Smart Electrical Technology A201 Textbook PDF

SMART ELECTRICAL TECHNOLOGY A201 MOE-ITE Applied Subject for Normal Course © 2016 Curriculum Planning and Development Division This publication is not for sale. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any mea...

SMART ELECTRICAL TECHNOLOGY A201 MOE-ITE Applied Subject for Normal Course © 2016 Curriculum Planning and Development Division This publication is not for sale. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the Ministry of Education, Singapore. This publication was developed in collaboration with the Institute of Technical Education. It is intended for educational purposes in MOE schools only. Smart Electrical Technology CONTENTS 1. ELECTRICAL PRINCIPLES AND CIRCUITS 1.1 Electrical Safety 1.2 Electric Circuits 1.3 Electric Circuit Laws 1.4 Electric Circuit Connections 1.5 Power and Energy in an Electric Circuit 1.6 Electric Power Sources 1.7 Electrical Hazards and Protection 1.8 Electrical Cables 1.9 Electrical Test Instruments 1.10 Conventional Lighting Circuits 1.11 Electrical Supply Systems 2. HOME AUTOMATION SYSTEMS 2.1 Home Automation Basics 2.2 Basic Devices, Topology and Communication 2.3 Wireless Network and Radio Frequency 2.4 Installing and Programming Devices 2.5 Switching and Dimming 2.6 Timing and Sensing 2.7 Scene Control 2.8 Central Control and Visualisation 2.9 Home Automation Controllers 2.10 Basic Networking and Installing Relevant Mobile Apps 2.11 Controlling Electrical Loads and Security Features via Mobile Phone ACKNOWLEDGEMENTS 1 1 6 10 13 21 28 35 42 46 49 58 63 63 72 75 82 86 92 95 97 99 104 106 108 Smart Electrical Technology UNIT 1 ELECTRICAL PRINCIPLES AND CIRCUITS At the end of the unit, students should be able to:  understand the basic principles of electricity  connect simple electrical lighting circuits for residential premises  use a multimeter to test for electrical continuity The section is organised into 11 sub-units 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.1 Electrical Safety Electric Circuits Electric Circuit Laws Electric Circuit Connections Power and Energy in an Electric Circuit Electric Power Sources Electrical Hazards and Protection Electrical Cables Electrical Test Instruments Conventional Lighting Circuits Electrical Supply Systems Electrical Safety Learning Objectives  Explain the two types of electric shock: • Direct contact • 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 Smart Electrical Technology 1. How an Electric Shock Happens An electric shock occurs when an electrical current flows through the human body (Figs. 1.1-1 and 1.1-2). From L supply N transformer E y 2. Fig. 1.1-1: A person who has received an electric shock y Fig. 1.1-2: How an electric shock occurs Types of Electric Shock There are two types of electric shock: • direct contact • indirect contact 2.1 Direct Contact Direct contact occurs when a person comes into contact with a conductor that is live under normal conditions (Fig. 1.1-3). Supply transformer P Earth metal N Protective conductor y Fig. 1.1-3: Electric shock due to direct contact 2 Smart Electrical Technology 2.2 Indirect Contact Indirect contact occurs when a person comes into contact with exposed conductive parts that have become live under fault conditions, i.e., parts which are not normally live but have become live due to faults such as insulation failure (Fig. 1.1-4). Supply transformer Fault P Earth metal N Protective conductor y 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. A shock current of 50 mA can be fatal. Figure 1.1-5 shows a hand burned by electricity. y Fig. 1.1-5: Electrical burn 3 Smart Electrical Technology 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, otherwise the current may pass through you too. 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. 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. 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. 6. General Safety Rules in the Electrical Laboratory 6.1 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 and replace frayed cords or broken power points. Use test equipment correctly. Read the instruction booklet and follow all instructions. 4 Smart Electrical Technology • 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 main electricity supply. • Keep electrical equipment away from water and wet areas to lower the risk of electric shock. 6.2 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. 5 Smart Electrical Technology 1.2 Electric Circuits Learning Objectives  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 current, voltage and resistance, and to check for continuity in an electrical installation  Exercise safety precautions when handling and using measuring instruments 1. Electric Circuits An electric circuit is the physical pathway for a 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 A Battery y Connecting wires Lamp Fig. 1.2-1: Simple electric circuit 6 Smart Electrical Technology 2. Voltage, Current and Resistance The three basic electrical quantities in a basic electric circuit are: • supply voltage or electromotive force • current • resistance 2.1 Supply Voltage / Electromotive Force This is the force required to cause the current to flow in a closed circuit. It is represented by V (supply voltage) or emf (electromotive force) and is measured in volts (V). 2.2 Current It is the electrical quantity that is needed for the electrical load to function, i.e., the flow of current allows the lamp in Fig. 1.2-1 to light up. It is represented by I and is measured in amperes (A). 2.3 Resistance Resistance helps limit the current flow in the circuit. 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. It is represented by R and is measured in ohms (Ω). 3. Ammeter An ammeter is used for measuring current flowing in the circuit. It is always connected in series to the load in the circuit. The symbol for an ammeter is A Figure 1.2-2 shows how an ammeter should be connected to measure the current in an electric circuit. A V y Lamp Fig. 1.2-2: Ammeter connection 7 Smart Electrical Technology 4. Voltmeter A voltmeter is used for measuring the emf of the battery, supply voltage and voltage across the load in the circuit. It is always connected in parallel to the load or supply source where the voltage is to be measured. The symbol for a voltmeter is V Figure 1.2-3 shows how a voltmeter should be connected to measure the voltage in an electric circuit. V y 5. V Lamp Fig. 1.2-3: Voltmeter connection Ohmmeter An ohmmeter is used for measuring the resistance of an electrical load in the circuit. It is always connected in parallel to the load where the resistance is to be measured. The symbol for an ohmmeter is Ω Figure 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 that the power is switched off. Otherwise, the user may receive an electric shock and the ohmmeter may be damaged. Ω y Electrical load Fig. 1.2-4: Ohmmeter connection 8 Smart Electrical Technology 6. Multimeter A multimeter is a device that can measure voltage, current and resistance. It can also be used to diagnose electrical problems. There are two kinds of multimeter: • 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 (-). y Fig. 1.2-5: Analogue multimeter y Fig. 1.2-6: Digital multimeter When using a multimeter to measure resistance, make sure that the 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. 