Overview
This lecture explains how electricity is generated, transmitted, and distributed to homes, focusing on domestic electric circuits, safety devices, and wiring standards.
Generation and Transmission of Electricity
- Electricity is generated at power stations, often located far from cities.
- The voltage generated is about 11,000 volts (11 kV) and is stepped up for transmission to reduce heat loss.
- Step-up transformers increase voltage (e.g., to 132 kV), and step-down transformers lower it for distribution (e.g., to 220 V in India, 110 V in the USA).
- Transmission uses long cables and multiple substations to regulate voltage before it reaches homes.
Domestic Electric Wiring Structure
- Two main wires (live/phase and neutral) supply alternating current (AC) to homes.
- The live wire carries 220 V (India), and the neutral wire is at 0 V, giving a potential difference of 220 V.
- These wires enter a main fuse, then an electricity meter (measures in kilowatt-hours) and a double-pole main switch.
- The distribution box contains multiple miniature circuit breakers (MCBs) for separate circuits.
Circuits and Sockets
- There are two types of circuits: lighting (max 5 A, for bulbs, fans, small sockets) and power (max 15 A, for large appliances, large sockets).
- Circuits are connected in parallel, so devices get full voltage and one device failure does not affect others.
- Each circuit has its own fuse or MCB for safety.
Earthing and Wire Identification
- A third Earth wire connects appliances to the ground for safety, preventing electric shock.
- Earthing involves burying a copper plate connected to the Earth wire near the meter.
- Old wire color codes: red (live), black (neutral), green (earth); new codes: brown (live), light blue (neutral), green/yellow (earth).
Three-pin Plugs and Appliance Safety
- Three-pin sockets/plugs: the top thick pin is Earth, the right is live, and the left is neutral.
- The Earth pin connects first for safety and is thicker/longer to prevent misplacement.
- Earthing is critical for appliances with metal bodies and high current draw (e.g., irons, kettles, refrigerators).
Electrical Safety: Faults and Protection
- Short circuit: live and neutral wires touch due to insulation failure, causing high current and fire risk.
- Overloading: too many high-power appliances used simultaneously, causing overheating and potential fire.
- Fuses protect wiring by melting and breaking the circuit if current is too high; MCBs automatically switch off power and can be easily reset.
- Appliances may have cartridge fuses, which must be replaced after blowing.
Key Terms & Definitions
- Live Wire — Wire carrying high voltage (220 V in India).
- Neutral Wire — Wire at zero potential relative to ground.
- Earthing — Connecting metal parts of an appliance to the ground for safety.
- Main Fuse — Safety device that breaks circuit if excessive current flows.
- MCB (Miniature Circuit Breaker) — Automatic switch that interrupts power during fault.
- Short Circuit — Fault where live and neutral wires touch, causing high current.
- Overloading — Excessive use of appliances causing too much current in wires.
- Parallel Connection — Each device gets full voltage; failure of one does not affect others.
Action Items / Next Steps
- Review the wiring and earthing setup at home for safety.
- Remember to call an electrician if a fuse blows and do not replace it until the cause is found and fixed.