SET Textbook 2023 30 - 65.pdf

Smart Electrical Technology 1.6 Electric Power Sources -28 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 sy...

Smart Electrical Technology 1.6 Electric Power Sources -28 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 Anode e) 1. Battery mocle-positive termral . Kathode : 11 cell III Negative - Batting 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. DC Direct : Current Current Current AC Alternating : current . Time y Fig. 1.6-1: DC waveform 28 e 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 single-phase three-phase 3. houses HDB flats = , industries - , classrooms . . factories . (higher powered machines -conveyer lifts , belts Smart Electrical Technology . escalators Single-Phase and Three-Phase Supply Cranes) . Two types of alternating power supply are commonly used in Singapore: • single-phase supply Ac suppl • 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). Three ↑one L phase wire -1 /// . L1 N L2 phase wires . L1 L3 L2 L3 N Lilive Phase N : Neutral 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. Es 3.1 Cells -> Batteries . Cells can be divided into two main types: • primary cells (non-rechargeable) • secondary cells (rechargeable) is * . I Fran - e ↳ neiffe 2H + 02 is X e + -> He0 non-reversible Y Are 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 SECONDARY PRIMARY NiCd NiMH Lithium Ion Zinc -Carbon Alkaline 4. Battery Types Lead - Smart Electrical Technology Acid Here are some types of batteries available in the market. 4.1 Zinc-Carbon Battery (Primary 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. - Don rechargable) . y Fig. 1.6-8: Zinc-carbon batteries 4.2 y 4.3 Fig. 1.6-9: Alkaline batteries Nickel-Cadmium Battery Alkaline Battery (Primary -I 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. Nargable) . (Secondary Rechargable) -- . 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 Nickel-Metal Hydride Battery (Secondary) y 4.5 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. Fig. 1.6-11: NiMH batteries Lithium-Ion Battery . (Secondary) . 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 Fig. 1.6-12: Lithium-ion battery Battery ~ Lead-Acid Battery car 4.6 . (Secondary) . 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 myE" 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. ef usually at coastal areas • 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. Photovoltaic x y cells = Fig. 1.6-14: Windmills creating wind power Solar Pacel 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 n INATER - - u - - - - Fig. 1.6-17: Hydroelectric plant tree = -DAM I = -T A I / 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 connect called to the earth grounding . Also Symbol : Zero volts The earth is considered a large conductor atzero 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. ·rics" iii. n. ( 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 230 V N = N If If CPC If I ① Earth fault RCCB a If L If Earth Earth fault current . With out earthing -electric shock there is a ② With Earth electrode if y Fig. 1.7-2: Installation with earthing fault current . earthing . Do electric shock . 36 CPC - Circuit (Earth Protective Wire) Conductor . 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. phase wire touches the 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. Earth wire . 3. -> Overcurrent Too much current MCB . . 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). 1 2. touch Neutral . Phase -> . short circuit . 4. Types of Protective Devices Phase/Neutral Earth Earth touches . - Fault . 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 2 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 types of power supply · · - Single-Phase Three-Phase E - - ↳ : 43 i - 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 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 safe for use by carrying out the tests listed below. (i) First, check that: • cable insulations and accessories are not damaged; • all terminations are secured; • cables are colour-coded correctly, such as brown for phase, blue for neutral, and green and yellow for earth; and • cables are of the correct sizes, such as 1.5 mm2 for a lighting circuit and 2.5 mm2 for a power circuit. (ii) Next, conduct the following tests with the supply disconnected: • Test the continuity of the protective (earth), phase and neutral conductors. Refer to Fig. 1.10-10. Ensure that the installation is not energised when carrying out the test. Set the multimeter to a low resistance range (around 1 Ω). Carry out the test between the earth terminal at the consumer unit and the earth terminal for all the accessories (in this case, a 13A switch socket outlet) one at a time. The reading on the multimeter should be around 0 Ω. Repeat the test for the phase and neutral conductors. Long test lead Continuity tester E N P Socket outlet used for test Consumer unit E P N Supply y Fig. 1.10-10: Continuity test of earth conductor 55 Smart Electrical Technology • Measure the insulation resistance of the installation. Apply the following settings: Consumer unit: All circuit breakers closed SSOs: No loads Lamp holders: No loads Switches: Turn on Using an insulation resistance tester, connect the test leads to the supply cables to measure the resistance: between phase and earth; between phase and neutral; and between neutral and earth. All the readings should have minimum values of 0.5 MΩ. Figures 1.10-11 and 1.10-12 show the insulation resistance tests between phase and neutral conductors, and between phase and earth conductors, respectively. E To N other P lights E N P Ceiling rose Ceiling rose Lamps removed Two-way switches on Switch on Lamps removed E NP Two-way switching Supply cables NOTE: MCBs closed Switches closed Insulation resistance tester y Fig. 1.10-11: Insulation resistance test between phase and neutral conductors 56 Smart Electrical Technology E N P E N P Ceiling rose To other lights Ceiling rose Lamps removed Switch on Two-way switches on Lamps removed EN P Two-way switching Supply cables NOTE: MCBs closed Switches closed Insulation resistance tester y Fig. 1.10-12: Insulation resistance test between phase and earth conductors (iii) Analyse the test results to identify and find the cause of any faults. • For the continuity of the protective, phase and neutral conductors, if the test values show unreasonably high resistance or even an open circuit: check for poor or loose connections; and fasten and secure all terminations and test again. • For the insulation resistance test, if any test values fall below 0.5 MΩ: check for partial short circuits or stray strands of cable terminations, especially at the lamp holders; and check the cable insulation resistance between the phase and earth, neutral and earth, or phase and neutral connections. 