering skin what?

37. Replace worn or damaged extension cords and power tools, when?

38. Power lines are very dangerous and should be avoided at all costs. How can you get a shock without touching the downed power line?

Safe Circuit design

As we saw earlier, a power system with no secure connection to earth ground is unpredictable from a safety perspective: there's no way to guarantee how much or how little voltage will exist between any point in the circuit and earth ground. By grounding one side of the power system's voltage source, at least one point in the circuit can be assured to be electrically common with the earth and therefore present no shock hazard. In a simple two-wire electrical power system, the conductor connected to ground is called the neutral, and the other conductor is called the hot or phase or live or active: usually phase.

As far as the voltage source and load are concerned, grounding makes no difference at all. It exists purely for the sake of personnel safety, by guaranteeing that at least one point in the circuit will be safe to touch (zero voltage to ground). The "Hot" side of the circuit, named for its potential for shock hazard, will be dangerous to touch unless voltage is secured by proper disconnection from the source (ideally, using a systematic lock-out/tag-out procedure).

This imbalance of hazard between the two conductors in a simple power circuit is important to understand. The following series of illustrations are based on common household wiring systems (using DC voltage sources rather than AC for simplicity).

If we look at a simple, household electrical appliance such as a toaster with a conductive metal case, we can see that there should be no shock hazard when it is operating properly. The wires conducting power to the toaster's heating element are insulated from touching the metal case (and each other) by rubber or plastic.

However, if one of the wires inside the toaster were to accidentally come in contact with the metal case, the case will be made electrically common to the wire, and touching the case will be just as hazardous as touching the wire bare. Whether or not this presents a shock hazard depends on which wire accidentally touches:

If the "hot" wire contacts the case, it places the user of the toaster in danger. On the other hand, if the neutral wire contacts the case, there is no danger of shock:

To help ensure that the former failure is less likely than the latter, engineers try to design appliances in such a way as to minimize hot conductor contact with the case. Ideally, of course, you don't want either wire accidentally coming in contact with the conductive case of the appliance, but there are usually ways to design the layout of the parts to make accidental contact less likely for one wire than for the other. However, this preventative measure is effective only if power plug polarity can be guaranteed. If the plug can be reversed, then the conductor more likely to contact the case might very well be the "hot" one:

Appliances designed this way, usually come with "polarized" plugs, they only can plug in one way). Power points are also designed like this. Consequently, the plug cannot be inserted "backwards”, and conductor identity inside the appliance can be guaranteed. Remember that this has no effect whatsoever on the basic function of the appliance: it's strictly for the sake of user safety.

Some engineers address the safety issue simply by making the outside case of the appliance nonconductive. Such appliances are called double-insulated, since the insulating case serves as a second layer of insulation above and beyond that of the conductors themselves. If a wire inside the appliance accidentally comes in contact with the case, there is little danger presented to the user of the appliance.

Other engineers tackle the problem of safety by maintaining a conductive case, but using a third conductor to firmly connect that case to ground:

The third prong on the power cord provides a direct electrical connection from the appliance case to earth ground, making the two points electrically common with each other. If they're electrically common, then there cannot be much voltage dropped between them. At least, that's how it is supposed to work. If the hot conductor accidentally touches the metal appliance case, it will create a direct short-circuit back to the voltage source through the ground wire, tripping any over current protection devices. The user of the appliance will still be in danger from explosion and shock due to the high currents involved, until the power is turned off.

This is why it's so important never to cut the third prong off a power plug when trying to fit it into a two-prong receptacle. If this is done, there will be no grounding of the appliance case to keep the user(s) safe. The appliance will still function properly, but if there is an internal fault bringing the phase wire in contact with the case, the results can be deadly

A final safeguard against electrical shock can be arranged on the power supply side of the circuit rather than the appliance itself. This safeguard is called a Residual Current Devices RCD, and it works like this:

In a properly functioning appliance (shown above), the current measured through the hot conductor should be exactly equal to the current through the neutral conductor, because there's only one path for electrons to flow in the circuit. With no fault inside the appliance, there is no connection between circuit conductors and the person touching the case, and therefore no shock.

If, however, the hot wire accidentally contacts the metal case, there will be current through the person touching the case. The presence of a shock current will be manifested as a difference of current between the two power conductors at the power point:

This difference in current between the "hot" and "neutral" conductors will only exist if there is current through the ground connection, meaning that there is a fault in the system. Therefore, such a current difference can be used as a way to detect a fault condition. If a device is set up to measure this difference of current between the two power conductors, a detection of current imbalance can be used to trigger the opening of a disconnect switch, thus cutting power off and preventing serious shock:

Such devices are called Residual Current Devices or RCD'S for short, and they are compact enough to be built into a power point. These receptacles are easily identified by their distinctive "Test" and "Reset" buttons. The big advantage with using this approach to ensure safety is that it works regardless of the appliance's design. Of course, using a double-insulated or grounded appliance in addition to a RCD point would be better yet, but it's comforting to know that something can be done to improve safety above and beyond the design and condition of the appliance.

Power systems usually have one side of the voltage supply connected to earth ground to ensure safety at that point.

