Mains Electricity

Every socket in the wall is the end of an astonishing journey. Turn on a kettle and, in the same instant, a power station tens of kilometres away works a fraction harder. The electricity that floods into your home — to run the lights, the fridge, the phone charger — is called mains electricity, and it behaves in a way that a battery never does.

In the United Kingdom the mains supply sits at about 230 volts and it flips back and forth 50 times every second. That "flipping" is the heart of this page: mains is alternating current, not the one-way current a battery gives. Meet it properly, and you also learn why a plug has three pins, why one wire can kill you even with the switch off, and how a tiny sliver of wire in a fuse quietly stands guard over your whole house.

A quick reminder of what a voltage is — the electrical "push" that drives charge around a circuit — sets the scene. Mains simply provides a very large, and rather special, push.

Alternating current vs direct current

A battery pushes charge one way and only one way: out of the + end, round the circuit, back into the − end, steadily, for as long as it lasts. That steady, one-direction flow is direct current — d.c. Draw its voltage against time and you get a flat, unchanging line.

Mains is different. Its voltage rises, falls, drops below zero, and climbs back — over and over, 50 full cycles a second. Because the voltage keeps reversing, the current it drives keeps reversing too: charge sloshes forwards, then backwards, forwards, then backwards, many times a second. This is alternating current — a.c. — and its trace is a smooth up-and-down wave. Flip the switch below to compare the two.

The number of cycles per second is the frequency, measured in hertz (Hz). UK mains is 50\,\text{Hz}, so one complete cycle takes

T = \frac{1}{f} = \frac{1}{50} = 0.02\ \text{seconds}.

Twenty thousandths of a second per wobble — far too fast to see a bulb flicker, but very real. a.c. is used for the grid because it can be pushed up to enormous voltages and down again with a transformer, which makes sending power across the country hugely more efficient. d.c. cannot be changed that easily — which is exactly why your phone charger's job is to turn the wall's a.c. back into the gentle d.c. its battery needs.

Inside the three-pin plug

Cut open a UK plug (carefully, in your imagination) and you find three wires, each a different colour and each with a different job. The colours are fixed by law so that an electrician anywhere knows instantly which is which:

Live wire — brown. This is the dangerous one. It carries the alternating supply, swinging up to around 230\,\text{V} and back, 50 times a second. It is the wire that delivers the push to the appliance.
Neutral wire — blue. This sits at roughly 0\,\text{V}. It is the return path — it completes the circuit, carrying the current back out of the appliance to the supply so charge can keep flowing round.
Earth wire — green-and-yellow stripes. In normal use this wire carries no current at all. It is a pure safety wire, connecting the appliance's metal case straight down to the ground. It only ever springs into action when something goes wrong.

A tidy way to remember the colours: bLue for neutraL, brown for the bRave (dangerous) live, and the stripy one is different because its job is different — safety.

A switch — and a fuse — is placed in the live wire, on purpose, and yet touching live can still be lethal when the appliance is off. How? The switch only breaks the loop between the live wire and the appliance. The live wire on the supply side is still connected all the way back to the power station, still swinging up to 230\,\text{V}.

You are standing on the ground, which is at 0\,\text{V}. Touch the live wire and you become a shortcut from 230\,\text{V} straight to earth through your own body — a big potential difference across you drives a dangerous current through your heart. The switch being off changes none of that. This is why you must isolate a circuit at the mains, not just flick the wall switch, before poking about inside.

Staying safe: the fuse and the earth wire

Now the clever part — how the earth wire and the fuse work as a team to protect you. Imagine a fault develops inside a metal-cased appliance, say a toaster: a frayed live wire touches the metal case. Without protection, the whole case would now sit at 230\,\text{V}, and the next person to touch it would get a shock.

The earth wire heads this off. Because it joins the case directly to the ground — a path of very low resistance — the fault current doesn't wait for a person. A huge current surges from live, through the case, down the earth wire to ground.

That surge trips the fuse: a short, thin piece of wire in the live wire, chosen to melt when the current climbs too high. It melts, the circuit breaks, and the appliance is cut off from the supply in a fraction of a second — long before anyone is harmed. The case is now safe and dead. Earth wire + fuse: one routes the fault safely away, the other slams the door.

A fuse is rated in amps, and you pick one just above the appliance's normal working current. To find that current, use the power equation P = IV, rearranged to I = \dfrac{P}{V}.

\text{A } 690\ \text{W heater on } 230\ \text{V mains: } \quad I = \frac{P}{V} = \frac{690}{230} = 3\ \text{A}.

It normally draws 3\,\text{A}, so you fit the next size up — a 5 A fuse. It sits happily through normal use, but the moment a fault sends the current soaring far past 5 A, it melts. Fit a fuse that is too big and it may never blow when it should; fit one too small and it blows during ordinary use.

These three catch people out again and again:

a.c. has no fixed + and − ends. A battery has a permanent positive terminal and a permanent negative one. Mains does not: the live wire is positive, then negative, then positive again, 50 times a second. Talking about "the positive wire of the mains" is a mistake — it alternates.
The earth wire normally carries no current. It is not part of the everyday circuit — neutral is the return path. Earth just sits there at 0\,\text{V} doing nothing, until a fault gives it a job. If you ever do measure current in the earth wire, something is wrong.
"Switched off" is not "safe". As the box above explained, the live wire is dangerous even when the appliance's switch is off, because it is still connected back to the supply. Off does not mean disconnected.

The pylons striding across the countryside carry electricity at up to 400{,}000\,\text{V} — nearly two thousand times your wall socket. Why so brutally high? Because for a given amount of power, a higher voltage means a smaller current (P = IV), and it is current that wastes energy heating the cables. Cranking the voltage up with a transformer and the current right down lets the grid ship power hundreds of kilometres while wasting very little of it as heat — then a transformer near your town drops it back to a safe-ish 230\,\text{V}.

So why is a bird perched on a bare 400 kV line perfectly fine? Because current only flows through something when there is a potential difference across it. Both of the bird's feet are on the same wire, at the same voltage — no difference, no push, no current through the bird. It is only when you bridge two different voltages — one wire and the ground, or two different wires — that current flows and it becomes deadly. The bird stays on one voltage and rides the wave in complete comfort.