ELECTRICAL SAFETY Part 1: Basic Electricity k groves /e haller

WHAT IS ELECTRICITY? Negatively charged particles (electrons) moving through a conductor. k groves /e haller

ELECTRIC CURRENT (I) Movement of electrons (electric charge) along a conductor. Ampere (I) - a measure of the number of electrons that are moving through a conductor every second. 1 amp = 6.24 x 1018 electrons per second. k groves /e haller

CONVENTIONAL CURRENT Electrons flow from negative to positive - attracted to the positive terminal and repelled from the negative terminal. Although it really doesn’t exist, conventional current assumes that current flows out of the positive terminal, through the circuit and into the negative terminal of the source. Many references use the conventional current direction. Note that Benjamin Franklin goofed in that he said that current flowed from positive to negative. Accepted until 1898 when the electron was discovered. Point out that it it makes no difference which way current is flowing as long as it is used consistently. The direction of current flow does not affect what the current does. k groves /e haller

ELECTRICAL CONDUCTOR A material that has a low resistance to the flow of electricity. Metals Concrete Graphite Dirty water Electricity needs a conductor to move. Most metals have electrons that can detach from their atoms and move around. These are called free electrons. Gold, silver, copper, aluminum, iron, etc., all have a large proportion of free electrons. The loose electrons make it easy for electricity to flow through these materials, so they are known as electrical conductors. They conduct electricity. The moving electrons transmit electrical energy from one point to another. k groves /e haller

ELECTRICAL INSULATOR A material that has a high resistance to the flow of electricity. Glass Rubber Porcelain Air Dry wood Ceramic Oil In many materials, the electrons are tightly bound to the atoms. Wood, glass, plastic, ceramic, air, cotton ... These are all examples of materials in which electrons stick with their atoms. Because the electrons don't move, these materials cannot conduct electricity very well, if at all. These materials are electrical insulators. k groves /e haller

RESISTANCE (R) The opposition to the flow of electrons. Measured in ohms (Ω) The amount of resistance depends on type of material diameter temperature length Material - Copper and silver offer negligible resistance because a large proportion of the electrons in these metals are free to move from atom to atom. Glass and porcelain have a great deal of resistance because a large proportion of their electrons are not free to move from atom to atom. Diameter - the smaller the diameter the higher the resistance (discuss the thin filament in a light bulb) Temperature - the higher the temperature, the higher the resistance. In general, electrical resistivity of metals increases with temperature, while the resistivity of semiconductors decreases with temperature. As the temperature of a metal is reduced, the resistance usually reduces until it reaches a constant value, known as the residual resistivity. Length - the longer the length, the higher the resistance. Wider wires have a greater cross-sectional area. the wider the wire, the less resistance that there will be to the flow of electric charge. Water will flow through a wider pipe at a higher rate than it will flow through a narrow pipe; this can be attributed to the lower amount of resistance which is present in the wider pipe. k groves /e haller

RESISTANCE (R) Discuss the relationship between wire gauge and size and resistance. k groves /e haller

POTENTIAL DIFFERENCE The difference in electrical charge between two points in a circuit expressed in volts. k groves /e haller

VOLTAGE (E) The force or energy that causes the electrical charge to move through a conductor. Pressure Volts Voltage is the electrical force that causes free electrons to move from one atom to another. Just as water needs some pressure to force it through a pipe, electrical current needs some force to make it flow. "Volts" is the measure of "electrical pressure" that causes current flow. Voltage is sometimes referred to as the measure of a potential difference between two points along a conductor. Voltage is typically supplied by either a generator or battery. Generators are analogous to a water pump in a water piping system, and batteries are similar to water towers. Both systems have a potential difference between the source of the power and someplace downstream from the source. Voltage is the measure of the potential difference between two points or the potential to move electrons. It is supplied by a battery or a generator. k groves /e haller

