Domestic circuits, electrical safety and LED lighting

How electricity reaches your home

Power comes from a pole or an underground cable to the electricity meter (kWh meter). It then passes the main fuse and main switch and a distribution board. From there, separate circuits run to the rooms. A 5 A circuit feeds low-power appliances like bulbs, tube lights and fans. A 15 A circuit with thicker wires feeds high-power appliances like the water heater, refrigerator, iron box and air conditioner. All appliances are connected in parallel, so each gets the full 220 V and has its own switch. In India, the supply is alternating current (AC) at a frequency of 50 Hz. Alternating current keeps reversing its direction: 50 full cycles each second.

The three wires

Faults and safety devices

• Short circuit: the live and neutral wires touch directly, often because the insulation is damaged. Resistance becomes almost zero, so a huge current flows and can start a fire.
• Overloading: too many appliances on one socket draw more current than the wires can safely carry.
• Electric fuse: a short, thin wire of tin-lead alloy with a low melting point, placed in series with the live wire. When the current goes above its rating, it melts and breaks the circuit. Many homes have porcelain 'kit-kat' fuses.
• Miniature circuit breaker (MCB): an automatic switch that trips (switches off) when the current is too high. After the fault is fixed, you push it back on: no wire to replace.
• Earthing: the metal body of an appliance is joined by the earth wire to a metal plate or pipe buried deep in the ground. Charcoal and salt around it keep the soil moist. If the live wire touches the body, current rushes to the earth instead of through the user. The fuse blows or the MCB trips.
• A three-pin plug has a longer, thicker earth pin. It enters the socket first, so the appliance is earthed before it gets current.

LED bulbs and LED displays

• LED stands for Light Emitting Diode. It is a small semiconductor device: a material whose conductivity lies between that of a conductor and an insulator.
• When current flows through it in the correct direction, it gives out light directly. There is no filament, so very little energy is lost as heat.
• The colour of the light depends on the semiconductor material used.
• Advantages: much less power for the same light; stays cool; lasts much longer; no mercury, unlike CFL tubes; switches on instantly; hard to break.
• An LED display is made of many tiny LEDs in rows and columns. Each dot of light is a pixel. Red, green and blue LEDs in a pixel mix to give any colour.
• LED displays are used in display boards at bus stands and railway stations, traffic signals, digital clocks, scoreboards and giant screens.
• A seven-segment display has 7 LED bars arranged in the shape of the figure 8. Lighting different bars shows numbers (0 to 9) and some letters. It is used in digital clocks, calculators and meters.
• An LED TV is usually a liquid crystal screen lit from behind by LEDs. It is thin, bright and uses less power than older TVs.

How to draw a domestic circuit

1. 1
   Draw the supply line from the pole entering a box labelled 'electricity meter (kWh meter)'.
2. 2
   Next, draw the main fuse and main switch, then the distribution board with MCBs.
3. 3
   From the board, draw the live (red) and neutral (black) wires as two long parallel lines.
4. 4★
   Join a bulb, a fan and a socket each between the two lines (in parallel), each with its own switch on the live wire.
5. 5
   Draw a green earth wire from the socket and the metal body of an appliance down to an earth plate in the ground. Label every part.

★ key step