1. Domestic Circuits – Learning Outcomes
Discuss plugs, fuses, MCBs, particularly in the home context.
Wire a plug.
Solve problems about fuses.
Discuss radial and ring circuits. Draw radial circuits.
Recognise the various parts of domestic circuits.
Discuss bonding, earthing, and general safety precautions in domestic circuits.
Discuss RCDs.
Solve problems about energy usage.

2. Plugs

3. Plugs
Live carries an a.c. voltage of -325 V to +325 V. It is dangerous. Do not touch it when it is connected to mains.
Neutral is at ~0 V and is used to complete circuits.
Earth is connected to ground (often literally).
In case a metal appliance becomes live, it provides a lower resistance route to ground than human bodies would.
This has the added effect of increasing current (V=IR) which may blow a fuse, disconnecting the appliance from the live.

4. Fail-Safes in Circuits
A fuse is a piece of wire designed to melt when a current of a certain size passes through it.
This will break the circuit, preventing damage to the components.
Fuses are attached to the live wire to avoid appliances becoming live.

5. Fail-Safes in Circuits
e.g. A lamp has a power rating of 100 W. It operates on the mains at 230 V. Should a 3 A or a 13 A fuse be fixed to the plug?
e.g. An electric cooker has four 500 W plates, a 2 kW grill, and a 3 kW oven. If it operates on the mains at 230 V, is a 40 A fuse suitable for this cooker?

6. Fail-Safes in Circuits
Miniature circuit breakers (MCBs) are switches on the distribution box (a.k.a. fusebox, breaker panel).
If the current flowing through them gets too high, they break the circuit.
A bimetallic strip will bend away from the circuit for small currents.
An electromagnet will pull a contact away for large currents.
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7. Fail-Safes in Circuits
A residual current device (RCD) is attached to the live and neutral at power sockets.
It detects the current flowing through each (which should be about the same as they are in series).
If there is a large difference, it is assumed that the live is leaking (either through earth or a person) and the circuit is broken at the live.
RCDs are quicker than fuses and MCBs.

8. Domestic Wiring

9. Domestic Wiring
In the distribution box, the mains live is separated into the various sub-circuits.
Each live has its own fuse / MCB, and the mains connection also has one.
A meter measures total energy usage.

10. Domestic Wiring
Lights take small current so several can be connected to one fuse in series.
Each light has its own switch (sometimes two) and is connected to the others in parallel.
Depending on the size of the house, there may be multiple light circuits with different fuses (e.g. upstairs and downstairs)

11. Domestic Wiring
Appliances that use large current get their own separate connection to the mains and their own fuse.
Examples include cookers and washing machines.
These are called radial circuits.

12. Domestic Wiring Sockets are connected together in a ring circuit.
Live and neutral are fed from both directions, so that one fault won’t break the circuit.
Modern buildings usually have a switch on each socket on the live.
As with lights, multiple sockets are on one fuse, but different areas of the house may be on different ring circuits.

13. Domestic Wiring
Appliances and sockets are connected to ground (called earthing).
If they become live, the current will be redirected.
Water and gas pipes are also earthed, which is called bonding.

14. Domestic Wiring

15. Units of Electricity
While joules are the SI unit of energy, electricity companies use a unit called the kilowatt-hour (kW h) instead.
e.g. A 2000 W appliance operates for 3 hours. How many kilowatt-hours does it use?
e.g. A 75 W lamp operates for 40 minutes. How many kilowatt-hours does it use?