57 Smart Electrical Technology 1.11 Electrical Supply Systems Learning Objectives  Explain how electricity is transmitted from a power station to consumers  Explain how electricity is distributed in residential, commercial and industrial premises  Explain the function of transformers in the transmission and distribution of electricity  State the voltages for the generation, transmission and distribution of electricity in Singapore 1. Introduction Electricity is generated in a power station. The type of fuel consumed by the power station depends extensively on the natural resources available, which include coal, oil, natural gas, nuclear power, water, diesel fuel and biomass (or waste). There are also more environmentally friendly ways of generating electricity on a smaller scale, such as by using wind power, solar power and tidal barrages. Some common types of power stations are: • thermal power stations • hydro-electric power stations • nuclear power stations The generation of electricity is a process of energy conversion. The stages of energy conversion in a thermal power station are shown in Fig. 1.11-1. Oil Coal Heat Mechanical Electrical Nuclear Natural gas y Fig. 1.11-1: Stages of energy conversion 58 Smart Electrical Technology 2. Components of a Typical Thermal Power Station Figure 1.11-2 shows the main components of a typical thermal power station. Waste gas to chimney Boiler Steam Electricity Turbine Water Condensed steam Condenser Generator Fuel Pump Furnace y 3. Sea water To sea Fig. 1.11-2: Typical thermal power station Generation of Electricity Electricity is generated in a thermal power station (Fig. 1.11-2) by the following processes: • Heat from the combustion of fuel oil is used to turn the water in the boiler to steam. • The powerful jets of steam are directed to drive the blades of the turbine. • The turbine, which is coupled to the rotor of the AC generator (or alternator), in turn drives the rotor of the generator. • The rotation of the rotor eventually results in electricity being produced by the alternator. The alternator produces AC electricity. The combination of the turbine and alternator is known as the turbo-alternator. The alternator in the power station produces a three-phase supply. 4. Transmission of Electricity The electricity generated at the power station must be increased to a very high voltage before it can be transmitted over a considerable distance through a network of cables to the main substations, which in turn feed secondary or smaller substations. A step-up transformer is used to increase the voltage. The cables used for the transmission of electricity are known as transmission lines. 59 Smart Electrical Technology Stepping up the voltage before transmission reduces the amount of current flowing in the cables, which results in: • smaller cable sizes and switchgear capacities; • a reduction in voltage drop in the transmission lines; and • lower power losses in the transmission lines. 5. Distribution of Electricity After transmission, the high-voltage electricity is fed to smaller substations, where its voltage is reduced before it is distributed to consumers. A step-down transformer is used to decrease the voltage. The cables used for the distribution of electricity are known as distribution lines. Distribution lines include: • feeders, which are conductors that connect one substation to another; • distributors, which are conductors that are tapped to supply consumers with electricity; and • service cables, which are conductors that connect the distributors to the consumers’ premises. Figures 1.11-3 and 1.11-4 show how electricity is transmitted and distributed to consumers, which can be done either by overhead lines or underground cables. Transmission lines Generating plant Transformer Distribution lines Consumers y Transformers Fig. 1.11-3: How generated electricity is transmitted and distributed 60 Smart Electrical Technology 6. Distribution System for Large Consumers Generating plant 400 kV 230 kV 230 kV 66 kV Transmission 22 kV 6.6 kV Feeders 6.6 kV 400 V OG box Distributors Service cable Distribution 66 kV, 22 kV, 6.6 kV and 230/400 V Transmission 400 kV, 230 kV y 66 kV 22 kV Consumers (e.g., apartment blocks) Fig. 1.11-4: Typical voltages used in the transmission and distribution network Figure 1.11-5 shows a typical distribution system for large consumers of electricity: • For heavy industries, the supply is taken at 66 kV directly from the secondary transmission lines. • For other commercial and industrial consumers, such as hotels, factories and multi-storey shopping complexes, the supply is taken at 22 kV. • For light industries, the supply is taken at 6.6 kV directly from the secondary transmission lines. A local substation is required at the premises to enable the consumers to reduce the voltage for their use. Heavy industries Light industries and commercial buildings 66 kV Transformer 230/66 kV y 22 kV Transformer 66/22 kV Light industries 6.6 kV Transformer 22/6.6 kV Fig. 1.11-5: Supplying electricity to large consumers 61 Smart Electrical Technology 7. Distribution System for Small Consumers Low-voltage consumers are provided with • three-phase, 230/400 V, 50 Hz supply • single-phase, 230 V, 50 Hz supply The system used for distribution is the three-phase, four-wire system shown in Fig. 1.11-6. The star point on the secondary side of the delta-star transformer is connected to the earth; it also serves as the neutral terminal for single-phase supply. On the secondary side, the voltage between any two phases is 400 V, 50 Hz, while the voltage between any phase and the neutral is 230 V, 50 Hz. L1 L2 L3 N L1 Power consumer 230/400 V L2 A L3 N Domestic consumers 230 V B A L1 L2 N Star-connected secondary LV winding 230/400 V C Service cable L3 N N * Distributors rection 6.6 or 22 kV substation Deltaconnected primary HV winding 6.6 kV or 22 kV #deation HV supply y Fig. 1.11-6: Distribution system for small consumers 62