OHM’S LAW Mathematical formula that describes the relationship between voltage, current, and resistance. Current I = E/R Resistance R = E/I Voltage E = IR (or V = IR) Many industries need employees that have the knowledge and skills necessary to install, operate, troubleshoot, service and repair electrical equipment.  A basic knowledge of the relationship between voltage, current, and resistance is basic to an understanding of electrical systems.  k groves /e haller

OHM’S LAW E E I R I R R = E/I E E I I R R I = E/R E = IR Explain that Ohm’s law is a useful tool for analyzing circuits. Discuss the relationship between I, R, and V. I I R R I = E/R E = IR k groves /e haller

OHM’S LAW Water Pipe Analogy Pressure increase Voltage increase Flow rate increase Current increase Resistance same Resistance same Pressure same Voltage same Flow rate decrease Current decrease Resistance increase Resistance increase Pressure decrease Voltage decrease Flow rate same Current same Resistance decrease Resistance decrease Discuss the water and pipe analogy: Ohm's Law also make intuitive sense if you apply if to the water-and-pipe analogy. If we have a water pump that exerts pressure (voltage) to push water around a "circuit" (current) through a restriction (resistance), we can model how the three variables interrelate. If the resistance to water flow stays the same and the pump pressure increases, the flow rate must also increase. If the pressure stays the same and the resistance increases (making it more difficult for the water to flow), then the flow rate must decrease. If the flow rate were to stay the same while the resistance to flow decreased, the required pressure from the pump would necessarily decrease. Distribute handout exercis and work through Ohm’s law problems with the class. k groves /e haller

ELECTRICAL POWER (P) P = IE Power is the rate at which electrical energy is converted to some other form of energy such as light, heat, or horsepower. It is expressed in watts or kilowatts. P = IE Power is the measure of how much work can be done in a given amount of time. Mechanical power is commonly measured (in America) in "horsepower.” Electrical power is almost always measured in "watts," and it can be calculated by the formula P = IE. Electrical power is a product of both voltage and current, not either one separately. Horsepower and watts are merely two different units for describing the same kind of physical measurement, with 1 horsepower equaling 745.7 watts. A 17-watt fluorescent bulb may produce more light than a 100-watt incandescent bulb. k groves /e haller

WATTS, AMPS, OHMS, VOLTS Note that V = E Discuss the formula wheel and explain the relationship between watts, amps, ohms, and volts. k groves /e haller

ELECTRICAL POWER Calculate the power I2 R = P E I = P E2 /R = P I = E/R = 18 /3 = 6 amps P = IE = 6(18) = 108 Watts Calculate the power E I = P E2 /R = P I2 R = P k groves /e haller

ELECTRICAL POWER Calculate the power I2 R = P E I = P E2 /R = P I = E/R = 36 /3 = 12 amps P = IE = 12(36) = 432 Watts Notice that the power has increased increased quite a bit more than the current. This is because power is a function of voltage multiplied by current, and both voltage and current doubled from their previous values, the power will increase by a factor of 2 x 2, or 4. You can check this by dividing 432 watts by 108 watts and seeing that the ratio between them is 4. Ohm's Law has limitations. For most conductors, resistance is a rather stable property, largely unaffected by voltage or current. In an incandescent lamp the resistance of the filament wire will increase dramatically as it warms from room temperature to operating temperature. If we were to increase the supply voltage in a real lamp circuit, the resulting increase in current would cause the filament to increase temperature, which would in turn increase its resistance, thus preventing further increases in current without further increases in battery voltage. Consequently, voltage and current do not follow the simple equation "I=E/R" Resistance changes with variations in temperature is one shared by almost all metals, of which most wires are made. For most applications, these changes in resistance are small enough to be ignored. In the application of metal lamp filaments, the change happens to be quite large. Calculate the power E I = P E2 /R = P I2 R = P k groves /e haller

ELECTROMAGNETIC FIELD The flow of electricity through a conductor produces an electric field and a magnetic field around the conductor. Electric Field The electric field is measured in volts per meter and the higher the source voltage, the higher strength of the field. It decreases with distance from the source. Electric fields are represented by straight lines with arrows showing the direction of the electric field. k groves /e haller