9 Smart Electrical Technology 1.3 Electric Circuit Laws Learning Objectives  Understand the relationship between current, resistance and voltage (Ohm’s law): V = I × R  Apply Ohm's law to determine current, resistance or voltage in an electric circuit  Connect a simple electric circuit comprising an ammeter, a voltmeter, a load and a power supply to verify Ohm’s law 1. Ohm's Discovery The relationship between V, I and R was discovered by scientist Georg 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. R I V y Fig. 1.3-1: Simple electric circuit The following formula is derived from Ohm’s law: V I R V=I×R where V is in volts (V) I is in amperes (A) R is in ohms (Ω) 10 Smart Electrical Technology 2. Worked Example R I V y 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? I= 3. V 230 V = = 0.46 A R 500 Ω Tutorial R I V y Fig. 1.3-3: Simple electric circuit (A) Refer to Fig. 1.3-3 for the following questions. Determine: (i) V if I = 0.5 A and R = 100 Ω (50 V) (ii) I if V = 110 V and R = 550 Ω (0.2 A) 11 Smart Electrical Technology (iii) R if V = 230 V and I = 1.2 A (191.67 Ω) (iv) I if V = 24 V and R = 10 kilo-ohms (kΩ) (0.0024 A) (B) A load of resistance 200 Ω is connected to a supply of 110 V. What is the current drawn? (0.55 A) (C) The current flowing in a circuit of 50 Ω is 0.2 A. What is the supply voltage? (10 V) 12 Smart Electrical Technology 1.4 Electric Circuit Connections Learning Objectives  Identify the three methods of connecting electrical loads: • Series • Parallel • Series-parallel  State the characteristics of a series circuit: • One path for current flow • Supply voltage is equal to the sum of all individual voltages • Total resistance is equal to the sum of all individual resistances  State the characteristics of a parallel circuit: • Supply voltage is the same as all branch voltages • Supply or total current is equal to the sum of all individual branch currents • Total resistance is lower than the lowest individual resistance  Determine the total resistance of a series circuit comprising two resistors using the formula: RT = R1 + R2  Determine the total resistance of a parallel circuit comprising two 1 1 1 resistors using the formula: = + RT R1 R2  Connect a series circuit comprising two resistors, a voltmeter and an ammeter for the purpose of verifying the characteristics of a series circuit  Connect a parallel circuit comprising two resistors, a voltmeter and an ammeter for the purpose of verifying the characteristics of a parallel circuit  Recognise that a series-parallel circuit has the characteristics of both series and parallel circuits 1. Types of Circuit There are basically three types of circuit, which are classified in terms of how their components (or loads) are connected to each other. They are: • series circuit • parallel circuit • series-parallel circuit 13 Smart Electrical Technology 2. Characteristics of a Series Circuit R1 I1 R2 I2 V1 V2 IS VS y (A) Fig. 1.4-1: Series circuit There is only one path for the current to flow, that is: IS or IT = I1 = I2 (B) The supply voltage, VS or VT , is the sum of all the individual voltages: VS or VT = V1 + V2 (C) The total resistance, RT , is the sum of all the individual resistances: RT = R1 + R2 Note that the total resistance is higher than the highest individual resistance in the circuit. 3. Applying Ohm's Law to a Series Circuit VT = IT × RT V1 = I1 × R1 V2 = I2 × R2 14 Smart Electrical Technology 4. Worked Examples R1 I1 I2 V1 R2 V2 IS VS y (A) Fig. 1.4-2: Series circuit Refer to Fig. 1.4-2. Let R1 = 10 Ω, R2 = 20 Ω and VS = 120 V. Determine: (i) the total resistance, RT RT = R1+ R2 = 10 Ω + 20 Ω = 30 Ω (ii) the supply current, IS IS = VS RT = 120 V 30 Ω =4A (iii) the voltage across R1 VS = IS × R1= 4 A × 10 Ω = 40 V (B) Refer to Fig. 1.4-2. Let R1 = 40 Ω, IS = 0.5 A and VT = 60 V. Determine: (i) the total resistance, RT VT 60 V = R = = 120 Ω T 0.5 A I S (ii) the resistance, R2 R2 = RT − R1= 120 Ω − 40 Ω = 80 Ω (iii) the voltage across R1 V1 = IS × R1= 0.5 A × 40 Ω = 20 V 15 Smart Electrical Technology 5. Characteristics of a Parallel Circuit I1 R1 V1 I2 R2 V2 IS VS y (A) Fig. 1.4-3: Parallel circuit The supply voltage is equal to all branch voltages, that is: VS or VT = V1 = V2 (B) The supply current or total current is equal to the sum of all individual branch currents: IS or IT = I1 + I2 (C) (i) The total or equivalent resistance is equal to the reciprocal of the sum of the reciprocal of individual resistances: 1 RT = 1 R1 + 1 R2 (ii) When there are only two resistances, the following formula can be used: RT = R1 × R2 R1 + R2 Note that the total or equivalent resistance is lower than the lowest individual resistance in the circuit. 16 Smart Electrical Technology 6. Applying Ohm’s Law to a Parallel Circuit VT = IT × RT 7. V1 = I1 × R1 V2 = I2 × R2 Worked Examples I1 R1 V1 I2 R2 V2 IS VS y (A) Fig. 1.4-4: Parallel circuit Refer to Fig. 1.4-4. Let R1 = 30 Ω, R2 = 20 Ω and VS = 120 V. Determine: (i) the total resistance, RT 1 1 = = 12 Ω RT = 1 1 1 1 + + R1 R2 30 Ω 20 Ω (ii) the supply current, IS VS 120 V = IS = = 10 A RT 12 Ω (iii) the current flowing in R1 V1 = VT = 120 A I1 = V1 120 V = =4A 30 Ω R1 (iv) the current flowing in R2 I2 = V2 120 V = =6A 20 Ω R2 17 Smart Electrical Technology (B) Two resistors of 80 Ω and 60 Ω respectively are connected in parallel across a 100 V supply. Determine the total resistance of the circuit and the current flowing in each of the branches. RT = 8. 