Mains Electricity and Safety

There are two kinds of electrical supply, defined by how the potential difference behaves.

Direct current (dc) flows in one direction only. It is produced by a steady potential difference, one that does not change direction, such as that from a cell or battery. The positive terminal stays positive, so the charge always drifts the same way.

Alternating current (ac) repeatedly changes direction. It is produced by a potential difference that constantly reverses, so the charge moves first one way, then the other, back and forth.

Mains electricity is an ac supply. Batteries and cells provide dc.

The essential difference to state in the exam: in dc the potential difference is steady and the current flows one way; in ac the potential difference keeps reversing, so the current keeps changing direction.

The electricity supplied to homes and businesses in the UK is an alternating supply with two values you must recall:

Quantity	UK mains value
Potential difference	about 230 V
Frequency	about 50 Hz

The frequency of 50 Hz means the alternating potential difference completes 50 full cycles every second (hertz, Hz, means cycles per second). In each cycle the potential difference rises, reverses and returns, so the current changes direction 50 times a second.

UK mains: about 230 V, frequency about 50 Hz, alternating current.

These two numbers are frequently tested directly, so memorise them together with the fact that mains is ac.

Most mains appliances connect through a three-core cable containing three wires, each with a coloured plastic coating so it can be identified. The colours are fixed by law and you must recall them.

Wire	Colour	Role
Live	Brown	Carries the alternating potential difference from the supply
Neutral	Blue	Completes the circuit back to the supply
Earth	Green-and-yellow striped	Safety wire; carries current only if there is a fault

The insulating coatings are colour-coded so that anyone wiring a plug connects each wire to the correct terminal. The outer layers of the cable are also insulators, protecting the user from the wires inside.

Live = brown, Neutral = blue, Earth = green-and-yellow. Learn these exactly; the specification requires the precise colours.

Understanding the potential of each wire explains both how the circuit works and why it can be dangerous.

• Live wire (brown): carries the alternating potential difference from the supply. It swings between positive and negative but is at about 230 V relative to earth. This is the dangerous wire.
• Neutral wire (blue): completes the circuit, returning the current to the supply. It is at or near 0 V.
• Earth wire (green-and-yellow): a safety wire at 0 V connected to the ground. It normally carries no current; it only carries current if a fault makes the metal case of an appliance become live.

The earth wire and a fuse work together for safety. If a fault connects the live wire to a metal case, the earth wire provides a low-resistance path to ground. A large current then flows, which melts the fuse and disconnects the appliance, making the case safe to touch.

Your body, and the ground, sit at about 0 V. The danger comes from any large potential difference between the live wire and you.

The live wire is dangerous even when the switch is open. A switch normally sits in the live wire, but even with it open the live wire on the supply side stays at about 230 V relative to earth. If you touch it, there is a potential difference of about 230 V between the wire and the ground, so a current flows through your body to earth, giving an electric shock.

A live-to-earth connection is dangerous. If the live wire touches an earthed object (or a person standing on the ground), the large potential difference drives a big current through that path. Through a person this can be lethal; through wiring it can cause overheating and fire.

A live wire can give a shock even when the appliance is switched off, because it remains at about 230 V relative to earth. Never assume a live wire is safe just because a switch is open.

1. Mixing up the wire colours

Live is brown, neutral is blue, earth is green-and-yellow. A common slip is putting blue on live; learn the exact colour for each of the three wires.

2. Saying mains is dc

UK mains is alternating current (ac) at about 230 V and 50 Hz. Only cells and batteries give dc.

3. Thinking a switched-off appliance is safe to touch inside

The live wire stays at about 230 V relative to earth even with the switch open, so it can still give a shock. This is exactly why you isolate at the mains before working on wiring.

4. Confusing the roles of neutral and earth

The neutral wire completes the normal circuit and carries current in use (about 0 V). The earth wire is a safety wire that carries current only during a fault.

5. Forgetting that current flows through you to the ground

A shock happens because there is a potential difference between the live wire (about 230 V) and the ground (0 V). Touching the live wire completes a path to earth through your body.