ELECTROMAGNETIC FIELD Magnetic Field The strength of a magnetic field is measured in units of gauss and varies with the amount of current moving through the conductor. The central yellow and red cylinder is the wire The arrow for the current shows the direction of flow of the charge. Magnetic field lines are in blue. The backs of the arrowheads are magenta (replacing the traditional x ) to distinguish a direction away from the viewer. Field strength is indicated by the opacity of the lines. Weaker field lines are more transparent. Demonstrate the left hand rule k groves /e haller

ELECTROMAGNETIC FIELD Electric fields are blocked by walls, houses, trees, soil, and other dense objects. Magnetic fields pass easily through most objects and are only blocked by structures containing large amounts of iron or iron alloy metals. k groves /e haller

ELECTROMAGNET Electromagnets are important in the operation of generators, motors, transformers and relays. They are made by wrapping an insulated conductor wire around an iron object and then passing an electrical current through the wire. k groves /e haller

DIRECT CURRENT (DC) Direct currents are produced when the electrons move in one direction. Direct current is produced by batteries, solar panels, fuel cells, and special DC generators such as wind turbines. k groves /e haller

ALTERNATING CURRENT (AC) An alternating current reverses direction in a circuit at regular intervals. + – Note that in Europe it changes 50 times every second. Describe how the wave would look for DC. AC power is represented by a sine wave which changes 60 times every second. k groves /e haller

ALTERNATING CURRENT (AC) Explain the five characteristics of AC power: amplitude, cycles, frequency (Hertz), peak to peak, and RMS. Voltage continually changes from positive to negative. The rate of change is measured in Hertz (cycles per second). k groves /e haller

THREE PHASE AC Explain the placement of each wave form. k groves /e haller

CIRCUITS The three components of an electrical circuit: source of power, a path for current, and a load. k groves /e haller

CIRCUITS Circuit with controller (switch) Is the circuit open or closed? On or off? k groves /e haller

CIRCUIT DIAGRAM A basic circuit diagram k groves /e haller

CIRCUIT SYMBOLS Distribute handout of symbols and discuss key symbols. k groves /e haller

DRAW THIS CIRCUIT Give the group a few minutes to draw the circuit and then look at the drawings. k groves /e haller

SERIES CIRCUITS A series circuit is a circuit where there is only one path from the source through all of the loads and back to the source. This means that all of the current in the circuit must flow through all of the loads. One example of a series circuit is a string of old Christmas lights. There is only one path for the current to flow. Opening or breaking a series circuit such as this at any point in its path causes the entire circuit to "open" or stop operating. That's because the basic requirement for the circuit to operate a continuous, closed loop path is no longer met. This is the main disadvantage of a series circuit. If any one of the light bulbs or loads burns out or is removed, the entire circuit stops operating. Many of today's circuits are actually a combination of elements in series and parallel to minimize the inconvenience of a pure series circuit. Discuss the effect of voltage and amperage in a series circuit. k groves /e haller

PARALLEL CIRCUITS A parallel circuit is a circuit in which there are at least two independent paths in the circuit to get back to the source. In a parallel circuit, the current will flow through the closed paths and not through the open paths. Consider a simple circuit with an outlet, a switch and a 60 watt light bulb. If the switch is closed, the light operates. When a second 60 watt bulb is added to the circuit in parallel with the first bulb, it is connected so that there is a path to flow through to the first bulb or a path to flow through to the second bulb. Note that both bulbs glow at their intended brightness, since they each receive the full circuit voltage of 120 volts. Every load connected in a separate path receives the full circuit voltage. If a third 60-watt bulb is added to the circuit, it also glows at its intended brightness since it also receives its full 120 volts from the source.One special concern in parallel circuits is that the amperage from the source increases each time another load is added to the circuit in parallel. Therefore, it is very easy to keep adding loads or plugging them in parallel and thereby overloading a circuit by requiring more current to flow than the circuit can safely handle. Discuss the effect of voltage and amperage in a parallel circuit. k groves /e haller

