Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Safely use electrical testing equipment like multimeters and insulation testers.
• Construct and troubleshoot basic electrical circuits using diagrams.
• Analyze test results to diagnose faults.
• Perform basic installations according to SANS 10142-1.
• Select appropriate tools and materials for electrical tasks.

Lesson summary

This week focuses on preparing you for the practical assessment tasks that form a significant part of your Grade 12 Electrical Technology mark. Practical skills are crucial not only for passing your exams but also for future employment or further study in electrical engineering and related fields. In South Africa, a skilled workforce is essential to maintain and expand our infrastructure, contributing to economic growth and improved living standards. Reliable electrical systems are vital for homes, businesses, industries, and public services. Mastering these practical skills ensures you are well-equipped to contribute to this vital sector.

Lesson notes

This week builds on previous knowledge and focuses on practical application. We'll revisit key concepts and expand on them with practical considerations: Safety First: Always prioritize safety. Never work on live circuits unless explicitly instructed and supervised for diagnostic purposes with strict safety measures in place. Use appropriate personal protective equipment (PPE) – safety glasses, insulated gloves, and appropriate footwear. Understand and adhere to lockout/tagout procedures. SANS 10142-1 highlights crucial safety aspects. Understand the implications of shock current levels and the dangers of working with electricity near water. Earth leakage protection is non-negotiable.

Testing Equipment Proficiency: Multimeters: Mastering multimeter usage is fundamental. Understand the difference between AC and DC voltage/current, resistance measurement, and continuity testing. Practice selecting the correct range for each measurement. Be aware of the multimeter's internal resistance and its potential impact on circuit readings.

Voltage Measurement: Connect the multimeter in parallel with the circuit component. Select the appropriate AC or DC voltage range. Read the voltage value from the display.

Current Measurement: Connect the multimeter in series with the circuit component. Select the appropriate AC or DC current range. Be aware of the maximum current the multimeter can handle.

Resistance Measurement: Isolate the component from the circuit before measuring resistance. Select the ohms range. Zero the meter if necessary.

Insulation Testers (Megohmmeters): These instruments are used to measure the insulation resistance of cables and equipment. A high insulation resistance indicates good insulation, while a low resistance indicates a potential fault or degradation of the insulation.

Procedure: Disconnect the cable or equipment from the power supply. Connect the tester to the conductors under test. Apply the test voltage (typically 500V or 1000V). Read the insulation resistance from the display. Acceptable values are typically above 1 megohm, depending on the application. Understand that temperature and humidity affect insulation resistance readings.

Circuit Construction and Troubleshooting: Wiring Regulations (SANS 10142-1): This standard is the bible for electrical installations in South Africa. Understand the requirements for wiring methods, conductor sizes, protection devices (circuit breakers, fuses, earth leakage protection), earthing, and bonding. Know the color codes for live, neutral, and earth conductors.

Circuit Diagrams: Be able to interpret schematic diagrams, ladder diagrams, and wiring diagrams. Understand the symbols used for various electrical components. Practice tracing circuits on diagrams to understand the flow of current.

Troubleshooting Techniques: Use a systematic approach to troubleshooting. Start by visually inspecting the circuit for obvious faults (loose connections, damaged components). Use a multimeter to check for voltage, current, and continuity. Divide the circuit into sections to isolate the fault. Understand the common faults in lighting circuits (blown lamps, faulty switches, loose connections) and motor control circuits (overloads, short circuits, open circuits).

Example: Troubleshooting a Lighting Circuit: Scenario: A light bulb in a domestic lighting circuit is not working.

Step 1 (Safety): Switch off the circuit breaker controlling the lighting circuit.

Step 2 (Visual Inspection): Check the light bulb for signs of damage (broken filament). If the bulb is blown, replace it and test the circuit.

Step 3 (Continuity Testing): Use a multimeter to check the continuity of the bulb filament. If the bulb is not blown, check the continuity of the switch contacts. If the switch is faulty, replace it.

Step 4 (Voltage Testing): With the circuit breaker switched on (exercise extreme caution!), use a multimeter to check for voltage at the light bulb socket. If there is no voltage, check the wiring connections between the switch and the bulb socket. Look for loose connections or broken wires.

Step 5 (Final Check): After making any repairs, switch off the circuit breaker and carefully inspect all connections. Switch on the circuit breaker and test the light.

Example: Calculating Conductor Size: Scenario: You need to determine the minimum conductor size for a 20A circuit feeding a geyser. The cable run is 15 meters. The cable is installed in conduit.

Step 1: Determine the design current (Ib): Ib = 20A Step 2: Determine the rated current (In) of the protective device: In = 20A (circuit breaker)

Step 3: Apply correction factors: Refer to SANS 10142-1 for de-rating factors for grouping and ambient temperature. For this example, assume a de-rating factor of 0.8.