The Fuse Box

1. The Fuse Box The circuits in our homes are connected in parallel. There are several circuits in our homes, for example, the lights, the sockets, the cooker, etc. Each circuit has its own fuse to protect the wiring from overheating if a fault occurs.

2. Circuit Breakers A circuit breaker can be used instead of a fuse to protect the wiring from overheating. A circuit breaker is an automatic switch which switches off if too large a current flows. Once the fault has been fixed it can be easily switched back on. Circuit Breakers v Fuses • Circuit breakers are reset by the flick of a switch. • Fuses need replaced so need supply of spare fuses. • Incorrect fuse size could be used allowing faulty circuit to work.

3. ÷ x Paying for Electricity A 2 kW heater is switched on for 3 hours. How much energy will it use? E P t

4. The Joule is too small a unit to measure the electrical energy we use at home. Energy suppliers use a much bigger unit for energy called the kilowatt-hour (kWh). 1 kWh = 3 600 000 J If each kWh costs 8p how much does it cost to run the heater for this time? number of kWh x price per unit = cost 6 x 8 p = 48 p

5. Example A 100 W lamp in a room is left on for six hours. Calculate the cost of the energy used by the lamp in this time (cost per unit of electricity is 8p). The lamp uses 0.6 kWh of electricity at a cost of 8p per unit so the cost of running it is:

6. Example Calculate the cost of running a 750 W drill for 30 minutes if each unit of electricity cost 9p. The drill uses 0.375 kWh of electricity at a cost of 9p per unit so the cost of running it is:

7. Yellow Book Pages 30 & 31 Q109 (a) (d) (g) Q110 Q114 Q116

8. Live Neutral The Lighting Circuit The lighting circuits are ordinary parallel circuits. They are each protected by a 5 A fuse in the fuse box. If one lamp goes out the others are unaffected. Lamps can be switched on/off individually.

9. Live Neutral The Sockets Thesockets are connected in special parallel circuits called Ring Main circuits. Each ring main is protected by a 30 A fuse in the fuse box. There are TWO paths to every socket. Half the current travels round the ring main one way, the other half travels round the ring the other way. The cables can be thinner than if an ordinary parallel circuit was used. Thinner cable is less expensive.

10. Advantages of Ring Main over Parallel • Since there are two paths for current to flow to a socket, cables used can be thinner and hence cheaper. • Less current in each part of the circuit, so less risk of overheating and hence is more safe. • More convenient as sockets can be placed anywhere on the ring.