Household circuits, earthing, fuse and MCB; electrical safety
Homes get live, neutral and earth wires, with appliances in parallel. A fuse or MCB cuts the current on overload or short circuit; earthing saves you from shocks.
From the power station to your home
- 1Power stations (hydro, thermal, nuclear, wind) make AC, usually at about 11 kV and 50 Hz.
- 2A step-up transformer raises the voltage to about 132 kV. For the same power, a higher voltage means a smaller current, so less energy is lost as heat in the lines.
- 3High-tension lines carry it across the country to substations.
- 4Step-down transformers at substations lower the voltage in stages. A last transformer near your street brings it to 230 V.
- 5Two wires reach the house from the pole: the phase (live) and the neutral. A third, the earth wire, is added for safety.
- 6Inside the house, the supply passes through the meter, the main switch and fuses or MCBs to the circuits of each room.Most towns now use a ring system of distribution, so a fault in one line does not cut off a whole area.
Wires and wiring inside the home
- The phase (live) wire is at 230 V. In your lesson its insulation is red.
- The neutral wire is at zero volts, the same as the earth. It has a colour other than red or green, usually black.
- The earth wire has green insulation. Newer wiring uses brown for live, blue for neutral and green with yellow stripes for earth.
- All appliances are joined in parallel between live and neutral. Each gets the full voltage and has its own switch.
- Switches and fuses are put in the live wire, so that switching off cuts the appliance off from the live supply.
- Lights and fans run on a 5 A circuit. Heavy appliances such as heaters and geysers use a separate 15 A power circuit with thicker wires.
Three causes of electrical accidents
| Cause | What happens | Protection |
|---|---|---|
| Leakage of current | Old or scraped insulation lets the live wire touch the metal body of an appliance. Anyone who touches it gets a shock. | Earthing; good quality wiring |
| Short circuit | The live and neutral wires touch directly. The resistance becomes tiny, the current shoots up, and a spark or fire can start. | Fuse or MCB |
| Overloading | Too many appliances on one circuit draw more current than the wires can safely carry. The wires overheat. | Fuse or MCB; do not put many heavy appliances on one socket |
Safety devices: fuse, MCB and earthing
| Device | How it works | Remember |
|---|---|---|
| Electric fuse | A short wire of tin and lead alloy with a low melting point and a high resistance (higher than the copper wiring), joined in series in the live wire. Too much current heats it, it melts, and the circuit breaks. | Use the right rating: 5 A for lights, 15 A for power. A 15 A fuse wire is thicker. |
| MCB (miniature circuit breaker) | A switch that turns off by itself when the current is too large, as in overloading or a short circuit. | Once the fault is fixed it can simply be switched on again; no wire to replace. |
| Earthing | The metal body of an appliance is joined by the earth wire to a copper plate buried deep in moist ground. | If current leaks to the body, it flows to the earth through this easy path, not through a person. |
Safety rules and what to do after a shock
- Switch off the main switch before working on any circuit, and in any electrical accident, fire or earthquake.
- Never touch a person who is stuck to a live wire. Separate him with a dry wooden stick, rubber or plastic, after switching off if you can.
- Never pour water on an electrical fire.
- Use wires of the right thickness and good insulation, with the ISI mark. Keep joints tight and covered with insulation tape.
- Wear rubber gloves and shoes when work on a live circuit cannot be avoided.
- Use an electric tester: its neon bulb glows if the point or the body of an appliance is live.
Activity (practical): assemble a household circuit with a fuse
You need two 3 V bulbs in holders, two switches, a fuse, four dry cells (or a battery eliminator) and connecting wires.
- 1Look carefully at the circuit diagram. Every part, the bulbs, switches, fuse and cells, has two terminals.
- 2Join bulb B₁ in series with switch S₁. Join bulb B₂ in series with switch S₂.
- 3Join the two pairs (B₁ with S₁, and B₂ with S₂) in parallel with each other.
- 4Join a fuse of suitable rating in series between this set-up and the positive terminal of the battery. Join the negative terminal to the other side.
- 5Switch on S₁ alone, then S₂ alone, then both. Record which bulbs glow each time.
- 6Conclude.Each bulb works on its own switch, because the bulbs are in parallel, just like the appliances in a home.
- 7To draw it: battery, fuse in the main line, and two parallel branches, each with a bulb (circle with a cross) and its switch.
Plugging a geyser, an iron and a heater into one extension board can overload the socket and trip the MCB. The longer, thicker third pin on the plug of an iron or a fridge joins its metal body to the earth wire.
2 marks: “The metal body of a washing machine is joined to the earth wire. How does this protect the user if the live wire touches the body?” The leaking current flows through the earth wire to the ground, an easy low-resistance path. The body stays at earth potential, so a person touching it does not get a severe shock.
2 marks: “Why is a fuse wire made of an alloy with a low melting point, and why is it joined in the live wire?” A low melting point lets it melt quickly when the current is too large, breaking the circuit. In the live wire, it cuts the appliance off from the live supply when it melts.
3 marks: “Distinguish between a short circuit and overloading. Name one device that protects against both.” Short circuit: live and neutral wires touch directly, so the current suddenly becomes very large. Overloading: too many appliances on one circuit draw more current than it can carry. A fuse or an MCB protects against both.
Red runs, black returns, green guards (live, neutral, earth). The fuse melts, the MCB trips, the earth wire saves.