Electrical Safety - Chapter 1-3 PDF

Learning Outcomes - Explain the two types of electric shock: o Direct contact o Indirect contact Explain the potential dangers in electrical work Understand the danger of hazardous work practices Explain the prec...

Learning Outcomes - Explain the two types of electric shock: o Direct contact o Indirect contact Explain the potential dangers in electrical work Understand the danger of hazardous work practices Explain the precautions and procedures for safe electrical work Explain the benefits of good housekeeping in electrical work Recommend measures to protect against electrical hazards 1.1.1 How an Electric Shock Happens An electric shock occurs when an electric current flows through the human body (Figs. 1.1-1 and 1.1-2). P Phase or Litive :. Electric current N : Neutral P or L N E Earth : E / # Symbol for Earth. Water Fig. 1.1-1: A person who has Fig. 1.1-2: An electric shock occurs when an received an electric shock electric current flows through a person Unit 1.1 | Electrical Safety 1  1.1.2 Types of Electric Shock There are two types of electric shock: - (a) Direct contact (b) Indirect contact (a) Direct Contact Direct contact occurs when a person comes into contact with a conductor (i.e., heating element of appliance in this example) that is live under normal conditions (Fig. 1.1-3). Supply Transformer Electric Current Heating element of P or L heater which is live Metal casing of appliance which is earthed N E Protective conductor against electric shock in the form of an earth wire Fig. 1.1-3: Electric shock due to direct contact Unit 1.1 | Electrical Safety 2  (b) Indirect Contact Indirect contact occurs when a person comes into contact with exposed conductive parts (i.e., metal casing of appliance in this example) that have become live under fault conditions (i.e., parts not normally live but have become live due to faults such as insulation failure). In Fig. 1.1-4, the metal casing of the appliance is not connected to a protective conductor (i.e., earth wire) which means it is not earthed. The person gets an electric shock when the live metal casing of the appliance is being touched. Supply Transformer fault due to insulation failure P or L Electric Current due to a stray live wire touching the metal casing Metal casing of appliance N E Fig. 1.1-4: Electric shock due to indirect contact The severity of injury from an electric shock depends on: the amount of current flowing through the body; and the length of time the current flows through the body. The severity of injury increases with a larger current and/or a longer duration of current through the body. A shock current of 50 mA can be fatal. Fig. 1.1-5 shows a hand burned by electricity. MA : milliampere Milli : x 10-3 GOnA = 50x10-s =H Fig. 1.1-5: Electrical burn Unit 1.1 | Electrical Safety 3  1.1.3 What to Do When Someone is Shocked or Burned by Electricity Switch off the electrical supply if the person is still in contact with the live circuit. At the same time, get someone else to call for help. If you cannot find the electrical supply source, do not take hold of the victim, as the current may pass through you too. Either use a dry towel or scarf to free the victim, or use a piece of wood to knock the victim’s hand free of the electrical equipment. As a last resort, take hold of the victim’s clothing – without touching the body – to pull the victim free. Do not try to move a victim who has fallen due to electric shock, except to shift the body into the recovery position, as the victim may have sustained other injuries. 1.1.4 How to Work Safely In addition to maintaining a safe work environment, we must also work safely. Safe work practices help reduce the risk of injury or death from workplace hazards. Here are some examples of safe work practices: Use and maintain tools properly. Inspect tools before using them. Switch off the power before working on a circuit. 