1 1 1 + R1 R2 = 1 1 1 + 80 Ω 60 Ω I1 = V1 100 V = = 1.25 A 80 Ω R1 I2 = V2 100 V = = 1.67 A 60 Ω R2 = 34.30 Ω Tutorial R1 I1 I2 V1 R2 V2 IS VS y (A) Refer to Fig. 1.4-5. Let R1 = 80 Ω, R2 = 160 Ω and IS = 0.2 A. Determine: (i) the total resistance, RT Fig. 1.4-5: Series circuit (ii) the supply voltage, VS (iii) the voltage across R1 (240 Ω) (120 V) (40 V) 18 Smart Electrical Technology (iv) the voltage across R2 (80 V) (B) Two resistors of 50 Ω and 70 Ω respectively are connected in series across a 100 V supply. Determine the total resistance of the circuit and the supply current. (120 Ω; 0.83 A) (C) Two resistors of 100 Ω and R Ω respectively are connected in series across a 110 V supply. If the current drawn is 0.5 A, determine the value of R. (120 Ω) (D) Refer to Fig. 1.4-6. Let R1 = 60 Ω, R2 = 80 Ω and IT = 1 A. I1 R1 V1 I2 R2 V2 IS VS y Fig. 1.4-6: Parallel circuit Determine: (i) the total resistance, RT (ii) the supply voltage, VS (34.29 Ω) (34.29 V) 19 Smart Electrical Technology (E) (iii) the current flowing in R1 (iv) the current flowing in R2 (0.43 A) Two resistors of 60 Ω and 90 Ω respectively are connected in parallel across a 48 V supply. Determine: (i) the total resistance of the circuit (ii) the supply current (iii) the current flowing in each of the two branches (F) (0.57 A) (36 Ω) (1.33 A) (0.8 A; 0.53 A) The total resistance of a parallel circuit with two resistors is 300 Ω. The resistance of one of the resistors is 500 Ω. If the supply voltage is 100 V, determine: (i) the supply current (ii) the current flowing in each of the two branches (iii) the resistance of the other resistor (0.33 A) (0.2A; 0.13 A) (769.23 Ω) 20 Smart Electrical Technology 1.5 Power and Energy in an Electric Circuit Learning Objectives  Define the terms “power” and “energy” in electric circuits  State the units of measurement for power (watt) and energy (joule)  State and apply the formula to determine the power in an electric circuit comprising one electrical load: P = V × I  State and apply the formula to determine the energy in an electric circuit comprising one electrical load: E = P × t  State the practical unit of energy consumption in a household (kilowatt-hour or kWh)  Calculate the energy consumption of an electrical load in kilowatt-hours 1. Power Power is the rate of doing work. The faster the work is done, the more power is required to do it. Power is represented by P and is measured in watts (W) or joules per second (J/s). In a direct current circuit, power can be determined by the following formulae: P = VI P P = I 2R V2 P= R 2. I V P-V-I Relationship Triangle y Fig. 1.5-1: Formulae for determining power Power Rating of Electrical Load Most electrical loads have ratings that include both voltage and power. The voltage rating of the load is an indication of the voltage the load is designed for. The power rating will determine how much work the load can do. Here are some common ratings of home appliances: • Light bulb (60 W, 230 V; 40 W, 230 V) • Fluorescent lamp (32 W, 230 V) • Water heater (3,000 W, 230 V) A 60 W bulb can do more work, and would thus be brighter, than a 40 W bulb. 21 Smart Electrical Technology 3. Worked Examples (A) Determine the rated current and resistance of a 60 W, 230 V bulb. (0.26 A, 884.62 Ω) P 60 W = 0.26 A I= V = 230 V R= (B) V 230 V = = 884.62 Ω I 0.26 A A resistance of 10 Ω is connected to a voltage of 220 V. Determine the power dissipated. (4.84kW) P= (C) V 2 (220 V)2 = = 4840 W = 4.84 kW R 10 Ω The power dissipated in a 1,000 Ω resistor is 50 W. Determine the supply voltage and current. (223.61 V; 0.22 A) P= V2 R V2 = P × R V= I= 4. P×R = 50 W × 1000 Ω = 50000 = 223.61 V P 50 W = = 0.22 A V 223.61 V Energy Energy is defined as the capacity to perform work. Energy exists in many forms and may be converted from one form into another. For example, a lead-acid battery converts chemical energy into electrical energy when it discharges, and vice-versa when it is being charged; a generator converts mechanical energy into electrical energy; and an electric radiator converts electrical energy into heat energy. Energy must be consumed in order to do work, and the amount of work done is directly proportional to the energy used. 22 Smart Electrical Technology The electrical energy taken from a source depends on the electric power of the appliance and the length of time used. Energy is represented by W and is measured in joules (J) or watt-seconds (Ws). 1 joule 1 watt 100 watts = 1 watt-second = 1 joule per second = 100 joules per second However, the joule or watt-second is too small to be used for computing the electrical energy consumed by households or industries. The practical unit used for this purpose is kilowatt-hour (kWh). Electrical energy in kWh can be determined by the following formula: Energy (W) = Power (P) × Time (t) where power is in kilowatts (kW) and time is in hours (h). 5. Cost of Energy (Energy Bill) The cost of electrical energy is calculated by the product of energy consumed in kWh and the rate per unit. 1 unit = 1 kWh Cost of Energy = Energy (kWh) × Rate ($) 23 Smart Electrical Technology 6. Worked Examples (A) A 1,000 W, 230 V heater is turned on for 10 hours. Determine the electrical energy used in: (i) joules W = P × t = 1000 W × (10 × 60 × 60) s = 36000000 J = 3.6 × 107 J (ii) kilowatt-hours W = P × t = 1 kW × 10 h = 10 kWh (B) A 2 kW refrigerator is turned on 24 hours per day. Determine: (i) the energy consumed (kWh) in one month (30 days) W = 2 kW × (24 × 30) h = 1440 kWh (ii) the cost of using the refrigerator at $0.25 per unit Cost = 1440 kWh × $0.25 = $360 (C) 7. What is the total energy consumed by a 1,000 W heater for 30 days if it is turned on for an average of 10 hours per day? If the rate of energy is $0.25 per unit, what is the bill for using this heater? W = 1 kW × (10 × 30) h = 300 kWh Cost = 300 kWh × $0.25 = $75 How to Interpret an Energy Bill Most households in Singapore pay their utilities bill to SP Services. The bill has three main components: • electricity services • gas services • water services 24 Smart Electrical Technology y Fig. 1.5-2: Utilities bill of a residential premise 25 Smart Electrical Technology 8. High-Energy Consumption Home Appliances The higher the power rating, the more energy an appliance consumes. Home appliances that consume a lot of energy include the: • refrigerator • air conditioner • water heater • electric iron 9. Strategies to Reduce Electrical Energy Consumption It is important for us to reduce our energy consumption. Not only does this reduce our energy bills, it also helps lessen the effects of air pollution and global warming. Making energy-efficient choices allows us to save electricity and money without compromising on our quality of life. Here are some easy energy-saving habits: • Unplug Unplug chargers that are not in use. Switch off the power supply to televisions, home theatre equipment and stereos when not in use. If you switch them off using only the on/off button, their combined stand-by consumption may be equivalent to that of a 75 or 100 W light bulb. • Set Computers to “Sleep” and “Hibernate” Enable the “sleep” mode on the computer to reduce power consumption during periods of inactivity. Set the computer to automatically “hibernate” after 30 minutes of inactivity. When you are done working for the day, shut down the computer. • Take Control of the Temperature Set the air conditioner to about 25 °C. • Use Appliances Efficiently Check that the seal on the oven door is undamaged and tight. When cooking with the oven, do not open the door unnecessarily. Use a microwave oven to reheat small amounts of food. When using the washing machine, set the water level to match the size of each load. Use cold water to wash and rinse clothes. • Use Energy-Saving Light Bulbs Replace incandescent light bulbs with energy-saving compact fluorescent bulbs or LED bulbs. • Flick a Switch Switch off the lights when you leave a room. 26 Smart Electrical Technology 10. Tutorial (A) A resistor of 500 Ω is connected to a supply voltage of 230 V. Determine the power dissipated. (105.8 W) (B) The power dissipated in a 200 Ω resistor is 1,000 W. Determine the supply voltage and current. (2.24 A; 448 V) (C) A 100 W, 230 V heater is turned on for 10 hours. Determine the energy used in: (i) joules (ii) kilowatt-hours (3,600,000 J) (1 kWh) (D) A 1 kW refrigerator is turned on for 24 hours per day. What is the energy consumed in one month (30 days)? (720 kWh) (E) A 1,000 W air conditioner in a residential premise is turned on for six hours per day. Determine: (i) the energy consumed in 30 days (ii) the cost of using the air conditioner for 30 days if the rate of energy is $0.25 per unit (180 kWh) ($45) 27 Smart Electrical Technology 1.6 Electric Power Sources Learning Objectives  State that supply sources can be alternating current (AC) or direct current (DC)  Understand the differences between AC and DC  Identify the common sources of AC and DC  Define the function of a cell  Draw the symbol for a cell  Explain the difference between a cell and a battery 1. Battery A battery is a number of cells connected together. A dry cell is a galvanic electrochemical cell with a pasty low-moisture electrolyte. This electrolyte paste, together with the two electrodes (the anode or positive terminal, and the cathode or negative terminal), provides the necessary chemical reactions to convert chemical energy into electrical energy. 2. Direct Current and Alternating Current The current in an electric circuit can either be direct current (DC) or alternating current (AC). DC flows in only one direction and has a constant value, while AC flows in both directions and has a value that changes all the time. Figure 1.6-1 shows the waveform of a DC. Current Current Time y Fig. 1.6-1: DC waveform 28 Smart Electrical Technology Figure 1.6-2 shows the waveform of an AC. Current Current Time One cycle y Fig. 1.6-2: AC sine waveform DC can be obtained from equipment such as batteries and DC generators. These DC sources have fixed plus (+) and minus (–) terminals as shown in Fig. 1.6-3. Plus or positive terminal y + – Minus or negative terminal Fig. 1.6-3: Battery terminals AC can be obtained from AC generators such as the ones shown in Fig. 1.6-4. Single phase generator, 100kVA y 3 phase generator, 150kVA Fig. 1.6-4: AC generators 29 Smart Electrical Technology 3. Single-Phase and Three-Phase Supply Two types of alternating power supply are commonly used in Singapore: • single-phase supply • three-phase supply Single-phase supply usually has two wires for connection, namely the phase (L) and neutral (N) conductors (Fig. 1.6-5). Three-phase supply has: • three wires (L1, L2 and L3; see Fig. 1.6-6); or • four wires (L1, L2, L3 and N; see Fig. 1.6-7). L N L1 L2 L3 L1 L2 L3 N Single-phase load Three-phase load Three-phase load y Fig. 1.6-5: Singlephase supply y Fig. 1.6-6: Three-phase, three-wire supply y Fig. 1.6-7: Three-phase, four-wire supply Although single-phase electricity is used to power common domestic and office electrical appliances, three-phase AC systems are universally used to distribute and supply electricity directly to higher-powered equipment. 3.1 Cells Cells can be divided into two main types: • primary cells (non-rechargeable) • secondary cells (rechargeable) In a primary cell, the chemical reaction is irreversible. This means that chemical energy is completely converted into electrical energy and the cell cannot be used again. Primary cells are most commonly used in smaller, portable devices. In a secondary cell, the chemical reaction is reversible. This means that chemical energy is converted into electrical energy when the cell is discharging, and electrical energy is converted into chemical energy when the cell is being charged. Secondary cells can be used in portable consumer products and in larger items and tools that take up more power. 30 Smart Electrical Technology 4. Battery Types Here are some types of batteries available in the market. 4.1 Zinc-Carbon Battery The zinc-carbon battery (Fig. 1.6-8) is packaged in a zinc can (electrode) with a carbon anode and a paste of zinc chloride and ammonium chloride. It is a primary cell because once the cell is discharged, it must be discarded. It has a cell potential (emf) of 1.5 V and is widely used because of its relatively low cost. It can be used in remote controls, flashlights, toys and transistor radios, which do not consume too much power. y Fig. 1.6-8: Zinc-carbon batteries 4.2 y 4.3 Fig. 1.6-9: Alkaline batteries Alkaline Battery The alkaline battery (Fig. 1.6-9) has an alkaline electrolyte of potassium hydroxide, unlike the acidic electrolyte of zinc-carbon batteries. Alkaline batteries have a nominal voltage of 1.5 V, and have a higher energy density and longer shelf-life than zinc-carbon batteries. Most alkaline batteries cannot be recharged. Sometimes, charging them will make the batteries leak hazardous liquids that will corrode the equipment. Nickel-Cadmium Battery The nickel-cadmium (NiCd) battery (Fig. 1.6-10) has electrodes using nickel oxide hydroxide and metallic cadmium. It is rechargeable, with a nominal voltage of 1.2 V. Small NiCd dry cells, made in the same size as primary cells, are used to power portable electronics and toys. Other NiCd batteries are specially designed for devices such as cordless and wireless telephones, power tools and emergency lighting. y Fig. 1.6-10: NiCd batteries 31 Smart Electrical Technology 4.4 y 4.5 Fig. 1.6-11: NiMH batteries Nickel-Metal Hydride Battery The nickel-metal hydride (NiMH) battery (Fig. 1.6-11) uses a hydrogen-absorbing alloy. It is rechargeable, with a nominal voltage of 1.2 V, and is widely used in most consumer electronics. Lithium-Ion Battery The lithium-ion battery (Fig. 1.6-12) has an electrolyte that allows lithium ions to move between the two electrodes. It is rechargeable, with a nominal voltage of 3.6 V. It is commonly used in digital cameras, watches and other types of portable electronics. y 4.6 Fig. 1.6-12: Lithium-ion battery Lead-Acid Battery The lead-acid battery (Fig. 1.6-13) has electrodes made of lead dioxide (cathode) and sponge metallic lead (anode), with a sulphuric acid solution as an electrolyte. It is a secondary cell, with a nominal voltage of 2 V. It is sturdy and often used to power motor vehicles, but must be properly disposed of due to its toxic nature. y 5. Fig. 1.6-13: Lead-acid batteries Recycling Batteries Many types of batteries, such as those used in digital cameras and mobile phones, can be recycled. NiMH batteries are environmentally friendly because they are made from non-toxic metals. However, NiCd batteries and others contain highly toxic heavy metals that are harmful to the environment, so these batteries must be disposed of properly. Visit the website of the National Environment Agency (www.nea.gov.sg) to find out more about e-waste recycling. 