Electrical Power & Mains Electricity

What electrical power is

• Power is the rate at which energy is transferred, i.e. the amount of energy moved each second
• Power is measured in watts (W); 1 watt is the same as 1 joule per second (1 W = 1 J/s)
• For an electrical device, the power depends on two things:
  • the voltage (potential difference) across the device
  • the current flowing through the device

The power equation

P = I × V

• where:
  • P = power transferred by the device (W)
  • I = current through the device (A)
  • V = voltage across the device (V)
• Rearrangements:
  • I = P / V (a high-power device at fixed voltage draws a large current)
  • V = P / I

Example — a 150 W electric toaster runs from a 230 V mains supply. Calculate the current it draws.

• Rearrange P = I × V to give I = P / V
• I = 150 / 230 = 0.65 A

Fuses

• A fuse is a safety component placed in the live wire of an appliance; it cuts the supply if the current grows dangerously large
• A fuse looks like a small glass or ceramic cartridge with a fine metal wire running through it; if the current passes the rated value, the wire heats up and melts ("blows"), breaking the circuit
• Once a fuse has blown it must be replaced, because it cannot reset itself
• Without a fuse, an internal short circuit could deliver enough current to overheat the cable, set fire to nearby material, or electrify the appliance's metal casing

Choosing the right fuse rating

• Common UK fuse ratings are 3 A, 5 A and 13 A
• The right fuse for an appliance is the smallest one that is still above the appliance's normal operating current:
  1. Use I = P / V to find the appliance's working current from its rated power
  2. Pick the next standard fuse rating up from that figure
• If the fuse is rated too low, it will blow during normal operation and the appliance will keep cutting out
• If the fuse is rated too high, a fault current can pass right through it before the wire melts, leaving the appliance vulnerable to overheating

Example — a 1800 W electric kettle runs from a 230 V mains supply. Decide which standard fuse (3 A, 5 A or 13 A) is the right choice.

• Normal current: I = P / V = 1800 / 230 = 7.83 A
• Either a 3 A or a 5 A fuse would melt the instant the kettle is turned on, because the normal working current already exceeds their rating
• A 13 A fuse is the next standard size above 7.83 A → choose the 13 A fuse