SET Chapter 10 PDF - Conventional Lighting Circuits

Learning Outcomes Define the term “final circuit” Analyse the design and characteristics of common final circuits: o Lighting final circuit o Power final circuit Understand the two methods of wiring a socket outlet final circuit: o Radial final circuit o Ring final c...

Learning Outcomes Define the term “final circuit” Analyse the design and characteristics of common final circuits: o Lighting final circuit o Power final circuit Understand the two methods of wiring a socket outlet final circuit: o Radial final circuit o 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: o One-way switch o Two-way switch o Dimmer switch Wire up and test a conventional lighting final circuit with: o One-way control o Two-way control o Dimmer control Wire up and test a 13 A radial power circuit Explain the method for testing wiring circuits for safe use Analyse test results to identify the type of faults 1.10.1 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. Unit 1.10 | Conventional Lighting Circuits 65  1.10.2 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. Fig. 1.10-1 shows a typical final circuit arrangement. Fig. 1.10-1: Typical installation for six final circuits Unit 1.10 | Conventional Lighting Circuits 66  1.10.3 Lighting Final Circuit All lighting circuits are primarily meant for on/off control. Lighting circuits are normally protected by a 6 A or 10 A 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 (circuit protective conductor – 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. 1.10.4 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 13 A 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) Fig. 1.10-2: Role of MCB and 13 A fuse in a power final circuit Unit 1.10 | Conventional Lighting Circuits 67  1.10.5 Types of Final Circuit for 13 A Socket Outlets (a) Radial Final Circuit A radial final circuit consists of a cable run from a consumer unit to a number of 13 A socket outlets connected in parallel in one circuit (Fig. 1.10-3). The overcurrent protection for a radial final circuit is a 20 A or 32 A MCB (or a 20 A or 30 A fuse). For a 20 A circuit, the PVC cable size should be a minimum of 2.5 mm2. For a 32 A 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. E E N L N L E E N L N L Fig. 1.10-3: Radial final circuit Unit 1.10 | Conventional Lighting Circuits 68 (b) 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 30 A fuse or 32 A 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. E E N L N L Fig. 1.10-4: Ring final circuit Radial Final Circuit vs Ring Final Circuit Table 1.10-1 states the advantages and disadvantages of radial and ring final circuits. Table 1.10-1: Advantages and disadvantages of radial and ring final circuits Type of Final Circuit Radial Ring The simplest system with a Cables are under less load as the low initial cost for a 20 A current to any of the socket circuit outlets has two paths to take Advantages If one path of the circuit is open (e.g., loose terminal in a socket outlet), the remaining path will still be able to safely provide current to the circuit An open path (e.g., loose More expensive to install if the terminal in any of the same wire size is used as it Disadvantages socket outlets) will cause requires more materials for cable an interruption of current supply to all socket outlets Radial final circuits are adequate for lighting as it is a low load. However, socket outlets are best fed from ring final circuits. Unit 1.10 | Conventional Lighting Circuits 69  1.10.6 Final Circuit for 15 A Socket Outlets A final circuit for a 15 A switched socket outlet should be connected individually and in a separate circuit from the consumer unit. It should be protected by a 16 A MCB (Fig. 1.10-5). The window-unit air conditioner is an example of a final circuit that can be found in residential premises. E N L P or L Minimum PVC cable size of 2.5 mm2 Fig. 1.10-5: 15 A socket outlet circuit Unit 1.10 | Conventional Lighting Circuits 70  1.10.7 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; lamp holder; socket outlet; and plug. A switch is a device used for making and breaking (closing and opening) an electric circuit. 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 single-pole switches and controls two lights separately. Some common switches are shown in Fig. 1.10-6. One-gang switch Two-gang switch Three-gang switch Fig. 1.10-6: Common switches For safety, all single-pole switches must be connected to the phase conductor. Some common types of switches include the: (a) one-way switch; (b) two-way switch; (c) intermediate switch; (d) ceiling switch; and (e) dimmer switch. Unit 1.10 | Conventional Lighting Circuits 71 (a) One-Way Switch A single-pole one-way switch is normally used to control lighting or equipment. It is a one- position control switch. E N L Fig. 1.10-7: One-way switch (b) 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. E N L Fig. 1.10-8: Two-way switches (c) Intermediate Switch 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. E N L Fig. 1.10-9: Intermediate switch Unit 1.10 | Conventional Lighting Circuits 72  1.10.8 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: (a) Test the continuity of the protective (earth), phase and neutral conductors Fig. 1.10-10: Continuity test of earth conductor 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 13 A 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. Unit 1.10 | Conventional Lighting Circuits 73  (b) 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 1 MΩ. Fig. 1.10-11 shows the insulation resistance test between phase and neutral conductors. Fig. 1.10-11: Insulation resistance test between phase and neutral conductors Unit 1.10 | Conventional Lighting Circuits 74  Fig. 1.10-12 shows the insulation resistance test between phase and earth conductors. 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. (a) 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. (b) For the insulation resistance test, if any test values fall below 1 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. Unit 1.10 | Conventional Lighting Circuits 75