Circuit Breakers (WJEC GCSE Science (Double Award)): Revision Note

Circuit Breakers

• Fuses and circuit breakers are safety devices designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)

• Domestic electricity is supplied at 230 V with a maximum current of 65 A

• The current enters the house at the consumer unit (sometimes referred to as a 'fuse box')

• The consumer unit consists of a series of circuit breakers which control the amount of current supplied to each circuit within the house

Example of a Domestic Circuit

The consumer unit distributes current to all the circuits in the house

• The circuit breakers within the consumer unit provide protection from current surges or faults

• There are two types of circuit breaker:

  • Miniature circuit breakers (mcb)

  • Residual current circuit breakers (rccb)

Miniature Circuit Breakers

• Miniature circuit breakers (mcb) control individual circuits, such as:

  • Upstairs plug sockets

  • Downstairs plug sockets

  • Upstairs ceiling lights

  • Downstairs ceiling lights

  • Electrical appliances, such as ovens and showers

• Each mcb can be switched on or off

• Each mcb limits the current flowing to its circuit

• Each mcb protects its circuit from surges or faults

  • If the current rises above the set limit of the mcb, the switch flips to its off position 'breaking' the circuit

• Miniature circuit breakers can be reset by turning them back to their on position

  • Nothing within the circuit breaker is permanently damaged (unlike a fuse)

Consumer Unit

A consumer unit consists of miniature circuit breakers (mcb) which control individual circuits, and residual current circuit breakers (rccb) which protect the household from electrocution

Residual Current Circuit Breakers

• Residual current circuit breakers (rccb) monitor the difference between the current drawn from the consumer unit and the current returning to it

• If the difference between the two is greater than the set limit of the rccb, the switch flips to its off position and 'breaks' the circuit

• Residual current circuit breakers can also be reset by switching them back to their on position

Fuses

• Fuses are used to protect individual appliances

• Fuses are located in the plug

• Fuses usually consist of a glass cylinder containing a thin metal wire

• If the current in the wire becomes too large:

  • The wire heats up and melts

  • This causes the wire to break, breaking the circuit and stopping the current

Circuit Symbol of a Fuse

The circuit symbol for a fuse has a wire running through it (not to be confused with a resistor)

• Fuses have ratings, which signify the maximum current that can flow through the fuse wire without it melting

• If the current exceeds that value, the fuse wire melts and the individual appliance is disconnected from the mains supply

• Fuses come in values of 3 A, 5 A and 13 A

• The correct fuse to use is the value just above the current required for the appliance