K323 / 2027
Practical electricity overview

Lesson 3 of 4

Live, neutral, earth and the plug

  • I can state what live, neutral and earth mean.Syllabus

    Syllabus K323, 16(g). State what live, neutral and earth mean.

  • I can describe the wiring in a mains plug.Syllabus

    Syllabus K323, 16(h). Describe the wiring in a mains plug.

  • I can explain why switches, fuses and circuit breakers interrupt the live wire.Syllabus

    Syllabus K323, 16(i). Explain why switches, fuses and circuit breakers interrupt the live wire.

Make a guess. You are not marked.

A heater is wired with its switch in the neutral wire instead of the live wire. The switch is turned off and the heater stops. Is it safe to touch the heating element?

  1. It is safe as long as the plug is earthed.
  2. Yes. The heater is off, so no current can flow.
  3. No. The element is still connected to live.
Show the answer

No. The element is still connected to live.

Switching the neutral only breaks the return path. The element stays joined to the live wire, so a person touching it can complete a path to earth and get a shock.

The three wires have different roles. Understanding where each connects explains both normal operation and why a switch must break the live connection.

A sustained current needs a complete conducting path. A break can stop current while a potential difference still exists. Review circuit connections if you need help following the paths.

Identify each conductor

Live: L, brown
Live is at a substantial alternating potential difference relative to earth. Together with neutral, it supplies the normal load circuit.
Neutral: N, blue
Neutral is the normal return conductor, maintained near earth potential in the simplified supply model. It carries normal load current. Its name does not make it safe to touch in every real circuit.
Earth: E, green and yellow
Protective earth connects exposed conductive parts, such as a metal casing, to the protective return path where required. It normally carries negligible current in a correctly operating appliance.

Mains is an alternating supply: current direction reverses. Calling neutral the return conductor describes its circuit role; it does not mean charge always travels in one direction through the appliance.

Read the plug connections

Identify terminals in an inside wiring view

The cable enters at the bottom. This labelled schematic shows connections inside the plug; it is not the pin-facing view or a complete circuit.

Live, neutral, earth, fuse and cable grip inside a three-pin plugAn educational inside view has the cable entry at the bottom, earth terminal E at the top, neutral N on the left and live L on the right. A blue insulated core connects to N. A green-and-yellow core connects to E. The brown live connection passes through the fuse before reaching the other live terminal connection. The fuse is therefore in series in live, with no bypassing wire. The three cores emerge from an intact grey outer cable sheath above the cable grip. The grip spans and holds that outer sheath rather than gripping three separate cores. Terminal letters and the written colour key identify the wires independently of colour. The geometry is schematic and supplies no wire-cutting dimensions.ENLFuseCable gripOutersheath
  • L: liveBrown
  • N: neutralBlue
  • E: earthGreen/yellow

The fuse is in the live connection. The grip holds the outer sheath so a pull on the cable is not taken directly by the terminal connections.

This is an inside wiring view with the cable entering from below. The terminal letters identify live, neutral and earth. The fuse is in the live connection and the cable grip holds the outer sheath.

The brown core connects to L through the fuse, the blue core to N, and the green/yellow core to E. Use the terminal letters and the stated viewpoint: looking at the pins from the opposite side reverses left and right in the drawing.

The grip secures the cable's outer sheath so that a pull on the cable is not transferred directly to the core connections. The insulating covering around each core keeps the conductors apart. The plug view shows these connections; the fault circuit explains how the earth connection works with protection.

Open the live connection

An open circuit can still contain parts connected to live

Both circuits have the same idealised a.c. supply and resistive load. An amber band marks connection to live potential; it is not a current arrow. Brown and blue still identify the cores.

Open a single-pole switch in live

Open a single-pole switch in liveAn a.c. source is on the left, with live above neutral. The live wire contains an open switch before the load. Neutral provides an unbroken connection from the other load terminal back to the source. No steady load current flows. An amber band marks the source-side live wire only; the load is disconnected from live. The band indicates a connection to live potential, not a direction of current. The neutral connection is not labelled safe to touch.LoadA.c.LiveNeutralOpenI = 0

The open gap disconnects the load from live. The load remains connected to neutral in this simplified circuit.

Open only the neutral wire

Open only the neutral wireThe same a.c. source and load are drawn, but the only open switch is now in neutral. The live wire to the load is unbroken. No steady load current flows, so there is no voltage drop across the ideal resistive load. An amber band marks the source live wire, the load and the load-side neutral segment up to the open switch. Those parts remain connected to live potential even though the appliance has stopped operating. The neutral core remains coloured blue; insulation colour has not changed its actual potential.LoadA.c.LiveNeutralOpenI = 0

Current is zero, but the load and the blue segment between the load and the open gap remain connected to live. Stopping operation has not disconnected live.

Live alternates in potential relative to earth. In this model neutral is maintained near earth potential; that does not make neutral universally safe to touch.

Both open switches stop the normal load current. Opening live disconnects the appliance from live supply; opening only neutral leaves a live connection to the appliance.

A single-pole switch opens one conductor. Placing it in live breaks the connection between the live supply and the appliance when the switch is open.

If the switch opens only neutral, the normal circuit is incomplete and the appliance stops operating. However, the load remains connected to live. In the ideal no-current model, there is no voltage drop across the load, so even the load-side segment of the opened neutral connection can be at live potential. Its blue insulation still identifies its intended wiring role.

For the same reason, a fuse or circuit breaker must interrupt live when it operates. Interrupting only neutral can stop operation while leaving the appliance connected to live. Some devices open more than one conductor; the essential protection here includes breaking the live connection.

Check your understanding

In a simple mains appliance circuit, a single-pole switch opens only the neutral wire. The appliance stops operating. Which explanation is correct?

  1. Stopping the current makes every part of the appliance have zero potential relative to earth.
  2. The normal current path is broken, but parts of the appliance can remain connected to live.
  3. The protective earth becomes the normal return wire.
Show the answer

The normal current path is broken, but parts of the appliance can remain connected to live.

Opening neutral stops the normal load current, but it does not disconnect the live connection. A single-pole switch belongs in live so that opening it disconnects the appliance from live supply.