39. What is the "grounded" conductor in a power system is called?

40. Grounding in power systems exists for the sake of what?

41. What is the purpose of double insulation?

42. RCD's work by sensing a difference in current between What?

43. There should be no difference in current at all. Any difference means that current must be entering or exiting the load by what means?

Before continuing, please check that you know the answers to these questions.

44. A person being shocked needs to be disconnected from what?

45. Locate the disconnecting switch/circuit- breaker and turn it off. Alternatively, if the disconnecting device cannot be located, the victim

can be pried or pulled from the circuit by what?

46. Victims need immediate medical response: check for breathing and pulse, and then apply what as necessary to maintain oxygenation?

47. If a victim is still conscious after having been shocked, they need what?

48. There is danger of physiological shock, what precautions should be taken?

49. Shock victims may suffer what up to several hours after being shocked?

50. When dose the danger for the victim of electric shock end?

51. Wet conditions increase risk of electric shock by lowering skin what?

52. Replace worn or damaged extension cords and power tools, when?

53. Power systems usually have one side of the voltage supply connected to earth ground to ensure safety at that point.

What is the "earthed" conductor in a power system called?

54. Earthing in power systems exists for the sake of what?

55. What is the purpose of double insulation?

56. RCD's work by sensing a difference in current between What?

57. There should be no difference in current at all. Any difference means that current must be entering or exiting the load by what means?

Safe meter usage

Using an electrical meter safely and efficiently is important for the sake of your personal safety and for proficiency in your trade. It can be daunting at first to use a meter, knowing that you are connecting it to live circuits, which may harbour life-threatening levels of voltage and current. This concern is not unfounded, and it is always best to proceed cautiously when using meters. Carelessness more than any other factor is what causes experienced technicians to have electrical accidents.

There are many types of meter and you must learn all of the safety precautions for the particular type that you intend to use, before using the meter.

A meter capable of checking for voltage, current, and resistance is called a multimeter. We occasionally use this type of meter and it must not be used until you have had proper training in its use. The appliance testing meters that we use incorporate the functions of a multimeter. If the need arises to test a live power point or lead, it is imperative that you not let the probe tips come in contact with one another or with anything else, while they are both in contact with their respective points on the circuit. If this happens, a short-circuit will be formed, creating an explosion and perhaps even a ball of flame, if the voltage source is capable of supplying enough current! The following image illustrates the potential for hazard:

This is just one of the ways that a meter can become a source of hazard if used improperly. Our appliance testing meters all produce 500+ volts to test the insulation of appliances. This voltage is lethal regardless of the fact that the meter is battery operated.

Appliance safety

Appliances that use electricity can become dangerous if they are damaged in any way. External damage can usually be seen but not recognised by most people as a danger. Internal damage requires special equipment and techniques to detect. This is the reason to test appliances on a regular bases. The vast array of appliance types means that we must adapt our testing procedures to suit each appliance to ensure that they are 100% safe. We have come across many tagged appliances that are in a lethal condition yet someone has passed them. As this part of the electrical industry dose not require electrical registration, there are unskilled people claiming to have the required knowledge to test for safety. This is clearly not the case. Tagging appliances for safety is a very responsible occupation, requiring a high level of understanding and integrity.

As your knowledge of how to test each appliance increases, your speed, and accuracy will increase.

The basic types of appliance are:

Earthed - class 1

Double Insulated - class 2

Earthed appliances have a minimum of one layer of insulation between the conductors and other parts. Any accessible conductive parts must be electrically attached to the earth pin of the appliance plug. (Some exceptions are allowed, such as metal nameplates and screws that are not likely to become alive due to other insulating material between them and the conductors braking down.)

Should the insulation be by-passed, the resulting high short circuit current to earth via the earth pin of the plug and the building wiring should cause the building circuit breaker to turn off fast. This short circuit current is still dangerous. It could cause a fire and burns. It will raise the voltage on the earthed parts of the appliances to high levels until the circuit breaker turns it off. It will also raise the voltage on any other earthed appliances in the building to varying degrees. This is the main reason to have a low resistance earth system for each earthed appliance. Some earthed appliances have no accessible metal such as a plastic fan. The internal metal motor may be earthed but the risk from this type of appliance is considered low and we do not dismantle any appliance to test it. Our test for this is a thorough inspection only.

Doubled insulated appliances rely on a minimum of two layers or equivalent , of insulation between the conductors and accessible parts. Some doubled insulated appliances have accessible metal parts, such as the chuck of a drill. When we test this type of appliance, we test the insulation between both conductors and the metal or conductive accessible parts. This is to check that the insulation is not bypassed, allowing voltage to be accessible anywhere on the appliance. If a double insulated appliance gets wet, its insulation can easily be bypassed.

Double insulated appliances should have - class two, double insulated, or more commonly a

symbol on them.

58. What is a class 2 appliance?

59. Why are earthed appliances earthed?

At this point you will have a basic understanding of electricity and its dangers.

Consider various appliances and the way they are constructed and wired to minimise danger from shock and fire.

List the ways that the danger could increase by:

60. Wear?

61. Damage?

62. Corrosion?

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