For the numbers behind these terms, see our UK home electrical statistics.
Consumer units and protection
- AFDD (arc fault detection device)
- An AFDD (arc fault detection device) is a protective device in a consumer unit that analyses the electrical waveform on a circuit for the signature of an arc. If it detects one, it cuts power to that circuit and could prevent a fire. It reacts to arcs far more sensitively than an ordinary circuit breaker.
- Circuit breaker (MCB)
- A circuit breaker, often called an MCB (miniature circuit breaker), is a switch in the consumer unit that cuts off a circuit automatically when it is overloaded or short-circuited. It is the modern replacement for the fuse. After it trips, it can be reset once the cause has been dealt with.
- Dual RCD (split-load) consumer unit
- A dual RCD consumer unit has a main switch and two RCDs, with the home's circuits divided between them. Each circuit also has its own circuit breaker. If a fault trips one RCD, the circuits on the other RCD stay on, so a single fault does not put the whole home in the dark.
- Fuses (rewireable and cartridge)
- A fuse is a protective device that contains a thin piece of wire or metal element. If too much current flows, the element melts and cuts the circuit. A rewireable fuse has a wire that is replaced after it blows. A cartridge fuse is a sealed unit that is replaced as a whole. Both protect against overcurrent.
- How often should a consumer unit be replaced
- There is no fixed time after which a consumer unit must be replaced. NICEIC says the best way to know is regular inspection and testing through an EICR. A registered electrician may recommend replacement if the unit no longer meets current regulations, lacks safety devices such as RCDs, or cannot handle extra circuits.
- How to reset a tripped consumer unit
- When a circuit breaker or RCD in the consumer unit trips, it switches off the circuit it protects. A householder can unplug the appliances on that circuit, reset the tripped switch, then plug the appliances back in one at a time to find a fault. If it will not reset, a registered electrician is needed.
- MCB vs RCBO
- An MCB (miniature circuit breaker) protects one circuit against overload and short circuit. An RCBO does the same and also detects current leaking to earth, which protects people from shock. With MCBs, an RCD shared by several circuits supplies the shock protection. With RCBOs, each circuit has its own.
- Metal consumer unit (Amendment 3)
- A metal consumer unit is a fuse box with a non-combustible enclosure. Amendment 3 to the 17th Edition of the Wiring Regulations (BS 7671:2008+A3:2015) required home consumer units to be made from non-combustible material, or housed in a non-combustible cabinet, so that a fault inside is contained and does not spread fire.
- RCD vs RCBO
- An RCD (residual current device) switches off a circuit when current leaks to earth, which protects people from electric shock. An RCBO does the same job and also protects the circuit against overload and short circuit, so it replaces both an RCD and a circuit breaker. A plain RCD does not protect against overload.
- Surge protection device (SPD)
- A surge protection device (SPD) is a protective device, usually fitted in the consumer unit, that guards a home's wiring and electronic equipment against transient overvoltages, which are short, sharp surges of electricity. These can come from lightning or from switching of motors and transformers, and they can damage equipment instantly or over time.
- Why does my RCD keep tripping
- An RCD trips when it detects current leaking to earth. If it keeps tripping, there is likely a fault, either in an appliance plugged into the circuit or in the wiring itself. Electrical Safety First advises getting a registered electrician to investigate rather than carrying on resetting it.
Testing, inspection and certificates
- Continuity test
- A continuity test checks that the conductors in a circuit are properly connected, with no breaks and no unnecessarily high resistance at joints and terminations. The electrician tests the earth conductor on every circuit and the live conductors on ring final circuits. It is a standard test in an EICR and when new work is certified.
- Earth fault loop impedance (Zs)
- Earth fault loop impedance, written Zs, is the resistance to the current that would flow if a live conductor touched earth. It covers the supply, the live conductor to the fault and the protective conductor back. The electrician checks it is low enough that a breaker or fuse would cut the supply fast.
- EICR observation codes (C1, C2, C3 and FI)
- EICR observation codes are the labels an inspector gives each problem on an Electrical Installation Condition Report. C1 means danger present. C2 means potentially dangerous. C3 means improvement recommended. FI means further investigation. A C1 or C2 makes the report unsatisfactory. C3 is advice only. FI needs checking against your report and the current rules.
- EICR vs EIC
- An EIC (Electrical Installation Certificate) is issued for a new installation or for new work such as a new circuit or consumer unit. An EICR (Electrical Installation Condition Report) is issued after inspecting and testing an existing installation, to say whether it is safe for continued use. They are different documents for different jobs.
