Lesson Notes By Weeks and Term v4 - SHS 2
ELECTRICAL SYSTEMS DESIGN
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ELECTRICAL SYSTEMS DESIGN
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Subject: Applied Technology
Class: SHS 2
Term: 2nd Term
Week: 14
Grade code: 2.5.1.LI.3
Strand code: 5
Sub-strand code: 1
Content standard code: 2.5.1.CS.1
Indicator code: 2.5.1.LI.3
Theme: ELECTRICAL AND ELECTRONIC TECHNOLOGY
Subtheme: ELECTRICAL SYSTEMS DESIGN
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In Ghana, almost every home, school, and office uses electricity from the Electricity Company of Ghana (ECG) or the Northern Electricity Distribution Company (NEDCo). The system that safely brings this power from the meter into our rooms to power our lights, fans, TVs, and charge our phones is the house wiring. Using the wrong materials, like a cable that is too thin for a big freezer, can cause it to overheat, melt, and start a fire. This lesson is crucial because it teaches us to identify the correct and safe materials needed for house wiring, a skill that protects lives and property.
This section breaks down the essential materials and accessories into logical groups. Group A: Conductors (The Pathways for Electricity) Electrical Cables A cable is the "road" for electricity. It consists of a metal conductor (usually copper) that allows electricity to flow easily, and an insulator (usually PVC plastic) that covers the conductor to prevent shock and short circuits. Conductor Material: In Ghana, we primarily use copper because it is an excellent conductor and less prone to corrosion compared to aluminium. Insulator Material: Polyvinyl Chloride (PVC) is the most common insulator. It is tough, flexible, and a very good insulator. Cable Sizes: The size of a cable is measured by the cross-sectional area of its copper conductor, in square millimetres (mm²). A bigger size means the cable can carry more current safely. 1.5 mm²: Used for lighting circuits. This includes ceiling lights, wall lights, and fans. It can safely carry up to about 15 Amperes (A). 2.5 mm²: The standard for socket outlet circuits (wall plugs). Used for general appliances like TVs, fridges, computers, and phone chargers. It can safely carry up to about 20-25 Amperes (A). 4.0 mm² & 6.0 mm²: Used for high-power, fixed appliances like air conditioners and water heaters. 10 mm² & 16 mm²: Used for very high-power appliances like electric cookers or as the main "tails" connecting the ECG meter to the consumer unit (distribution board). Current-Carrying Capacity (Ampacity) This is the maximum amount of current (in Amperes) a cable can carry continuously without its temperature exceeding a safe limit. If you force more current through a cable than it is designed for, it will overheat, melt the insulation, and can cause a fire. Simplified Ampacity Table: | Cable Size (mm²) | Typical Application | Approx. Current Capacity (A) | | :--------------- | :------------------ | :---------------------------- | | 1.5 | Lighting, Fans | 15 A | | 2.5 | Socket Outlets | 25 A | | 4.0 | Air Conditioners | 32 A | | 6.0 | Water Heaters | 40 A | | 10.0 | Cookers, Sub-mains | 55 A | Group B: Containment (Protection for Cables)
These materials are used to house and protect the cables from physical damage. Conduits: These are PVC or metal pipes that cables are drawn through. They are embedded in walls or run on the surface. Use: To provide strong mechanical protection for cables, especially inside concrete walls or floors. Trunking: These are rectangular PVC or metal channels with a removable cover. Use: To neatly house many cables running along the same path, especially on the surface of walls. They make it easy to add or remove cables later. Pattress Boxes / Back Boxes: These are plastic or metal boxes installed in the wall. Use: To mount switches and socket outlets securely and provide a space for connections behind them. Group C: Accessories (Control, Connection, and Protection) Switches: Use: To open or close a circuit, allowing us to turn lights or appliances ON and OFF. Types: One-way (controls a light from one location), Two-way (controls one light from two different locations, e.g., top and bottom of stairs). Socket Outlets: Use: To provide a connection point for plugging in portable appliances. Type: The most common in Ghana is the 13A rectangular pin socket. Ceiling Rose / Lamp Holder: Use: A fitting in the ceiling used to connect a pendant light fixture (hanging bulb) to the circuit wiring. Consumer Unit (or Distribution Board - DB): Use: This is the main distribution centre for electricity in the house. It's a metal or plastic box that contains the main switch and all the protective devices (circuit breakers). It takes the main supply from the ECG meter and distributes it to the different circuits (lights, sockets, etc.). Main Switch: Use: A large switch inside the consumer unit that can disconnect the entire house from the electricity supply at once. It's used for safety during maintenance or in an emergency. Circuit Breakers (MCBs - Miniature Circuit Breakers): Use: An automatic protective device. It protects a specific circuit from damage caused by excess current (overload or short circuit). When it detects a fault, it "trips" (switches off) automatically. It can be reset manually after the fault is cleared. They are rated in Amps (e.g., 6A for lights, 20A for sockets).
Guided Practice (With Solutions)
Question 1: A new bedroom is being wired. It will have one ceiling light and two 13A socket outlets. List five essential materials/accessories needed for this job, specifying the correct cable sizes.