Domestic Electricity and the National Grid
Domestic Electricity and the National Grid
The electricity we use at home is very different from the direct current (d.c.) supplied by batteries. Understanding mains electricity, wiring, and how the National Grid works is important for safety and for the exam.
Alternating and Direct Current
Direct current (d.c.) flows in one direction only. Batteries and solar cells supply d.c.
Alternating current (a.c.) repeatedly reverses direction. The UK mains supply is a.c. at:
- 230 V (potential difference)
- 50 Hz (frequency — the current changes direction 50 times per second)
On an oscilloscope trace, a.c. appears as a wave (sinusoidal), while d.c. appears as a straight horizontal line.
The Three-Pin Plug
UK appliances use a three-pin plug with colour-coded wires:
| Wire | Colour | Connection | Purpose |
|---|---|---|---|
| Live | Brown | Right pin (looking at plug front) | Carries the alternating p.d. from the supply (switches between +230 V and -230 V) |
| Neutral | Blue | Left pin | Completes the circuit; stays at approximately 0 V |
| Earth | Green and yellow stripes | Top pin | Safety wire; carries current to ground if a fault occurs |
The live wire is dangerous even when a switch is off, because it still carries a high potential difference relative to earth. Touching it can cause a fatal electric shock.
The cable grip holds the outer insulation of the cable so pulling the cable does not pull wires from their terminals.
The fuse is connected to the live wire. It is a thin wire that melts and breaks the circuit if the current exceeds its rated value, protecting the appliance and wiring from overheating.
Fuses and Circuit Breakers
A fuse protects a circuit by melting if the current is too high. Choose a fuse rating just above the normal operating current.
Example: A 500 W heater at 230 V draws I = P / V = 500 / 230 = 2.2 A. Choose a 3 A fuse (the next rating above 2.2 A). Common ratings are 3 A, 5 A, and 13 A.
A circuit breaker does the same job but can be reset (a fuse must be replaced). Circuit breakers also react faster.
An RCCB (residual current circuit breaker) detects a difference between the current in the live and neutral wires. If current is leaking to earth (e.g. through a person), it trips almost instantly. This is faster and more sensitive than a fuse.
Earthing and Double Insulation
If a fault causes the live wire to touch the metal case of an appliance, the earth wire provides a low-resistance path to the ground. A large current flows through the earth wire, which blows the fuse and disconnects the supply before anyone gets a shock.
Double-insulated appliances (marked with a square-within-a-square symbol) have cases made entirely of plastic (an insulator). They do not need an earth wire because the user can never touch a live metal part.
Power and Energy Calculations
power = potential difference x current
P = V x I
power = current squared x resistance
P = I squared x R
energy transferred = power x time
E = P x t
energy transferred = charge flow x potential difference
E = Q x V
Example: A kettle rated at 2,300 W is used for 3 minutes.
E = P x t = 2,300 x (3 x 60) = 2,300 x 180 = 414,000 J (414 kJ)
The National Grid
The National Grid is a system of cables and transformers that connects power stations to consumers across the UK.
Power stations generate electricity at about 25,000 V. This is then:
1. Step-up transformers increase the voltage to 275,000 V or 400,000 V for transmission through overhead power lines
2. Step-down transformers decrease the voltage to 230 V for domestic use
Why transmit at high voltage?
For a given power, increasing the voltage decreases the current (since P = V x I). A lower current means less energy is wasted as heat in the cables (because power lost = I squared x R). Transmitting at high voltage makes the system much more efficient.
Exam Tips
- Always state which wire the fuse is connected to (live) and explain WHY (so the appliance is disconnected from the high-voltage supply)
- For National Grid questions, link high voltage to low current to less heating to more efficient — write the full chain of reasoning
- Remember: the live wire is dangerous even when the switch is off