Fuses (SQA National 5 Physics): Revision Note

Fuses

• A fuse is a safety device designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)

Fuse circuit symbol

The circuit symbol for a fuse - take care not to confuse this with a resistor

• 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 breaks the circuit and stops the current from flowing

• This is an important safety feature in electrical appliances to prevent damage to the appliance and to protect the user from fire

Fuse sizes

• Fuses come in a variety of sizes, typically 3 A, 5 A and 13 A

  • In order to select the right fuse for the job, the current through an appliance needs to be known

• If the electrical power of the appliance is known (along with mains voltage), the current can be calculated using the equation:

• Where:

  • I = current in amperes (A)

  • P = power in watts (W)

  • V = voltage in volts (V)

• The fuse should always have a current rating that is slightly higher than the current needed by the appliance

  • Because of this, the rule of thumb is to always choose the next size up

• If the fuse current rating is too low, it will break the circuit even when an acceptable current is flowing through

• If the fuse current rating is too high, it will not break the circuit in enough time before damage occurs