Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Interpret SANS 10142-1 for wiring and earthing.
• Safely wire a single-phase distribution board.
• Troubleshoot common electrical faults.
• Conduct mandatory tests and record results.
• Demonstrate safe working practices and use of PPE.

Lesson summary

This week focuses on preparing you for the practical assessment tasks that form a crucial part of your Grade 12 Electrical Technology mark. The practical assessments are designed to evaluate your ability to apply the theoretical knowledge you've gained throughout the year to real-world electrical problems. This is not just about memorizing facts; it's about demonstrating competence and problem-solving skills essential for your future, whether you choose to pursue further education, enter the electrical trade, or simply become a responsible homeowner capable of basic electrical troubleshooting.

Lesson notes

This section covers the core concepts that will be tested in the practical assessment. 2.1 SANS 10142-1 (The Wiring Code): SANS 10142-1 is the South African National Standard for the wiring of premises. It’s the bible for any electrician and is enforced by law. Understanding this code is not optional; it's essential for safety and legal compliance.

Wiring Methods: The standard specifies acceptable methods for installing cables, including: Surface wiring: Cables run along the surface of walls or ceilings, typically in trunking or conduit.

Concealed wiring: Cables run within walls or ceilings, usually in conduit. This is the most common method in new construction.

Underground wiring: Cables buried directly in the ground or in ducts.

Cable Types: SANS 10142-1 specifies cable types for different applications (e.g., surfix cable for general wiring, armored cable for underground). It defines the current carrying capacity (ampacity) based on cable size, insulation type, and installation method.

Earthing and Bonding: This is critical for safety. The earth wire provides a path for fault current to flow back to the source, causing the overcurrent protection device (MCB or RCBO) to trip and disconnect the faulty circuit.

Earthing: Connecting exposed metal parts of electrical equipment to earth (the general mass of the earth).

Bonding: Connecting conductive parts of a building (e.g., metal water pipes, structural steel) to the earthing system to create equipotential bonding.

Overcurrent Protection: MCBs (Miniature Circuit Breakers) and RCBOs (Residual Current Circuit Breakers with Overcurrent protection) are designed to protect circuits from overloads and short circuits.

MCBs: Trip based on excessive current flow. They have different trip characteristics (e.g., B, C, D) depending on the type of load.

RCBOs: Trip based on earth leakage current (residual current). They protect against electric shock. The standard trip current is typically 30mA. 2.2 Single-Phase Distribution Board Wiring: The distribution board (DB board) is the heart of the electrical installation. It distributes power to individual circuits throughout the building.

Components: Main Switch: Isolates the entire installation.

Incoming Supply Cable: Feeds power from the municipal supply.

Earth Bar: Provides a connection point for earth wires.

Neutral Bar: Provides a connection point for neutral wires.

MCBs/RCBOs: Protect individual circuits.

Busbars: Conductors that distribute current to the MCBs/RCBOs.

Wiring Procedure: Isolate the supply: Ensure the main switch is off and locked out/tagged out.

Connect the incoming supply cable: Connect the live and neutral wires to the main switch. Connect the earth wire to the earth bar.

Connect the busbars: Connect the busbars to the output terminals of the main switch.

Connect the MCBs/RCBOs: Connect the live wire from the busbar to the input terminal of each MCB/RCB

O. Connect the circuit wiring: Connect the live wire from the output terminal of the MCB/RCBO to the live conductor of the circuit. Connect the neutral wire to the neutral bar. Connect the earth wire to the earth bar.

Double-check all connections: Ensure all connections are tight and secure. 2.3 Electrical Fault Finding and Testing: Common Faults: Short Circuit: A low-resistance path between live and neutral or live and earth.

Open Circuit: A break in the circuit.

Earth Leakage: Current flowing from the live conductor to earth.

Testing Equipment: Multimeter: Measures voltage, current, and resistance.

Insulation Tester (Megger): Measures the insulation resistance between conductors. Used to detect insulation breakdown.

Earth Loop Impedance Tester: Measures the impedance of the earth fault loop path. This ensures that the overcurrent protection device will trip quickly in the event of a fault.

Mandatory Tests (SANS 10142-1): Earth Continuity Test: Ensures that all earth conductors are properly connected.

Insulation Resistance Test: Ensures that the insulation between conductors is adequate.

Polarity Test: Ensures that live and neutral conductors are correctly connected.

Earth Loop Impedance Test: Verifies the effectiveness of the earth fault loop path. 2.4 Worked Example 1: Calculating Cable Size A geyser with a power rating of 3kW is to be installed. The supply voltage is 230V. The cable length is 20m. The ambient temperature is 30°C. Determine the minimum cable size required, assuming PVC insulated copper conductors installed in conduit.

Calculate the current: I = P/V = 3000W/230V = 13.04A Apply a correction factor for ambient temperature: Consult SANS 10142-1 or a cable sizing chart. For PVC cables at 30°C, the correction factor is typically around 0.94 (check the specific table).

Therefore, the corrected current is: 13.04A / 0.94 = 13.87A Select a cable size: Consult SANS 10142-1 or a cable sizing chart. Look for a PVC insulated copper conductor cable with a current carrying capacity greater than 13.87A.