Mains Electricity

Basics of Mains Electricity

In the UK, mains electricity is supplied at a standard voltage of 230 V with a frequency of 50 Hz. This means the voltage alternates direction 50 times per second, producing an alternating current (AC).

The mains cable contains three wires, each with a specific function and colour code:

• Live wire (brown): carries the alternating voltage from the supply.
• Neutral wire (blue): completes the circuit and carries current away.
• Earth wire (green and yellow): a safety wire that prevents electric shocks by providing a path to ground.

The live wire alternates between positive and negative voltage, causing the current to reverse direction regularly. The neutral wire is kept close to earth potential (0 V), so the potential difference between live and neutral is what powers appliances.

For example, in a UK plug, the live wire supplies 230 V AC, and the neutral wire completes the circuit back to the power station. The earth wire is connected to the metal casing of an appliance to protect users if a fault occurs.

AC supply is used because it is easier to transform voltages using transformers, allowing efficient transmission over long distances with less energy loss.

For instance, if a UK kettle is plugged in, it receives 230 V AC at 50 Hz, powering the heating element to boil water.

Safety Features in Mains Electricity

Mains electricity can be dangerous, so several safety features are built into electrical systems to protect users and appliances.

Fuses are safety devices placed in the live wire of a plug. They contain a thin wire that melts if the current exceeds a certain value, breaking the circuit and stopping the flow of electricity. This prevents overheating and potential fires.

Circuit breakers perform a similar function but can be reset after tripping. They detect excess current and automatically switch off the circuit.

Earth wire provides a low-resistance path to the ground. If a fault causes the live wire to touch the metal casing of an appliance, the earth wire carries the current safely away, causing a large current that blows the fuse or trips the circuit breaker.

Double insulation is used in some appliances instead of an earth wire. These appliances have plastic casings and internal insulation to prevent any live parts from touching the outside, reducing the risk of electric shock.

For example, a hairdryer with a metal casing will have an earth wire, but a plastic-cased radio may be double insulated and have no earth wire.

Dangers of Mains Electricity

Mains electricity can cause electric shocks, burns, or fires if not handled correctly. The main risks include:

• Electric shock: Occurs when a person becomes part of the circuit, allowing current to pass through the body, which can be fatal.
• Short circuits: Happen when the live wire touches the neutral or earth wire, causing a sudden surge of current that can cause overheating or fire.
• Faulty insulation or damaged cables: Expose live wires, increasing the risk of shock or short circuits.

Proper insulation around wires and earthing of metal parts are essential to reduce these dangers. Always use appliances with intact cables and plugs, and never touch electrical devices with wet hands.

AC vs DC in Mains Electricity

Alternating Current (AC) is an electric current that changes direction periodically. In the UK mains supply, the current reverses direction 50 times every second (50 Hz).

Direct Current (DC) flows in one direction only. Batteries and cells provide DC.

AC is used in mains electricity because:

• It is easy to change the voltage using transformers, allowing electricity to be transmitted efficiently over long distances at high voltage and low current, reducing energy loss.
• The voltage alternates, which suits the design of many electrical devices and motors.

In contrast, DC is used in low-voltage applications like batteries, electronic circuits, and some transport systems.

For example, a mobile phone charger converts AC from the mains into DC to charge the battery.