Household electric circuits and safety

What is behind the wall?

Look at a switch board. The wires go into the wall, and they join the supply of the town. How is a house wired? And what keeps the people in it safe?

The three wires

The supply cable that brings electricity to a house has three wires.

• The live wire (usually red) carries the current to the house. It is at a high potential, $220\ \text{V}$ above the neutral wire.
• The neutral wire (usually black) takes the current back. Its potential is near zero.
• The earth wire (usually green) is joined to a metal plate that is buried deep in the ground near the house. It does not carry a current in normal use. It is for safety.

The colours of the wires are different in some countries. For example, in many countries the live wire is brown, the neutral wire is blue, and the earth wire is green and yellow. Always check before you touch a wire. And do not touch the wires without an adult who knows how.

From the pole to the plug

1. The supply from the pole goes through the main fuse and the electricity meter (which counts the units, in kWh).
2. Then it goes to the distribution box, where it is divided into separate circuits.
3. One circuit, of $5\ \text{A}$, is used for bulbs and fans, which take small currents.
4. Another circuit, of $15\ \text{A}$, is used for heavy appliances, such as an air conditioner, a water heater (geyser) or an iron, which take large currents.
5. All the appliances in a circuit are joined in parallel, so each gets the full $220\ \text{V}$, and each has its own switch.

Each circuit has its own fuse (or a circuit breaker) in the live wire.

Two dangers

Short circuit. If the live wire and the neutral wire touch each other, for example when the insulation is worn, the resistance of the path becomes very small. By Ohm's law, the current becomes very large. The wires heat up quickly, and this can start a fire.

Overloading. If too many appliances are used on one circuit, the total current becomes more than the wires can carry safely, and the wires get hot. The current is the total power divided by the voltage: $I = \dfrac{P}{V}$.

What does the fuse do?

The fuse is a thin wire with a low melting point. When the current is too large, in a short circuit or in overloading, the fuse wire melts, and the circuit is broken. The current stops before the wires of the house can burn. A modern house also has a miniature circuit breaker (MCB), which switches itself off in the same case, and can be switched on again.

You should find that the kettle and the heater together are too much for a $10\ \text{A}$ fuse, while all the bulbs together take less than $3\ \text{A}$.

Earthing

Many appliances, such as a refrigerator, an iron or a washing machine, have a metal body. If the live wire touches the body by accident, the body is at $220\ \text{V}$, and a person who touches it gets a severe shock.

The earth wire joins the metal body to the ground, through the third pin of the plug. If the live wire touches the body, the large current goes through the earth wire to the ground, and it melts the fuse. The body of the appliance is kept at the potential of the earth, so the person is safe. The earth wire is also the reason why the third pin of a plug is longer and thicker.

Safety rules

• Never touch an electric switch or an appliance with wet hands, or when you stand on a wet floor.
• Use plugs with three pins for appliances with a metal body.
• Never use too many appliances on one socket.
• Use a fuse of the right rating. Never replace a fuse with a thick wire.
• Switch off before you repair anything, and ask a qualified electrician.

A worked example

On a $15\ \text{A}$ circuit at $220\ \text{V}$, a refrigerator of $150\ \text{W}$, an iron of $1000\ \text{W}$ and a water heater of $2000\ \text{W}$ are used together. Is the circuit safe? What happens if a heater of $500\ \text{W}$ is added?

Step 1. The total power is $150\ \text{W} + 1000\ \text{W} + 2000\ \text{W} = 3150\ \text{W}$.

Step 2. $I = \dfrac{P}{V} = \dfrac{3150\ \text{W}}{220\ \text{V}} = 14.3\ \text{A}$. This is less than $15\ \text{A}$, so the circuit is safe.

Step 3. With the heater of $500\ \text{W}$, the power is $3650\ \text{W}$, and $I = \dfrac{3650\ \text{W}}{220\ \text{V}} = 16.6\ \text{A}$. This is more than $15\ \text{A}$, so the fuse melts.

Practise with new numbers

Try it in Python

The program adds the appliances one by one on a $15\ \text{A}$ circuit and checks the current.

Remember these

• A house has a live wire, a neutral wire and an earth wire. The appliances are in parallel, in separate circuits of $5\ \text{A}$ and $15\ \text{A}$.
• A short circuit and overloading make a large current. The fuse melts and breaks the circuit.
• The earth wire joins the metal body of an appliance to the ground, so that a fault does not give a shock.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why is a thin wire with a low melting point used for a fuse?

2. Why does an iron have an earth wire?

3. Why are the appliances of a house joined in parallel?

4. Can a $5\ \text{A}$ circuit at $220\ \text{V}$ run an iron of $1320\ \text{W}$?