- Electrical Installation Certificate (EIC)
- An Electrical Installation Certificate (EIC) is the document an electrician issues after new electrical work, such as a new circuit, a new consumer unit or a rewire. It confirms the work was designed, constructed, inspected and tested to BS 7671. It comes with schedules of circuit details and test results. It is not used for existing installations.
- How long is an EICR valid
- An EICR has no expiry date that applies to every home. The inspector sets the date of the next inspection on the report. For a rented home in England the law requires an inspection at least every 5 years. For an owner-occupied home, Electrical Safety First recommends one at least every 10 years, but no law requires it.
- Insulation resistance test
- An insulation resistance test, often called a megger test, measures the resistance of the insulation around the live parts of a circuit. The electrician compares the reading with the minimum in BS 7671 to check the insulation has not broken down through damage or age. It is one of the standard tests in an EICR.
- RCD trip test
- An RCD trip test checks that a residual current device switches off the supply when it detects a leak to earth. An electrician applies a test current with an instrument and records how quickly the device trips. Householders can also press the test button on the device. The certificate guidance recommends doing that every six months.
- What does an EICR test
- An EICR checks the fixed electrical installation in a property: the wiring, consumer unit, socket outlets, light fittings, protective devices and fixed equipment such as showers. The electrician inspects it visually, then tests continuity, insulation resistance, polarity, earth fault loop impedance and RCDs. They report the condition and give each problem a code.
Regulations and registration
- 18th Edition
- The 18th Edition is the current edition of BS 7671, the IET Wiring Regulations. It is the UK national standard for electrical installations and is published by the IET and the British Standards Institution. Electricians use it to design, install, inspect and test wiring. Numbered amendments update it, and Amendment 4 of 2026 is now in use.
- Building Regulations compliance certificate (electrical)
- A Building Regulations compliance certificate for electrical work is issued by an electrician registered with an authorised competent person scheme. It states that notifiable work, such as a new circuit or a consumer unit replacement, complies with the Building Regulations. The installer or their scheme must send a copy to the occupier and to building control within 30 days.
- NICEIC vs NAPIT
- NICEIC and NAPIT are two organisations that register and assess electricians. Both are authorised competent person scheme operators for electrical work in dwellings, so a registered member of either can self-certify work under Part P of the Building Regulations. They differ in size, history and branding, but not in that legal ability.
- TrustMark
- TrustMark is the government-endorsed quality scheme for work that a consumer chooses to have done in or around their home. It is a not-for-profit organisation that licenses scheme providers, such as trade associations and certification bodies, who vet and monitor their registered businesses for technical competence, customer service and trading practices. It also has a three-stage complaints process.
Landlords and rented homes
- Remedial work deadline (28 days)
- The remedial work deadline is the time limit in the Electrical Safety Standards regulations for fixing faults found in an electrical inspection of a rented home in England. The landlord must complete the work within 28 days of the inspection, or sooner if the report names a shorter period, then confirm it in writing.
Wiring, circuits and faults
- Amps, volts, watts and kWh
- Amps measure how much electric current flows. Volts measure the force that pushes it. Watts measure power, found by multiplying amps by volts. A kilowatt hour (kWh) is the energy used by 1,000 watts running for one hour, and it is the unit on your electricity bill. A kW is power at a moment, a kWh is energy over time.
- Burning smell from socket
- A burning smell from a socket is a smell of hot plastic or burning near a socket or plug. Electrical Safety First lists it as a danger sign, alongside scorch marks, sparks and a socket that feels hot. It can point to overloading, a loose connection or a damaged appliance, and it needs checking by a registered electrician.
- Cable sizes (1.5mm, 2.5mm, 6mm)
- A cable size such as 1.5mm, 2.5mm or 6mm is shorthand for the cross-sectional area of each copper conductor, measured in square millimetres (mm²). It is not the cable's width. A larger area carries more current, so the right size depends on the load, the protective device and how the cable is installed.
- Earth fault
- An earth fault is a fault in which electric current flows to earth along an unintended path, such as when a live conductor touches the metal casing of an appliance. If someone touches the live metal, current can pass through them. Earthing and a residual current device are designed to disconnect the supply quickly.
- Earthing systems (TN-S, TN-C-S and TT)
- An earthing system is how a home's electrical installation connects to earth. In the UK there are three: TN-S, where the supplier provides a separate earth, usually the cable sheath; TN-C-S (PME), where the earth comes from the supply neutral; and TT, where the home has its own earth rod. The supplier can tell you which applies.
