Mains Safety

Mains Safety

The electricity from a battery and the electricity in your wall sockets are not the same thing. One is gentle and one-directional; the other reverses 50 times a second and can kill you. Understanding the difference, how a plug is wired, and what actually happens when something goes wrong, is what keeps you safe.

Two kinds of current

Every supply you meet is one of these two, and which one it is changes almost everything about how it behaves and how dangerous it is:

Which supply?

A torch runs from a battery, so its current always flows in one direction: that is _____ current. A washing machine plugged into the wall runs on _____ current, which reverses direction over and over. UK mains reverses _____ times every second, which is why its frequency is quoted as 50 _____.

The UK mains

UK mains electricity is supplied at about 230 volts and 50 hertz, and reaches appliances through a three-core cable. Each wire has its own colour and its own job, and the exam wants the job stated precisely:

Wire the plug correctly

Here is the inside of a three-pin plug. Use the wire colours to drag each label onto the correct wire.

Name the wire

The wire that carries the alternating potential difference from the supply is the _____, insulated in brown. The wire that completes the circuit and sits near 0 V is the _____, coloured blue. The _____ wire, in green and yellow, carries no current at all in normal use: it is there purely for _____.

Why live is the dangerous one

A switch does not make a live wire safe. All a switch does is break the circuit: the live wire on the supply side of it is still at about 230 V relative to earth. You are standing on the earth. Touch the live wire and you become the connection between 230 V and 0 V, so a large potential difference appears across you and a current flows through you to earth. That is the shock. It is also why touching the neutral wire is usually far less dangerous: it sits near 0 V, so there is little potential difference between it and you.

The hidden danger

Why is the live wire dangerous to touch even when the appliance is switched OFF?

• It is still at about 230 V relative to earth, so touching it puts a large p.d. across you and drives a current through you
• It permanently stores charge, like a fully charged battery
• It only carries current when there is a fault
• It sits at 0 V, so it slowly heats your hand

What happens in a fault

Picture a metal-cased toaster whose live wire works loose and touches the case. Without an earth wire the case is now at 230 V, and the first person to touch it becomes the path to earth. The earth wire is what prevents that, and it does not work alone: - The earth wire offers a path of very low resistance from the case straight to earth, so the current takes it rather than going through anybody. - Because the resistance is so low, that current is very large, far bigger than the appliance normally draws. - That surge melts the fuse, which breaks the circuit and disconnects the appliance from the live supply. This is why the fuse must be in the live wire. Put it in the neutral and the fuse would still blow, but the appliance would stay connected to 230 V.

Trace the fault

Inside a metal-cased heater, the live wire works loose and touches the metal case. Follow what happens.

• Suppose there were NO earth wire. What is the situation now?
• Now with the earth wire connected. Where does the current go?
• That path has very low resistance, so the current is very large. What does it do?

Mark the claims

Five statements about mains wiring. Tap the TWO that are WRONG.

• The neutral wire completes the circuit and sits at about 0 volts.
• The earth wire carries the working current to the appliance during normal use.
• The live wire is at about 230 volts relative to earth even when the switch is off.
• The fuse is fitted in the neutral wire, so that a fault disconnects it.
• In normal use, no current flows in the earth wire at all.

Explain the safety system

A metal-cased electric heater develops a fault: the live wire comes loose and touches the metal case. Explain how the earth wire and the fuse together stop this from being dangerous.

• Say what would happen to somebody touching the case if there were no earth wire
• Say what path the current takes with the earth wire connected, and why it takes that one
• Say what the SIZE of that current does to the fuse
• Say which wire the fuse must be fitted in, and why that matters

Safety summary

Mains electricity is _____ current at about 230 V and 50 Hz. In a three-core cable the _____ wire (brown) carries the supply, the neutral wire (blue) completes the circuit near 0 V, and the _____ wire (green and yellow) is a safety wire that carries current only in a _____, when the large current it lets through melts the fuse.