32 Smart Electrical Technology 6. Emerging and Future Types of Energy 6.1 Renewable Energy Climate change and the uncertainty of oil prices have inspired more research on renewable energy, which includes major natural resources such as solar and wind power. • Wind Power Wind is the ideal source of energy because it is clean and environmentally friendly, and increasingly more countries are using wind turbines to generate power for electricity. The United States, India, Germany, France, Denmark and China are some of the countries that have invested much in this technology. y Fig. 1.6-14: Windmills creating wind power Sunlight • Solar Power Solar power converts sunlight into useful electricity. Light energy is Current flow Anti-reflection Front collected and captured on solar coating contact or photovoltaic (PV) cells, which in turn generate renewable energy. Initially, the PV cell was used to power small devices such as calculators and watches, and was later used in rooftop panels to N-type power water heaters. Today, you semiconductor Back contact can find PV panels at chalets in P-type semiconductor East Coast Park and at various public-sector buildings in Singapore. y Fig. 1.6-15: Solar cell Other countries have used this technology to build solar power stations capable of supplying tens of megawatts of electrical energy. y Fig. 1.6-16: Solar cell made from a monocrystalline silicon wafer 33 Smart Electrical Technology • Hydropower Hydropower uses water as a source of energy and is usually available in countries with large rivers, such as China, India and the United States. In a hydroelectric plant, specially designed turbines convert the kinetic energy of falling or running water into mechanical energy, which is in turn converted into useful electrical energy. y Fig. 1.6-17: Hydroelectric plant 34 Smart Electrical Technology 1.7 Electrical Hazards and Protection Learning Objectives  Explain the electrical hazards caused by overcurrent and earth fault  Understand the need to protect people and properties against electrical hazards  Explain the importance of earthing to avoid the risk of electric shock  Describe the application and selection of different electrical protective devices for residential use (fuses, circuit breakers, residual current circuit breakers)  Explain how protective devices can protect against electrical hazards 1. Introduction All electrical installations (that is, the conductors and apparatus) must be protected against overcurrent and earth fault (leakage). This is because if a current in excess of the rating of the circuit flows indefinitely, it would result in fire and damage. In addition, if there is an earth fault or current leaking to the metal casing of the apparatus, the metal casing will become live and anyone who touches the live part will receive an electric shock. Protective devices are thus installed to disconnect the faulty section from the supply before damage and electric shock occur. 2. Earthing The earth is considered a large conductor at zero potential. The neutral at the supply transformer, shown in Fig. 1.7-1, is connected to the earth. This is done by connecting a conductor from the neutral at the supply origin to a rod driven into the ground. This is known as earthing. 35 Smart Electrical Technology 2.1 Importance of Earthing Consumer Supply If L Earth fault Protective device 230 V N N If If If If Earth Earth fault current Earth electrode y Fig. 1.7-1: Installation without earthing In Fig. 1.7-1, the exposed metallic part of the apparatus at the consumer’s installation is not earthed. A phase to earth fault occurs when the live (phase) terminal touches the exposed metallic part. This causes the metallic part to become live, and anyone who comes into contact with the live part will become part of the earth fault current path and will thus receive an electric shock. Consumer Supply If 230 V N N Earth fault RCCB L If If CPC If If Earth Earth fault current Earth electrode y Fig. 1.7-2: Installation with earthing 36 Smart Electrical Technology In Fig. 1.7-2, the exposed metallic part of the apparatus at the consumer’s installation is earthed. • When a phase to earth fault occurs in the electrical equipment, most of the earth fault current will flow through the circuit protective conductor (CPC). • Anyone who comes into contact with the live part will receive a negligible earth fault current, as the human body’s resistance is comparatively much higher than the resistance of the CPC. • If the earth leakage current is high enough, it will trigger the residual current circuit breaker (RCCB) inserted between the phase and neutral. In order to avoid the risk of electric shock, it is important to: • provide a path for the earth leakage current to operate the circuit protection effectively and rapidly; and • maintain all metalwork at the same potential. 3. Overcurrent An overcurrent is any current that exceeds the current capacity of the circuit. There are two main types: • An overload current is the result of adding too many electrical loads to a circuit, causing a current higher than what the circuit is designed to handle. • A fault current is the result of two or more live conductors touching one another (short circuit), or of a phase conductor touching the protective conductor directly or indirectly (earth fault). 4. Types of Protective Devices Protective devices may be classified as: • overcurrent protective devices; or • residual current protective devices. 4.1 Overcurrent Protective Devices Overcurrent protection in a circuit can be by means of: • a fuse; or • an excess current circuit breaker. 