SHORT CIRCUIT A short circuit is a circuit in which the electricity has found an alternative path to return to the source without going through an appropriate load. You can demonstrate this easily by taking a fine piece of wire and connecting it to both the positive and negative terminals of a small battery. The wire will heat instantly and probably melt. In most circuits, this high amperage represents a dangerous situation that could cause a fire or electrocute someone. Consider the circuit shown where the source is the outlet, the path is the extension cord, and the load is the drill. If the wire inside the drill comes loose and touches the other wire, a new path exists where the current can return to the source without going through a load the drill motor. Thus, the name short circuit because the electrons have found a shorter path to take to get back to the source. Another type of short circuit occurs when some conductive object accidentally gets into an overhead power line. If the object touches both the lines at the same time, the electricity has a short circuit path available to return to the source before it goes to the customer's electric service. If the object is connected to the ground, the earth can act as a short circuit path. k groves /e haller

SHORT CIRCUIT k groves /e haller

CIRCUIT WIRING Electrically common points: 1 and 2 and 3 and 4 If connecting wire resistance is very little or none, we can regard the connected points in a circuit as being electrically common. That is, points 1 and 2 in the above circuits may be physically joined close together or far apart, and it doesn't matter for any voltage or resistance measurements relative to those points. The same goes for points 3 and 4. It is as if the ends of the resistor were attached directly across the terminals of the battery, so far as our Ohm's Law calculations and voltage measurements are concerned. This is useful to know, because it means you can re-draw a circuit diagram or re-wire a circuit, shortening or lengthening the wires as desired without appreciably impacting the circuit's function. All that matters is that the components attach to each other in the same sequence.It also means that voltage measurements between sets of "electrically common" points will be the same. That is, the voltage between points 1 and 4 (directly across the battery) will be the same as the voltage between points 2 and 3 (directly across the resistor). Electrically common points: 1 and 2 and 3 and 4 k groves /e haller

CIRCUIT WIRING Voltage same between points 1 and 4 (across battery), and between points 2 and 3 (across resistor) k groves /e haller

CIRCUIT WIRING Electrically common points: 1, 2, and 3 and 4, 5, and 6 k groves /e haller

GROUNDING If there is a short circuit, grounding enables the electricity to take an alternate path back to the circuit breaker and then to a grounding rod driven into the ground. k groves /e haller

GROUNDING The term "ground" refers to a connection to the earth, which acts as a reservoir of charge. A ground wire provides a conducting path to the earth which is independent of the normal current-carrying path in an electrical appliance. As a practical matter in household electric circuits, it is connected to the electrical neutral at the service panel to guarantee a low enough resistance path to trip the circuit breaker in case of an electrical fault (see illustration below). Attached to the case of an appliance, it holds the voltage of the case at ground potential (usually taken as the zero of voltage). This protects against electric shock. The ground wire and a fuse or breaker are the standard safety devices used with standard electric circuits. k groves /e haller

CIRCUIT BREAKER A circuit breaker has a bimetal strip that heats and bends during a circuit overload. It then trips the breaker and opens the switch. Circuit breakers act to limit the current in a single circuit in most household applications. Typically a single circuit is limited to 20 amperes, although breakers come in many sizes. This means that 20 amps of current will heat the bimetallic strip to bend it downward and release the spring-loaded trip-lever. Since the heating is fairly slow, another mechanism is employed to handle large surges from a short circuit. A small electromagnet consisting of wire loops around a piece of iron will pull the bimetallic strip down instantly in case of a large current surge. k groves /e haller

CIRCUIT BREAKER Another type circuit breaker has an electromagnet. Increasing current boosts the electromagnet's magnetic force, and decreasing current lowers the magnetism. When the current jumps to unsafe levels, the electromagnet is strong enough to pull down a metal lever connected to the switch linkage. k groves /e haller

FUSES Plug fuses are round and screw into a base in a fuse holder to complete the circuit. It contains a soft wire or metal that will carry a given amount of current. If more current flows in the circuit than the fuse is designed to carry, the metal strip melts or “burns out” which opens the circuit. k groves /e haller

