Domestic Uses and Safety

Direct current (DC) flows always in the same direction. Examples: torch batteries, mobile-phone batteries, USB outputs.

Alternating current (AC) flows first one way then the other, reversing many times per second. The UK national grid supplies AC at 50 Hz — meaning 50 complete cycles per second.

Why AC for mains? AC can be transformed up or down in voltage using a transformer (covered in 4.2.4.3). This lets electricity be transmitted across the National Grid at very high voltage (low current = low energy loss) and stepped down for safe domestic use.

UK mains values to memorise:

• Voltage: 230 V (this is the rms or 'effective' voltage; the peak is about 325 V).
• Frequency: 50 Hz.
• Period: $T = 1/f = 1/50 = 0.02\,\text{s}$ = 20 ms.

An oscilloscope plots voltage against time. Quick recognition:

• DC trace: a flat horizontal line above or below the zero line (depending on polarity).
• AC trace: a smooth sinusoidal wave alternating between positive peak and negative peak, crossing zero twice per cycle.

Reading a trace (HT):

• Each grid square has a known time-base setting (e.g. 5 ms/div).
• Count the squares for one complete cycle to find T; then f = 1/T.
• Voltage axis (V/div) tells you the amplitude — peak voltage.

Inside a UK three-pin plug (BS 1363):

Wire	Colour	p.d. (relative to earth)	Function
Live	Brown	230 V (alternating)	Carries the current that drives the appliance
Neutral	Blue	~0 V	Returns the current to the local substation
Earth	Green and yellow stripes	0 V	Safety — connects the metal case to ground

The live wire is the dangerous one — if you touch it, current flows through your body to earth (which you are connected to via the floor). Even though neutral is also part of the AC circuit, it sits at approximately 0 V.

The earth wire connects the metal case of an appliance to the earth pin. If the live wire frays and touches the case:

• Without earth: case becomes live → dangerous shock when touched.
• With earth: a huge current flows from live through the case down the earth wire — this trips the fuse/breaker, disconnecting the appliance.

The fuse is a thin wire inside a glass tube. If current exceeds the rating, the wire melts (blows) and breaks the live circuit. After blowing, it must be replaced.

Fuse selection (UK):

• 3 A fuse: for appliances under 720 W (lamps, chargers, TVs).
• 13 A fuse: for appliances over 720 W (kettles, irons, heaters).
• Calculate from $P = VI$: $I = P/V$.

Circuit breaker (RCD/MCB): does the same job as a fuse but trips electromagnetically. Resettable rather than replaceable. Faster than fuses.

Worked example. A 2.4 kW kettle. $I = P/V = 2400 / 230 = 10.4\,\text{A}$. A 13 A fuse is appropriate (not 3 A).

Double insulation. Appliances with plastic cases (e.g. hair dryers) often have NO earth wire — instead they have two layers of plastic insulation that prevent the live wire ever reaching anything you can touch. Symbol: two squares one inside the other.