1.1.5 Good Housekeeping In order to work safely, we should keep our workspaces tidy and well-arranged. Good housekeeping: lowers the risk of accidents and fire; improves productivity; makes better use of space; and reflects a well-managed operation. Unit 1.1 | Electrical Safety 4  1.1.6 General Safety Rules in the Electrical Laboratory Dos Use only insulated tools. All electrical work must be completed by a suitably qualified person. Always switch off the power and remove the plug before any electrical work. Ensure that your electrical equipment is properly inspected and maintained. Disconnect broken appliances from the power supply and replace frayed cords or broken power points. Use test equipment and tools correctly. Read the instruction booklet (if available) and understand the instructions before following them. Keep electrical cords off the floor to prevent them from being damaged from dragging or contact with sharp objects. A damaged electrical cord can cause a fatal electric shock. Know the location of the mains supply. Keep electrical equipment away from water and wet areas to lower the risk of electric shock. Don’ts Never take risks. Do not re-close a tripped circuit breaker or replace a blown fuse until the cause has been found and rectified. Do not misuse electrical equipment and appliances. Keep them dry. Do not use flammable solvents near an electrical apparatus unless the apparatus is labelled “flameproof”. Do not use a fire extinguisher on electrical fires unless it is an appropriate type, such as a carbon-dioxide or dry-powder extinguisher. Switch off the power as soon as possible. Do not overload circuits and fuses by plugging in too many appliances into one power point. Use a power board with individual switches instead of double adapters. Unit 1.1 | Electrical Safety 5  Learning Outcomes Explain how an electric circuit works State the three basic electrical quantities: voltage, current and resistance State the units of measurement for voltage, current and resistance Describe the use of a voltmeter for measuring voltage Describe the use of an ammeter for measuring current Describe the use of an ohmmeter for measuring resistance State the different uses of a multimeter Use the multimeter to measure voltage, current and resistance, and to check for continuity of an electrical installation Exercise safety precautions when handling and using measuring instruments 1.2.1 Electric Circuits An electric circuit is the physical pathway for current to flow. A simple electric circuit consists of: a source of electromotive force (emf), such as a battery or generator; a load (which has resistance), such as a lamp; conducting/connecting wires to connect the various parts of the circuit; and additional components such as a switch, fuse or circuit breaker, and a measuring instrument. Switch Connecting A Wire t Battery - Lamp - - open : off circuit Fig. 1.2-1: Simple electric circuit - closed : On circuit Unit 1.2 | Electric Circuits 6  1.2.2 Voltage, Current and Resistance The three basic electrical quantities in a basic electric circuit are: (a) supply voltage or electromotive force; (b) current; and compare to height of waterfall. enta ↳ (c) resistance. Voltage is the driving force to cause current (a) Supply Voltage / Electromotive Force ↑ to flow i n a circuit. It is the electrical quantity that causes current in a closed circuit. It is represented by V (supply voltage) or emf (electromotive force) and is measured in volts (V). > - volts is the unit (b) Current electrial load= electrical equipment for ↳ (lightbulb fan charger air con). voltage/ef moving charges , , ,. It is the electrical quantity that is needed for the electrical load to function, i.e., current allows the lamp in Fig. 1.2-1 to light up. It is represented by I and is measured in amperes (A). ↳ unit for current (c) Resistance is amperes. Resistance helps limit the size of current in the circuit. It is represented by R and is measured in ohms (Ω). When there is no or very little resistance in the circuit, such as when a short circuit occurs, the current will be very high and will damage the equipment if the circuit is not protected from this high current. current flowing short circuit will have a high. This causes a heat and lot of eventually fire 1.2.3 Voltmeter cause a. may voltage = electromotive force f A voltmeter is used for measuring the emf of the cell, battery, supply voltage and voltage across theO load in the circuit. It is always connected in parallel to the load or supply source, -S and at its two ends, where the voltage is to be measured. Eg Lamp Resistance Voltmeter get.. > - The symbol for a voltmeter is - V. must be connected in PARALLEL. Fig. 1.2-2 shows how a voltmeter can be connected to measure the voltage across the lamp in an electric circuit. across the lamp parallel. V