Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Identify components of a house wiring system.
• Explain the purpose of safety devices like circuit breakers and ELCBs.
• Describe the three-core cable colour coding (SANS 10142).
• Calculate the correct circuit breaker size for appliances.

Lesson summary

House wiring is a fundamental aspect of Electrical Technology, directly impacting our daily lives. From the lights that illuminate our homes to the appliances that make our lives easier, electricity is integral to modern living in South Africa. Understanding how electricity is safely distributed within a building is crucial for preventing accidents, ensuring the longevity of electrical systems, and contributing to energy efficiency. This week, we will delve into the basics of house wiring and the essential safety devices that protect us from electrical hazards.

Lesson notes

2. 1. Components of a House Wiring System A typical South African house wiring system consists of the following key components: Supply Authority Connection: This is where the electricity enters your property from Eskom or the local municipality. It usually involves a meter box outside the house.

Distribution Board (DB Board): This is the central control point for all the electrical circuits in your house. It houses the main switch, circuit breakers (MCBs), and earth leakage circuit breaker (ELCB). Think of it as the electrical "brain" of your home.

Main Switch: A large switch that allows you to disconnect all electrical power to the house in case of an emergency or for maintenance. Circuit Breakers (MCBs - Miniature Circuit Breakers): These are automatic switches that protect individual circuits from overloads and short circuits. When a fault occurs, the breaker trips, cutting off the electricity to that circuit. They are reusable, unlike fuses. Earth Leakage Circuit Breaker (ELCB) / Residual Current Device (RCD): This device is designed to detect current leakage to earth. If a fault develops where current is flowing through the earth wire (or through a person touching a live part), the ELCB will trip very quickly, preventing electric shock.

Wiring: Conductors (wires) made of copper or aluminium that carry electricity throughout the house. They are insulated to prevent accidental contact and short circuits.

Earthing System: A critical safety feature that connects the metal parts of appliances and electrical installations to the earth. This provides a low-resistance path for fault current to flow, causing the ELCB or MCB to trip, and preventing electric shock. The South African standard requires effective earthing.

Outlets (Sockets) and Switches: Points where you can connect appliances to the electrical supply and control the flow of electricity to lights and appliances. 2.

2. Electrical Safety Devices Circuit Breakers (MCBs): As mentioned earlier, MCBs protect circuits from overloads and short circuits. An overload occurs when too much current flows through a circuit, usually because too many appliances are plugged in. A short circuit occurs when there is a direct, unintended connection between the live and neutral wires. MCBs are rated in Amperes (A). The rating indicates the maximum current the circuit can safely handle before the breaker trips. Earth Leakage Circuit Breakers (ELCBs) / Residual Current Devices (RCDs): These are vital for preventing electric shock. They detect small imbalances between the current flowing in the live and neutral wires. This imbalance indicates that some current is leaking to earth, potentially through a person. ELCBs are much more sensitive than MCBs and trip at a much lower current (typically 30mA).

Fuses: Older technology compared to MCBs. A fuse contains a thin wire that melts and breaks the circuit when the current exceeds its rating. Fuses are single-use and must be replaced after they blow. While still found in some older installations, MCBs are generally preferred due to their reusability and convenience. 2.

3. Three-Core Cable and Colour Coding (SANS 10142) In South Africa, the standard wiring system uses three-core cable: Live (Brown): Carries the electrical current from the supply to the appliance.

Neutral (Blue): Provides a return path for the current back to the supply.

Earth (Green/Yellow): A safety wire that provides a path for fault current to flow back to the supply, causing the ELCB or MCB to trip. This prevents electric shock. It's extremely important to adhere to this colour coding for safety and to avoid confusion when working with electrical circuits. 2.

4. Calculating Circuit Breaker Size To calculate the appropriate size of a circuit breaker, you need to know the power rating (in Watts) of the appliances that will be connected to the circuit and the voltage of the supply (230V in South Africa).

The formula to use is: Current (I) = Power (P) / Voltage (V)

Example 1: A geyser has a power rating of 3000W. What size circuit breaker is required? I = 3000W / 230V = 13.04A Therefore, you would need a circuit breaker rated slightly higher than 13.04A. A 15A or 20A circuit breaker would be suitable. Always choose a circuit breaker with a rating slightly higher than the calculated current to allow for inrush current and prevent nuisance tripping.

Example 2: A microwave oven is rated at 1200W, and a kettle is rated at 2000

W. Can they both be safely connected to a circuit protected by a 10A circuit breaker?

Microwave Current: I = 1200W / 230V = 5.22A Kettle Current: I = 2000W / 230V = 8.70A Total Current: 5.22A + 8.70A = 13.92A Since the total current (13.92A) exceeds the circuit breaker rating (10A), they cannot both be safely connected to the circuit simultaneously. This would likely cause the circuit breaker to trip.

Example 3: Calculate the maximum power that can be safely drawn from a circuit protected by a 20A circuit breaker at 230V.