37 Smart Electrical Technology Figure 1.7-3 shows some examples of overcurrent protective devices. Cartridge fuse High-breaking capacity fuse y Miniature circuit breaker Fig. 1.7-3: Overcurrent protective devices It is necessary to install fuses and single-pole circuit breakers in the phase conductors of systems with earthed neutrals. This is to ensure the safety of the user when an overcurrent occurs in the installation, as shown in Fig. 1.7-4a. If the neutral is fused and when there is an overload or a short circuit, the neutral fuse will blow. This will pose a danger to the user (Fig. 1.7-4b). Hence, the neutral must be constructed with a solid link to ensure discrimination of the fuse, i.e., the fuse must cut off the supply to protect the user when there is a fault. P N y Phase fuse blown Neutral link Fig. 1.7-4a: Correct installation of fuse P N y Neutral fuse blown Fig. 1.7-4b: Wrong installation of fuse 38 Smart Electrical Technology 4.1.1 Fuse A fuse consists of a thin wire connected in series with the circuit to be protected. The wire is thick enough to carry a normal current without overheating. However, if the current exceeds its nominal value, the fuse wire will melt, breaking the circuit. A fuse consists of three main parts: • a fuse element, which is designed to melt when the fuse operates; • a fuse carrier, which is made of incombustible material, such as porcelain or moulded plastic, and has two contacts for connecting the fuse element or fuse link with screws or clips; and • a fuse base, which is made of incombustible material and encloses the fixed contacts to which the incoming and outgoing cables are connected. The three main types of fuse are: • rewirable or semi-enclosed fuse • cartridge fuse • high-rupturing or high-breaking capacity fuse a. Rewirable Fuse A rewirable fuse, shown in Fig. 1.7-5, consists of a fuse link or fuse element and a fuse base. The fuse link has two sets of contacts, which can be fitted onto the base. The fuse element, which is usually made of Fuse base tinned copper wire, is connected between the terminals of the fuse link. An asbestos tube or pad is fitted to reduce the effects of arcing when the fuse element melts. However, the fuse element can be too easily replaced with one of the wrong size and, compared to other fuses, the rewirable fuse takes the longest to cut off a faulty circuit. Rewirable fuses are thus not used in Singapore today. Fuse element Fuse carrier y Fig. 1.7-5: Rewirable fuse 39 Smart Electrical Technology Fuse element End cap Porcelain tube b. Cartridge Fuse A cartridge fuse, shown in Fig. 1.7-6, consists of a sealed tube with metal end caps. The fuse element passes through the tube from cap to cap, and is welded or soldered to the inside of the cap. A more expensive fuse may have an indicator on the outside to show whether the fuse element is open-circuited or not. y Fig. 1.7-6: Cartridge fuse When the fuse is blown, the whole cartridge must be replaced. It is commonly used in the 13A plug and in electronic equipment. Ceramic tube c. High-Rupturing Capacity Fuse Elements End cap A high-rupturing capacity (HRC) fuse (Fig. 1.7-7), sometimes called a high-breaking capacity (HBC) fuse, is used extensively in industrial and commercial installations. It is designed to protect a circuit against heavy overload. It is also capable of opening a circuit under both overload and short-circuit conditions. Fixing slot Indicator y Quartz filling Fig. 1.7-7: HRC fuse 4.1.2 Excess Current Circuit Breaker An excess current circuit breaker is a device for making and breaking a circuit under normal and abnormal conditions. It is an automatic switch that opens in the event of an excess current. The switch can be closed again when the current returns to normal, as the device is not damaged during normal operation. The two common types of excess current circuit breaker are: • miniature circuit breaker • moulded-case circuit breaker 40 Smart Electrical Technology a. Miniature Circuit Breaker A miniature circuit breaker (MCB), shown in Fig. 1.7-8, is used as an alternative to the fuse. It gives both overload and short-circuit protection. The MCB is fitted with a thermal device for overload protection and a magnetic device for speedy short-circuit protection. It is commonly used to protect lighting and power circuits in residential premises. y Fig. 1.7-8: Miniature circuit breaker b. Moulded-Case Circuit Breaker A moulded-case circuit breaker (MCCB), shown in Fig. 1.7-9, is commonly used as an alternative to the HRC fuse. y 4.2 Fig. 1.7-9: Moulded-case circuit breakers The MCCB has high current and breaking capacities, which make it popular for use in commercial and industrial installations. Residual Current Protective Devices y Fig. 1.7-10: Residual current circuit breaker Look at the consumer unit in your home and you can find a residual current circuit breaker (RCCB), or residual current protective device, similar to the one shown in Fig. 1.7-10. It is specially designed to protect us if there is an earth leakage current (in the range of 5 to 30 mA) in an electrical appliance or any part of the house. An RCCB can cut off the supply within 40 milliseconds to prevent an electric shock. 41 Smart Electrical Technology 1.8 Electrical Cables Learning Objectives      1. State the function of a cable State the three main parts of a cable Describe the specific roles of the three main parts of a cable State the common materials used to make conductors and insulators Identify the common sizes of cable used in residential premises Conductor A conductor is any material that allows a current to flow through easily. Conductors for everyday use should be: • of low electrical resistance; • mechanically strong and flexible; and • relatively cheap. Silver has the lowest resistivity and thus better conductivity compared to copper and aluminium. However, it is too expensive to install cables made of silver. The most commonly used material for cables is copper, as it is mechanically stronger and has better conductivity than aluminium. 2. Insulator An insulator is any material that has high electrical resistance. Its function is to confine a current to the conductor, minimising the risk of electric shock and fire. Many materials can be used to insulate cable conductors, accessories and equipment. The type of insulation used depends on the supply voltage it is subject to and the operating environment. Different insulating materials are used for different applications (see Table 1.8-1). 