Light sources and fittings
A light source turns electrical energy into light. Three common kinds:
- Filament (incandescent) bulb: a thin wire gets white-hot. Most energy becomes heat, not light.
- Fluorescent tube / CFL: a gas glows with invisible light and a coating turns it into visible light. It wastes less energy.
- LED: a tiny semiconductor lights up directly. It is the most efficient and lasts the longest.
Brightness is measured in lumen (lm). Power is measured in watt (W). Efficacy = lumen ÷ watt: how much light you get for each watt. A 60 W filament bulb gives about 12 lm/W; a 9 W LED gives about 89 lm/W.
Fittings hold and connect the lamp: the lamp holder (screw or bayonet), the luminaire (the whole lamp housing with its shade), the ceiling rose and the switch. Colours of wires differ by country, so always follow the local code.
Electrical machines: overview
An electrical machine changes energy from one form to another, or changes voltage.
- Motor: electrical → mechanical energy (a fan, a pump, a lift).
- Generator: mechanical → electrical energy (a power station, a bicycle dynamo).
- Transformer: changes AC voltage up or down. It has no moving parts. A bell transformer lowers 230 V to 12 V.
Machines can be AC (alternating current, from the mains) or DC (direct current, from batteries or rectifiers). Every machine has a rating plate showing its voltage, power and current. For transformers, V₁ ÷ V₂ = N₁ ÷ N₂ (turns of the coils). Machines are covered fully in the lesson on electrical machines.
Building lighting installations
Power reaches a building through the meter and then the distribution board. The board splits the supply into several circuits, such as lights, sockets and the air conditioner. Each circuit has its own MCB (miniature circuit breaker), which trips if the current is too high (overload or short circuit). An RCD also trips when current leaks, and the earth wire carries faults safely to the ground.
- The switch is placed in the live wire. Then, when it is off, the lamp holder is not live.
- The neutral wire goes straight to the lamp.
- Lamps on one circuit are connected in parallel, so each gets full voltage and one lamp failing does not stop the others.
- Lighting circuits use thinner wire and a smaller MCB than socket circuits, because lamps draw less current.
- A two-way switch pair lets you switch one lamp from two places, such as the top and bottom of a staircase.
Safety: switch off the MCB before touching any wire. Real wiring must be done by a trained electrician.
Low-current installations
Low-current (extra-low-voltage) installations carry signals or small power at a safe low voltage, usually 12 V or 24 V or less. Examples: doorbell, intercom, fire and burglar alarm, CCTV, telephone, TV aerial and computer network cables.
- Many use a transformer to bring 230 V down to 12 V.
- They use thin cables, often in a separate conduit from mains cables. This keeps people safe and stops electrical noise from spoiling signals.
- Network and fibre cables carry data, not power.
Even though the voltage is low, the transformer's input side is still mains, so it must be installed correctly.
Try it: wire a room
In the 3D, pick each lamp type and read its watts and lumen. Predict first: which one has the most lumen per watt? Then press Overload circuit 2 and see which devices stop. At home, count how many lamps and appliances share one MCB switch on your board (do not open the board).
Key formulas and definitions
- Energy (kWh) = Power (kW) × Time (h)
- Efficacy = Luminous flux (lm) ÷ Power (W)
- Power P = V × I, so I = P ÷ V
- Transformer: V₁ ÷ V₂ = N₁ ÷ N₂
- Maximum power on a circuit = V × MCB rating
Worked examples
1. A 60 W lamp burns for 5 hours. How much energy does it use?
60 W = 0.06 kW. Energy = 0.06 × 5 = 0.3 kWh.
2. A 60 W filament bulb is replaced by a 9 W LED. They run 1000 hours. How much energy is saved?
Filament: 60 kWh. LED: 9 kWh. Saved = 60 − 9 = 51 kWh. At 8 per kWh the saving is 408.
3. An LED gives 800 lm using 9 W. Find its efficacy.
Efficacy = 800 ÷ 9 ≈ 89 lm/W.
4. A 115 W fan runs on 230 V. Find the current.
I = P ÷ V = 115 ÷ 230 = 0.5 A.
5. A lighting circuit has a 6 A MCB at 230 V. What is the largest total power it can supply? How many 60 W bulbs is that?
Maximum power = 230 × 6 = 1380 W. Number of bulbs = 1380 ÷ 60 = 23.
6. A bell transformer has 1150 turns on the 230 V side and gives 12 V. How many turns on the 12 V side?
N₂ = N₁ × V₂ ÷ V₁ = 1150 × 12 ÷ 230 = 60 turns.
7. Sort these into mains installations and low-current installations: ceiling fan, doorbell, CCTV camera, electric iron.
Mains: ceiling fan, electric iron. Low-current: doorbell, CCTV camera.
Common mistakes
- Putting the switch in the neutral wire. Then the lamp holder stays live even when the switch is off.
- Thinking a higher-watt LED is "brighter" without checking lumen. Brightness is in lumen, not watt.
- Replacing an MCB with a bigger one because it keeps tripping. The wire may overheat. Find the overload instead.
- Believing low-current lines never need care. The transformer feeding them still has a mains input.