Fuses, Circuit-breakers
When electricity from the power station reaches your house, the cable goes into a fuse box on the wall. From the fuse box, it is connected to many wires that go to different parts of the house. Some goes directly to the ceiling lights. Others go to mains sockets on walls, for you to plug in your appliances.
Appliances like television, kettle, light bulb, etc. are usually well insulated. This means that they are insulated from the voltage in the wires so that if you touch it, the voltage is not conducted to your hand. However, it can happen that the wires are worn out, or the appliance gets went. Then the voltage from the wires can come into contact with the casing of the appliance. If you touch this, a current would flow through your body.
There are two common ways to avoid this danger. One way is to use a fuse. The other way is to use double insulation.
A fuse is a thin wire that melts and breaks when too much current flows through it. To understand how, it works, consider a circuit where a battery is connected in series with a fuse and a resistor. The same current flows through both fuse and resistor. If I now connect a wire across the resistor, nearly all current would go through the wire because it has very low resistance. Very little will go through the resistor. So the resistance to the current becomes very small, and a large current would flow. This way of skipping over the resistor is called a SHORT CIRCUIT. When this happens, the fuse get very hot, melts and breaks. Then current stops.
To see why this is useful, think of the electric kettle. It is connected to the mains socket by a cable. If one of the wires is worn, or if the plug is wet, current can flow from wire to the casing. The casing becomes 240 V (or 100 V, depending on your country). If you touch the casing, a large current flows through you.
If there is a fuse along the wire, this large current would cause the fuse to melt and break. Then the large current stops and you are safe.
In some countries, like the UK and Singapore, the mains socket provides a third contact point, called earth. In addition to the two wires in the cable that carry the normal current and form the circuit, there could be a third wire in the cable called earth wire. If the worn or wet wire is in contact with the casing, large current flows through the earth wire to earth, and the fuse breaks immediately. This makes it safer.
Note that some countries, like Japan, may not have the earth wire. Also, many electronic appliances also do not have earth wires. In such cases, these appliances must have double insulation. This is an additional layer of insulation inside the appliance to prevent electrical contact of the inside with the casing. Even if there is earth wire, double insulation should make the appliance safer.
The two wires that carry the current and the earth wire (assuming that there is one) are usually bunched together and go from the kettle to the electric plug. This bunch of wire is the CABLE. The plug is usually a small plastic box with three pins sticking out (or two pins if there is no earth wire). The mains socket on the wall has three holes, with electrical contact points inside each hole. When the pins from the plug are inserted into the holes (or plugged in), current flows.
Only two wires carry the current. The EARTH wire normally does not. Of the two wires that carry current, one is at 240 V, one is at 0 V. The one at 240 V is called the LIVE wire. The one at 0 V is called the NEUTRAL wire. 0 V is the same potential as the earth wire, so the neutral wire should be safe to touch (though I would not touch any of the wires when the electricity is on).
The fuse is normally inside the plug, inserted in series with the live wire. If the live wire is in contact with the kettle's casing, the casing would be at 240 V. It is said to have become "live", and is dangerous to touch. A large current then flows to the earth wire, which is at 0 V. The fuse melts and breaks, cutting off the 240 V live wire. The casing returns to 0 V of the earth wire and is safe.
The circuit breaker is in the fuse box, which connects directly to the cable from the power station.
It switches off if the total current drawn by all the appliances in the house is too large. In that sense, it is like a fuse. The difference is that it can be switched on again - nothing is actually broken. This is unlike the fuse, which would melt and break and have to be replaced.
The circuit breaker is very similar to a relay. The mains current passes through an electromagnet. The electromagnet is placed next to a switch. If the current is too large, the electromagnet would be strong enough to open the switch. This stops the current to the whole house.
This gives an additional measure of safety to the fuse in each appliance. Once the problem has been fixed, the circuit breaker can be switched on again.