42 Smart Electrical Technology Table 1.8-1: Common insulating materials and applications 3. Material Application Bakelite Accessories Ceramic Support for switchgears Polyvinyl Chloride (PVC) Cable insulation or sheaths Mica Insulation for heating elements Rubber Cable insulation Paper Cable insulation and capacitors Cable A cable consists of at least two parts: • a conductor • an insulator Conductor Insulator y Fig. 1.8-1: Cable conductor and insulator The conductor is normally made of copper and aluminium. The insulator can be made of: • polyvinyl chloride (PVC); • vulcanised rubber (VIR); • mineral insulation (MI); or • cross-linked polyethylene (XLPE). A cable is always named after the insulation used. Hence, the PVC cable got its name because its insulator is made of PVC. 43 Smart Electrical Technology 4. Mechanical Protection Some cables have an additional sheath and/or armour. These are provided to prevent mechanical damage to the cables during installation and throughout their subsequent operation. Mechanical protection Insulation Conductor y Fig. 1.8-2: Parts of a cable Sheathing involves covering the cable insulation with materials such as PVC, copper and aluminium. Apart from giving mechanical protection, the sheath also prevents moisture from damaging the insulation of the cable. Armouring involves wrapping the cable with metal in the form of steel wire or metal tape. Very often, cables are sheathed and armoured to give complete protection (Fig. 1.8-3). a. b. c. d. e. a b c y 5. d Copper conductor PVC insulation PVC bedding Steel-wire armour PVC sheath e Fig. 1.8-3: PVC cable with steel-wire armour Stranding Stranding refers to the conductor being divided into a number of smaller wires that are twisted together in a spiral fashion, forming a core equivalent to a single wire of the required size (Fig. 1.8-4). Conductor with seven strands of wire y Fig. 1.8-4: Cable stranding Conductors are often stranded to ensure flexibility and ease of handling. 44 Smart Electrical Technology 6. Cable Size The size of a cable can be specified by: • the number of strands and the diameter of each strand (e.g., 7/0.67, where “7” refers to seven strands of wire and “0.67” refers to each strand of wire having a diameter of 0.67 mm); and • the nominal cross-sectional area of its conductor (e.g., 2.5 mm2). A cable is normally indicated by the cross-sectional area of its conductor. Some common cable sizes for residential wiring are 1.5 mm2, 2.5 mm2, 4 mm2, 6 mm2 and 10 mm2. The size of a cable will determine how much current it can carry. 7. Identification To ensure ease of connection, every conductor should be identifiable at its terminations and preferably throughout its length (see Table 1.8-2). Table 1.8-2: Standard colour identification for cables Function Alphanumeric Number Colour Phase of Single-Phase Circuit L Brown Neutral of Single-Phase or Three-Phase Circuit N Blue Phase One of Three-Phase Circuit L1 Brown Phase Two of Three-Phase Circuit L2 Black Phase Three of Three-Phase Circuit L3 Grey Protective (Earth) Conductor - Green and yellow 45 Smart Electrical Technology 1.9 Electrical Test Instruments Learning Objectives  Explain the purpose of test instruments in electrical work (insulation resistance tester, continuity tester, earth tester)  Explain the functions of a socket-outlet polarity tester  Explain the method to test for the correct polarity of a socket outlet 1. Introduction Electrical personnel use many different types of test instruments. These instruments are used to determine whether a circuit is correctly wired, sound or faulty. Some common instruments are the continuity tester, the multimeter, the socket-outlet polarity tester and the insulation resistance tester. 2. Continuity Tester A continuity tester can be used to check the electrical continuity of an electric circuit, measure its resistance and check the condition of the fuses. The instrument carries its own batteries, giving it an independent power supply. If the conductive path is formed, the continuity tester will beep If the conductive path is broken, the continuity tester will not beep y Fig. 1.9-1: Continuity tester Before taking a reading, ensure that: • the circuit to be measured or tested is not connected in parallel with another circuit; • the circuit to be measured is not live; • the instrument has been set on infinity (∞) with the leads disconnected; and • the instrument has been set to 0 Ω with the leads connected together. 46 Smart Electrical Technology 3. Multimeter A multimeter can measure many types of electrical quantities, such as: • current • voltage • resistance A multimeter that is set to measure resistance (Fig. 1.9-2) is basically a continuity tester. If there is continuity, the meter will buzz and it will give a resistance reading of about 0 Ω Touch the other test lead to the screw base of the bulb Touch the test lead to the centre contact of the bulb y Fig. 1.9-2: Measuring resistance When using a multimeter to check the continuity of a circuit, ensure that the circuit is not energised or live. 4. Socket-Outlet Polarity Tester y Fig. 1.9-3: Checking a socket outlet connection Figure 1.9-3 shows a 13A socket-outlet polarity tester. It has three lights to indicate whether the outlet voltage or outlet wiring connection is correct; or whether there are open earth, open neutral, open live, reversed live and earth, or reversed live and neutral wires. 47 Smart Electrical Technology 5. Insulation Resistance Tester An insulation resistance tester is used to measure very high values of resistance (Figs. 1.9-4 and 1.9-5). It is also used to check the conditions of circuit cables and motor winding by measuring the resistance of their insulation. It measures resistance in mega-ohms (MΩ). One mega-ohm is equal to one million ohms. As with using a continuity tester, ensure that the circuit is not energised when using an insulation resistance tester. y Fig. 1.9-4: Digital insulation tester y Fig. 1.9-5: Analogue insulation tester To motor winding connections Insulation resistance tester y AC motor To motor frame Fig. 1.9-6: Measurement with insulation resistance tester 48 Smart Electrical Technology 1.10 Conventional Lighting Circuits Learning Objectives  Define a final circuit  Analyse the design and characteristics of common final circuits: • Lighting final circuit • Power final circuit  Understand the two methods of wiring a socket-outlet final circuit: • Radial final circuit • Ring final circuit  State the cable sizes commonly used for lighting and power circuits  State the standard cable colour codes for a single-phase circuit  State the protective device sizes commonly used for lighting final circuits  State the application of the following electrical accessories: • One-way switch • Two-way switch • Dimmer switch  Wire up and test a conventional lighting final circuit with: • One-way control • Two-way control • Dimmer control  Wire up and test a 13A radial power circuit  Explain the method for testing wiring circuits for safe use  Analyse test results to identify the type of faults 1. Introduction Household electrical systems consist of a number of separate final circuits; some supply power socket outlets while others supply fixed lighting. There are also separate circuits for individual high-power appliances such as electric stoves and air conditioners. Each circuit starts at the consumer unit, has its own MCB or fuse and is sized accordingly. 