Lesson Notes By Weeks and Term v4 - SHS 3
CIRCUITS AND MACHINES
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CIRCUITS AND MACHINES
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Engineering
Class: SHS 3
Term: 2nd Term
Week: 4
Grade code: 3.2.1.LI.4
Strand code: 2
Sub-strand code: 1
Content standard code: 3.2.1.CS.1
Indicator code: 3.2.1.LI.4
Theme: ENE RGY SYSTEMS
Subtheme: CIRCUITS AND MACHINES
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In Ghana, from the local corn mill operator in our communities to the large-scale factories in Tema, electrical machines are the backbone of our industry and daily life. We use generators during power outages ("dumsor"), water pumps in our homes, and motors in countless appliances. However, these powerful machines can be extremely dangerous if not handled with care and respect. This lesson focuses on the critical safety measures required when working with various electrical machines. Understanding these principles is not just for passing exams; it is essential for preventing injury, protecting property, and ensuring a long, productive career in engineering.
This topic can be broken down into two main areas: general safety principles that apply everywhere, and specific precautions for particular types of machines. Part A: General Safety Principles ("The Golden Rules")
These are fundamental rules that must be followed whenever you are near or working on any electrical machine. They can be grouped into three categories: Personal, Workspace, and Equipment safety. Personal Safety (Yourself): Personal Protective Equipment (PPE): This is your first line of defence. Always wear the correct PPE for the job. Safety Goggles/Face Shield: Protects your eyes from flying debris, sparks, or chemical splashes. Insulated Gloves: Protects you from electric shock when handling live or potentially live parts. Safety Boots (Steel-toed): Protects your feet from falling objects and provides insulation from the ground. Overalls/Aprons: Protects your clothes and skin from dirt, grease, and entanglement. Avoid loose clothing, chains, or ties that can get caught in rotating parts. Stay Alert: Never work on machines when you are tired, ill, or distracted. Do not engage in horseplay in the workshop. Remove Jewellery: Rings, watches, and metallic necklaces are excellent conductors of electricity and can get caught in machinery. Remove them before starting work. Workspace Safety (Your Environment): Good Housekeeping: Keep the floor clean, dry, and free of oil spills, tools, and cables. A tidy workspace prevents slips, trips, and falls. Proper Ventilation: Ensure good airflow, especially when working with machines that produce fumes (like generators) or heat. Adequate Lighting: You must be able to see clearly what you are doing. Poor lighting leads to mistakes and accidents. Fire Safety: Know the location of fire extinguishers and ensure they are the correct type for electrical fires (e.g., CO₂, Dry Powder). Never use water on an electrical fire. Equipment Safety (The Machine): De-energise Before Work (Lockout/Tagout - LOTO): This is the most important rule. Before any maintenance, repair, or inspection, you MUST disconnect the machine from the power source. Lockout: Apply a physical lock to the switch or breaker so no one can accidentally turn it on. Tagout: Attach a tag to the lock that says "DANGER: Do Not Operate" with your name and the date. Use the Right Tools: Use insulated tools (screwdrivers, pliers) that are in good condition. A damaged tool can fail and cause an accident. Machine Guarding: Ensure all moving parts (belts, pulleys, gears, shafts) are covered by a proper guard. Never operate a machine with the guards removed. Grounding (Earthing): All stationary electrical machines must be properly grounded. This provides a safe path for fault current to flow to the earth, preventing the machine's body from becoming live and causing a fatal shock. Part B: Specific Safety Measures for Different Machines
While the general rules apply to all, different machines have unique hazards.
| Machine Type | Common Hazards | Specific Safety Measures | | --------------------- | --------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Electric Motors | Entanglement: Loose clothing or hair getting caught in the rotating shaft or pulley. Overheating: Leading to insulation failure, fire, or burns. Electric Shock: From faulty wiring or improper connections in the terminal box. | 1. Guarding: Ensure all rotating parts (shaft, fan, belts) are fully guarded. 2. Ventilation: Keep cooling fins and vents clear of dust and debris to prevent overheating. 3. Proper Wiring: Ensure all connections are tight and the terminal box cover is secure. 4. Overload Protection: Use correctly rated fuses or circuit breakers. | | Generators | Carbon Monoxide (CO) Poisoning: The exhaust contains this deadly, odourless gas. Electric Shock/Electrocution: From improper connection to a building's wiring ("back-feeding"). Fire: From refuelling while the engine is hot. Noise: Can cause hearing damage over time. | 1. Outdoor Use ONLY: NEVER operate a generator indoors, in a garage, or near windows/doors. The CO gas can kill. 2. Use a Transfer Switch: To connect to a building, a qualified electrician must install a transfer switch to prevent dangerous back-feeding into the ECG grid. 3. Refuel Safely: Turn the generator off and let it cool down completely before refuelling. 4. Grounding: Ensure the generator frame is properly grounded using a grounding rod. | | Transformers | High Voltage: The primary and secondary windings can carry lethal voltages. Oil Leaks: The cooling oil (sometimes containing harmful PCBs) can leak and cause environmental damage or fire. Explosion: Severe internal faults can cause a catastrophic failure. | 1. Enclosures & Fencing: Transformers (like those on ECG poles or in substations) must be fenced off with clear "DANGER: High Voltage" signs. 2. De-energise & Discharge: Before working on a transformer, it must be completely disconnected from both the primary and secondary sides. Capacitors inside might also need to be safely discharged. 3. Inspect for Leaks: Regularly check the tank and cooling fins for any oil leaks. 4. Keep Clear: Maintain a safe distance, especially during wet weather conditions. |