Lamp Fig. 1.2-2: Voltmeter connected parallel to the lamp and at its two ends Unit 1.2 | Electric Circuits 7  Parallel circuit : the current will split path. Series circuit : the current only have ONE = - 1.2.4 Ammeter path. Does not split up An ammeter is used for to measure current flowing in a circuit. It is always connected in series to the load in the circuit. *Ammeter connected The symbol for an ammeter is A. in SERIES. - ! Fig. 1.2-3 shows how an ammeter should be connected to measure the current in an electric circuit. A Lamp Fig. 1.2-3: Ammeter connected in series to lamp 1.2.5 Ohmmeter An ohmmeter is used for measuring the resistance of an electrical load in a circuit. It is always connected in parallel to the load and at these two ends, where the resistance is to be measured. Ohmmeter connected The symbol for an ohmmeter is - Ω. in PRALLEL Fig. 1.2-4 shows how an ohmmeter should be connected to measure the resistance of an electrical load. NOTE: When using an ohmmeter to measure resistance, make sure its power is switched off. Otherwise, the user may receive an electric shock and the ohmmeter may be damaged. Fig. 1.2-4: Ohmmeter connection Unit 1.2 | Electric Circuits 8  1.2.6 Multimeter ① ③ A multimeter is a device that can measure voltage, current and resistance. It can also be used to diagnose electrical problems. -- check if 2 ↳ continuity tester There are two kinds of multimeter: points are connected. the analogue multimeter (Fig. 1.2-5), which uses an indicator needle with a measurement scale; and the digital multimeter (Fig. 1.2-6), which uses a numeric LCD display. The multimeter uses a rotating switch to select the quantity to be measured. It has two metal- tipped wires called probes, one red (+) and one black (-). probes. ↓ t > - Fig. 1.2-5: Analogue multimeter Fig. 1.2-6: Digital multimeter When using a multimeter to measure resistance, make sure its power is switched off. However, the measurement of voltage can only be carried out with the power on. Due to the risk of electric shock, only trained individuals should conduct voltage tests. Unit 1.2 | Electric Circuits 9  Learning Outcomes Understand the relationship between voltage, current and resistance (Ohm’s law): V=I ×R Apply Ohm’s law to determine voltage, current or resistance in an electric circuit Connect a simple electric circuit comprising a voltmeter, an ammeter, a load and a power supply to verify Ohm’s law 1.3.1 Ohm’s Discovery The relationship between V, I and R was discovered by scientist, Georg Simon Ohm. This discovery, known as Ohm’s law, is the basic formula used in all electric circuits. Ohm’s law states that the current (I ) flowing through a conductor is directly proportional to the potential difference (V ) applied across its ends, provided the temperature remains constant. ↳ voltage/EMF (Voltage increase current , increase) Fig. 1.3-1: Simple electric circuit The following formula is derived from Ohm’s law: V=I×R I = = where V is voltage in volts (V) I is current in amperes (A) R R is resistance in ohms (Ω) Unit 1.3 | Electric Circuit Laws 10  1.3.2 Worked Examples Fig. 1.3-2: Simple electric circuit (a) Refer to Fig. 1.3-2, determine V if I = 0.5 A and R = 20 Ω. V=I ×R = 0.5 A × 20 Ω = 10 V (b) A load of resistance 500 Ω is connected to a supply of 230 V. What is the current drawn from the supply? V V I = R - 230 V F = 500 Ω = 0.46 A Notes ! M : micro -100 000 louA 100000 = = 0. 001A M : willi = 1000 Int = += 0 005A. kilo +1000 20km = 20x1000 20000 - = k : M : Mega x 100000 0 3 MV. = 0 3 +100000. = 30000 Unit 1.3 | Electric Circuit Laws 11  1.3.3 Practice Questions ⑮D V = [xR V I = it I R R = Fig. 1.3-3: Simple electric circuit (a) Referring to Fig. 1.3-3, determine: (i) V if I = 0.5 A and R = 100 Ω. [50 V] V = IXR = 0. 5/ x100m = (ii) I if V = 110 V and R = 550 Ω. [0.2 A] I (iii) R if V = 230 V and I = 1.2 A. [191.67 Ω] V R = I = 4x1000 (iv) I if V = 24 V and R = 10 kΩ (kilo-ohms). [0.0024 A] = =1 ↳ convert to otims I = 0 0024A.. R = 10ke I = 10 X 1000 - 10000 = 100002 (b) A load with resistance of 200 Ω is connected to a supply of 110 V. What is the current drawn from the supply? R 2001 V [0.55 A] llOV I =? =. =. I - (c) The current flowing in a circuit with a load of resistance 50 Ω is 0.2 A. What is the supply voltage? [10 V] R = 50r I 0. =. 2A V IXR = 50x0 2 V ? = =. ↓V. I Unit 1.3 | Electric Circuit Laws 12