2. Final Circuit A final circuit refers to a circuit connected directly to current-using equipment, or to one or more socket outlets or other outlet points for the connection of such equipment. When an installation comprises more than one final circuit, each final circuit must be connected separately, via an MCB or a fuse, in the consumer unit or distribution board. Figure 1.10-1 shows a typical final circuit arrangement. 49 Smart Electrical Technology Final circuits Lighting (upstairs) Lighting (downstairs) Heating (immersion heater) Radial (upstairs) Ring (downstairs) Cooker Energy meter Consumer unit N E kWh P N P P N N P P N P Service fuse MCBs Double-pole isolator Double-pole RCCB Bonding to other services Ground P N E Water pipe Gas pipe y 3. N Fig. 1.10-1: Typical final circuit arrangement Lighting Final Circuit All lighting circuits are primarily meant for on/off control. Lighting circuits are normally protected by a 6A or 10A MCB. The standard PVC cable size for a lighting final circuit is 1.5 mm2. The cable colour codes for single-phase lighting final circuits are: • brown for phase (live) • blue for neutral • green and yellow for earth (CPC) In normal installations, good planning usually limits the number of lights in each circuit to about 10, with more than one lighting circuit for each house. This ensures that a building would be less likely to be plunged into darkness when the MCB is in operation. 50 Smart Electrical Technology 4. Power Final Circuit Power circuits supply electricity to sockets that electrical appliances are plugged into. However, appliances that use a lot of electricity, such as electric stoves and water heaters, are connected directly to the power circuits. Like lighting circuits, the power circuits start at the consumer unit and each has its own MCB or fuse. The fuse in the 13A plug protects the flexible cord and the appliance, so the power circuit MCB or fuse now protects only the circuit cables and the socket outlets (Fig. 1.10-2). The cable colour codes for single-phase power circuits are: • brown for phase (live) • blue for neutral • green and yellow for earth (CPC) MCB protects ring circuit cables, spur cables and sockets outlets Socket outlet To other socket outlets Fuse protects flexible cord and appliance 32 A Minimum PVC cable size of 4 mm2 y To other socket outlets Plug (cover removed) 13 A Appliance Fig. 1.10-2: Role of MCB and 13A fuse in ring and radial circuits Types of Final Circuit for 13A Socket Outlets • Radial Final Circuit A radial final circuit consists of a cable run from a consumer unit to a number of 13A socket outlets connected in parallel in one circuit (Fig. 1.10-3). The overcurrent protection for a radial final circuit is a 20A or 32A MCB (or a 20A or 30A fuse). For a 20A circuit, the PVC cable size should be a minimum of 2.5 mm2. For a 32A circuit, the PVC cable size should be a minimum of 4 mm2. There is no limit to the number of socket outlets that can be connected to the radial final circuit. However, the total current drawn by all the loads must not exceed the rating of the protective device or MCB. 51 Smart Electrical Technology 20A MCB P Minimum PVC cable size of 2.5 mm2 NOTE: For simplicity, neutral and protective conductor connections are omitted. 32A MCB P Minimum PVC cable size of 4.0 mm2 y Fig. 1.10-3: Radial final circuit • Ring Final Circuit A ring final circuit is arranged in the form of a ring and connected to the supply. It is always protected by a 30A fuse or 32A MCB with a PVC cable size of 2.5 mm2 (Fig. 1.10-4). There is no limit to the number of socket outlets that can be connected to the ring final circuit. However, the total current drawn by all the loads must not exceed the rating of the protective device or MCB. 32A MCB L Minimum PVC cable size of 4.0 mm y 4.1 2 Minimum PVC cable size of 2.5 mm2 Fig. 1.10-4: Ring final circuit Final Circuit Using a 15A Socket Outlet A final circuit for a 15A switched socket outlet should be connected individually and in a separate circuit from the consumer unit. It should be protected by a 16A MCB (Fig. 1.10-5). The window-unit air conditioner is an example of a final circuit that can be found in residential premises. 52 Smart Electrical Technology N E P P Minimum PVC cable size of 2.5 mm2 y 5. Fig. 1.10-5: 15A socket outlet circuit Electrical Accessories An accessory is a device associated with current-using equipment or with the wiring of an installation. Some common accessories include the: • switch • lampholder • socket outlet and plug A switch is a device used for making and breaking (closing and opening) an electric circuit. Some common switches include the: • one-way switch • two-way switch • intermediate switch • ceiling switch • dimmer switch Switches can be single-pole, double-pole or three-pole. Single-pole one-way switches are available in single or multi-gang units. For example, a two-gang switch holds two switches and controls two lights separately. For safety, all single-pole switches must be connected to the phase conductor. Some common switches are shown in Fig. 1.10-6. One-gang switch Two-gang switch Three-gang switch y Fig. 1.10-6: Common switches 53 Smart Electrical Technology 5.1 One-Way Switch A single-pole one-way switch is normally used to control lighting or equipment. It is a oneposition control switch. L1 C C L1 One-way switch y 5.2 Fig. 1.10-7: One-way switch Two-Way Switch In Fig. 1.10-8, two two-way switches are used to control the lamps from two positions independently. It has three terminals, including a common terminal that makes contact with either one of the strapping points. L1 C L1 L2 L2 Two-way switch y 5.3 L1 L2 Two-way switch Fig. 1.10-8: Two-way switches Intermediate Swtich The intermediate switch is a four-terminal switch. Switch-contact positions are either vertical or diagonal. In Fig. 1.10-9, two two-way switches are always used in conjunction with at least one intermediate switch to provide lighting control from three or more positions. L1 L2 L3 L4 Supply Two-way switch Two-way switch Intermediate switch y Fig. 1.10-9: Intermediate switch 54 Smart Electrical Technology 6. Testing Wiring Circuits for Safe Use Upon completion of the electrical installation or circuit, check that it is