FUSES Cartridge fuses fit in between two holders on each end of the fuse. the metal ends of the fuse connect to the fuse link inside the cartridge. Show examples of circuit breakers and fuses and explain how they work. k groves /e haller

CAPACITOR Capacitors are voltage storage devices. Capacitors are essentially "voltage storage devices" commonly found in controls, motors and welding circuits, and many other places. They are "charged up" by another source and hold that charge even when the circuit is open, unlike typical resistance loads that lose their voltage as soon as the current goes to zero. Like a battery, a capacitor has two terminals. Inside the capacitor, the terminals connect to two metal plates separated by a dielectric. The dielectric can be air, paper, plastic or anything else that does not conduct electricity and keeps the plates from touching each other. The plate on the capacitor that attaches to the negative terminal of the battery accepts electrons that the battery is producing. The plate on the capacitor that attaches to the positive terminal of the battery loses electrons to the battery.Once it's charged, the capacitor has the same voltage as the battery (1.5 volts on the battery means 1.5 volts on the capacitor). For a small capacitor, the capacity is small. But large capacitors can hold quite a bit of charge. A capacitor in an ac circuit limits the current flow in a similar manner to a resistor. The amount of opposition to current flow is quantified as the capacitive reactance. Capacitive reactance is measured in units of ohms. Camera flash capacitor k groves /e haller

TRANSFORMERS Large power transformer Power cube transformer This transformer steps up voltage to as high as 765,000 volts so it can travel long distances This transformer converts normal 120 volt AC current to 3 volts k groves /e haller

TRANSFORMERS This transformer on the utility pole receives voltage from a substation where the voltage was reduced. This transformer transforms 7,200 volts to 220-240 volts. It is then sent to your home over 3 wires: one ground and 2 positive. k groves /e haller

TRANSFORMERS Secondary Coil Primary Coil Large power transformer Power Cube Transformer: 120 volts comes in on the primary winding which has an iron core and iron around the outside. The AC current in the primary winding creates an alternating magnetic field in the iron core. The secondary winding is wrapped around the same iron core and the magnetic field in the core creates a current. The voltage in the secondary core is controlled by the ratio of the number of turns in the two windings. If the primary and secondary windings have the same number of turns, the voltage will be the same. If the secondary has half the number of turns, the voltage will be half of the primary. Note that the primary has very fine wire while the secondary uses much thicker wire. To drop to 3 volts there needs to be 40 times more turns in the primary. Large power transformer Power cube transformer k groves /e haller

TRANSFORMERS Reverse side of power cube transformer Power Cube Transformer: 120 volts comes in on the primary winding which has an iron core and iron around the outside. The AC current in the primary winding creates an alternating magnetic field in the iron core. The secondary winding is wrapped around the same iron core and the magnetic field in the core creates a current. The voltage in the secondary core is controlled by the ratio of the number of turns in the two windings. If the primary and secondary windings have the same number of turns, the voltage will be the same. If the secondary has half the number of turns, the voltage will be half of the primary. Note that the primary has very fine wire while the secondary uses much thicker wire. To drop to 3 volts there needs to be 40 times more turns in the primary. Reverse side of power cube transformer Two diodes wrapped in rubber insulation turn AC current into DC current. k groves /e haller

INSULATED TOOLS Cable Shears Pliers Screwdrivers Knives Discuss the differences between insulated and non-insulated tools - advantages and disadvantages and safety issues. Show examples of approved insulated tools and note the VDE certification marking. Pliers Screwdrivers Knives k groves /e haller

TESTERS Non- Contact AC Voltage Detector Clamp-On Ammeter Socket Tester Discuss how and when theses testers should be used and any Safety related issues in their use. Show examples of these testers and demonstrate use. Allow participants to use the testers. Non-Contact Current Detector Voltage Tester k groves /e haller

MEASURING INSTRUMENTS Show examples of the instrument, demonstrate use on a motor, lamp, battery, etc. Allow participants to practice. Megohmmeter Multimeter Battery/Bulb/Fuse/ Continuity Tester k groves /e haller