Every time you plug in a television or a laptop charger, you are connecting to the UK mains electricity. The UK mains supply is an alternating current (AC) with a frequency of 50 Hz and a potential difference of 230 V. Most appliances connect to this supply using a three-core cable, which contains three separate copper wires bundled inside a flexible plastic insulating layer.
If you open a standard UK plug, you will see three distinct wires, each with a specific colour and role. To easily remember their spatial positions in a plug, you can use the second letter of each wire's colour: Blue is on the Left, Brown is on the Right, and Striped is at the Top.
The live wire is brown and carries the alternating potential difference from the supply at approximately 230 V. The neutral wire is blue and completes the electrical circuit by providing a return path to the local substation, sitting at an earth potential of 0 V.
The final wire is the earth wire, which has green and yellow stripes. This is a crucial safety wire connected to the metal casing of the appliance to stop it from becoming live. Under normal conditions, the earth wire sits at 0 V and does not carry any current; it only carries a current if there is a fault.
The human body naturally sits at 0 V. If a person accidentally touches a live wire, a large potential difference (230 V) is created across their body. This large potential difference causes a current to flow through the person to the ground, resulting in a dangerous electric shock.
We can calculate the current that flows through a person during a shock by rearranging the equation for resistance:
Step 1: Identify the values.
Step 2: Substitute into the equation.
Step 3: Calculate the final answer.
Even if a wall socket or appliance switch is open (turned "off"), the danger remains. The circuit is broken, but the segment of the live wire between the mains supply and the open switch is still held at 230 V. If a person touches this part of the wire, a large potential difference relative to earth is still present, and a shock will occur.
Connecting a live wire directly to the earth wire (or any earthed object) creates a short circuit. Because the earth wire provides a very low-resistance path to the ground, a massive surge of current flows instantly. This huge current can cause wires to rapidly overheat, creating a severe fire hazard.
When explaining the danger of an open switch, examiners want you to explicitly state that the wire up to the switch remains at a potential difference of 230 V.
Always use the precise phrase 'potential difference relative to earth' rather than just 'voltage' when describing the hazards of the live wire.
Examiners frequently ask you to identify wires in a plug diagram. Remember the mnemonic using the second letter of the colour: Blue is Bottom-Left (Neutral), Brown is Bottom-Right (Live), and Striped is Top (Earth).
Three-core cable
A cable containing three separate insulated copper wires bundled together within an external insulating layer.
Live wire
The brown wire that carries the alternating potential difference of 230 V from the mains supply to the appliance.
Neutral wire
The blue wire that completes the electrical circuit by providing a return path for the current, kept at or close to 0 V.
Earth potential
The potential difference of the ground, defined as 0 V.
Earth wire
A safety wire with green and yellow stripes connected to the metal casing of an appliance to provide a low-resistance path to the ground.
Electric shock
A physiological effect caused by a large potential difference driving an electric current through a person's body to the earth.
Short circuit
An unintended low-resistance connection between two points in an electric circuit, such as a connection directly between the live and earth wires.
Put your knowledge into practice — try past paper questions for Physics
Three-core cable
A cable containing three separate insulated copper wires bundled together within an external insulating layer.
Live wire
The brown wire that carries the alternating potential difference of 230 V from the mains supply to the appliance.
Neutral wire
The blue wire that completes the electrical circuit by providing a return path for the current, kept at or close to 0 V.
Earth potential
The potential difference of the ground, defined as 0 V.
Earth wire
A safety wire with green and yellow stripes connected to the metal casing of an appliance to provide a low-resistance path to the ground.
Electric shock
A physiological effect caused by a large potential difference driving an electric current through a person's body to the earth.
Short circuit
An unintended low-resistance connection between two points in an electric circuit, such as a connection directly between the live and earth wires.