This is an overview of a typical UK home wiring. All circuits begin from the consumer unit.
There are 3 main kinds of circuits:
- Ring main
- Radial circuit
- Hard wired appliance circuit
- Product must be installed by a competent person (e.g. a qualified electrician) in accordance with the relevant clauses of the current edition of the IEE Wiring Regulations (BS 7671: Requirements for Electrical Installations) and appropriate statutory regulations.
- Switch off main power at the consumer unit.
- Remove the circuit fuse to isolate the circuit you are working on and keep the fuse in a secure location to avoid accidental replacement.
- Turn off the circuit breaker of the circuit you are working on and lock it if possible.
- Place a note in a visible location to advise that you are working on the circuit.
- Make sure the circuit is not live by checking it with a socket tester or voltage tester.
The consumer unit or commonly known as fuse box is the heart of a home wiring installation. It is usually mounted near the energy meter. A modern consumer unit as pictured above houses modular electrical devices called circuit breakers. There are 4 main kinds of circuit breakers in a common household: Main switch, MCB and RCD.
The main switch functions like a master switch that is able to isolate the incoming electrical power supply to the house.
After the main switch the incoming power supply is split to several circuits such as power, light and appliance.
Each circuit is protected & controlled by a miniature circuit breaker MCB. When the circuit draws current above the MCB rated current, the MCB trips and turns off the power to protect the devices on that circuit.
In modern homes, consumer units are also equipped with a residual current device RCD. RCD is a sensitive safety device that switches off electricity when it detects a fault. It is designed to prevent electrical shocks & fire caused by earth faults. Circuits that powers outdoor devices must be installed with RCD protection.
Every circuit breaker is then labelled carefully so that the circuits can be identified and isolated when work needs to be done on them.
Cables that are used in home wiring are separated into three copper wires, live, neutral and earth.
Electrical current flows to electrical devices along the live wire and flows back along the neutral wire. The earth wire which is located between the live and neutral wire provides a safe escape route for leakage current from a circuit as a result of electrical fault or poor connection.
With the progression of wiring standards over the years, there are several electrical wiring colour coding used in homes around UK. The colour coding for current wiring standard is highlighted above. Brown for Live, Blue for Neutral and Green/Yellow for Earth. This colour coding is implemented in 2006 Amendment 2 of 17th Edition BS7671 to harmonise the UK & Europe wiring colours to maintain consistency & avoid confusion.
It is important that the wiring is done in accordance with BS7671 Wiring Regulations. To ensure safety, it is important to make sure the wiring is in good condition. The average lifespan for wiring is around 30-40 years and the condition deteriorates over time.
Some properties in UK are currently still using wiring with old colour coding. This is a sign that the wiring is quite old and it needs to be tested or updated.
Radial circuit is a single cable run from a consumer unit circuit breaker to a number of electrical devices and terminates at the last device.
The current rating of a radial circuit limits the area the circuit can supply. For example, a 20A radial circuit can supply up to 50 square metres and 32A radial circuit can supply up to 100 square metres.
Ring main circuit starts from the consumer unit, connects to a number of electrical devices (typically sockets) and then returns to the consumer unit. Electrical current can flow from either end of the ring.
This increases the current carrying capacity without increasing the wiring size. The ring main is typically 32A in rating which allows it to power an area of 100 square metres.
Spur is a short branch of cable from existing ring main circuit. The branch can stem from an existing electrical device or from a junction box as pictured above.
Every unfused spur can only supply one socket or connection unit. The total number of spurs cannot exceed the number of devices on a ring main circuit.
High current hard wired appliance such as cooker, hot water unit and electric shower require dedicated circuits from the consumer unit. These circuits have higher gauge wiring and protected by high rating circuit breakers.
Cookers are typically controlled by a cooker control unit and other hard wired appliances controlled by connection units. The switches for the connection unit are typically double pole, meaning it switches both live and neutral at the same time.
Earthing & Bonding
Earthing and bonding must be implemented in accordance with BS7671 to ensure the safety of the wiring installation. Both earthing and bonding uses the yellow and green wire colour coding.
Earthing is required in every home wiring installation to protect you against electrical shocks. Electrical current always uses the shortest path to flow from the electrical fault to the ground. If a circuit does not have earthing & a fault happens, the metal body of the faulty appliance can become live. If the human body comes in contact, it can become the shortest path to ground and lead to potentially fatal incident. If a fault happens on a properly earthed circuit, the electric current will flow through earth wire to ground and this will trigger the circuit breaker in consumer unit to cut off the power supply instantaneously.
Bonding is a connection of all metal components to the earth circuit. Taking the image above as an example, if you were to touch the faulty cooker and then touch a metal tap in your kitchen, you would be the earth path and could receive a potentially life threatening electric shock. The lack of bonding could create a chain effect that makes all metal parts, including metal water pipes, in a home to carry live electrical current.
Electrical Wiring Devices
Appliance Pull Switch
Switched Socket with USB Charger
Unfused Connection Unit
Fused Connection Unit
Cooker Control Unit
Metal Back Box
Surface Pattress Box
Dry Lining Box