- Electric shock
- Electric shock happens when a person's body becomes part of an electrical circuit and current passes through it. At home it usually follows contact with a live part, such as a faulty appliance, a damaged cable or a fault in the wiring. The effects are not always obvious straight away and can include internal injuries.
- Flickering lights
- Flickering lights are lights that dim, pulse or cut out on their own. One flicker on one bulb may be the bulb. Lights that flicker constantly, or bulbs that keep blowing, can point to a loose connection or other fault in the wiring that a registered electrician should investigate.
- Full rewire vs partial rewire
- A full rewire replaces the whole electrical installation in a home: cables, sockets, switches, light fittings and the consumer unit. A partial rewire replaces the wiring on selected circuits or in selected rooms and leaves the rest in place. An electrical inspection report shows which one a property needs.
- Overload
- An electrical overload is a current larger than a circuit was designed to carry, flowing in a circuit that is otherwise electrically sound. It happens when too much equipment draws power at once. A circuit breaker or fuse should disconnect the circuit before the cable overheats and damages its insulation.
- Power surge
- A power surge is a brief spike of electricity above the normal voltage, which the IET calls a transient overvoltage. It can come from lightning on nearby overhead lines or from switching motors and transformers. A surge can damage wiring and sensitive electronics, and a surge protective device in the consumer unit is the usual protection.
- Protective bonding
- Protective bonding is a low-resistance connection between metal parts that could carry a voltage, such as incoming gas and water pipes, and the main earthing terminal. It keeps those parts at the same potential, so no dangerous voltage can appear between them in a fault. Main bonding is whole-installation. Supplementary bonding is local.
- Ring final circuit vs radial circuit
- A ring final circuit is a socket circuit wired in a loop that starts and ends at the same protective device in the consumer unit. A radial circuit runs out from the consumer unit to its sockets and stops at the last one. Current wiring rules allow either layout to supply sockets.
- Short circuit
- A short circuit is a fault in which current takes an unintended, very low-resistance path, for example when a live conductor touches a neutral conductor. The current is far larger than the circuit normally carries. A circuit breaker or fuse is designed to disconnect it almost at once, before cables and equipment are damaged.
- Spur (fused spur)
- A spur is a branch of cable taken off a ring or radial socket circuit to feed an extra socket or a piece of fixed equipment. A fused spur is one that passes through a fused connection unit, which holds its own fuse. An unfused spur has no separate fuse and relies on the circuit's protective device.
- Twin and earth cable
- Twin and earth cable is the flat PVC-insulated and sheathed cable used to wire most UK homes. It holds two insulated cores, live and neutral, plus a bare copper earth conductor called the circuit protective conductor. Its British Standard type is 6242Y, and it is used for sockets, lighting, cookers and showers.
EV charging
- DNO notification (Distribution Network Operator)
- DNO notification is the step of telling your Distribution Network Operator, the company that brings electricity to your property, about a large new device such as an EV charge point or heat pump. For many home installations the installer notifies within 28 days after fitting. Others need the operator's approval before the work starts.
- Do I need planning permission for an EV charger
- In England, most home EV chargers on a house or block of flats do not need a planning application, because permitted development rules cover them. The rules set limits on height, size and number, and they exclude listed buildings, scheduled monuments and, for some installations, conservation areas. If a charger breaks a limit, a planning application is needed.
- Granny charger (3-pin)
- A granny charger is the informal name for a portable cable that charges an electric car from a standard 3-pin household socket. It is very slow, at about 2.3kW, and a socket is not built for the steady load of EV charging, so manufacturers treat it as an occasional back-up rather than a routine way to charge.
- How long does it take to charge an electric car at home
- Home charging time is the battery size in kWh divided by the charging rate in kW, so a bigger battery or a slower charger takes longer. On a typical 7.4kW home charger a 60kWh battery fills from empty in just under 8 hours, which is why most drivers charge overnight and top up rather than wait.
- Off-peak EV tariff
- An off-peak EV tariff is an electricity tariff that charges a lower unit rate at set low-demand times, usually overnight, so charging an electric car at home costs less. It is a type of time of use tariff. You normally need a smart meter, and you save only if you move your charging into the cheap hours.
- PEN fault and O-PEN device
- A PEN fault, or open PEN, is a break in the combined protective earth and neutral conductor of a supply network that uses PME earthing. It can put a dangerous voltage on earthed metalwork. An O-PEN device is the protection built into many EV chargers that detects the fault by monitoring supply voltage and disconnects the vehicle.
- Smart charger
- A smart EV charger is a home charge point that can send and receive information and respond to signals, so it can charge your car when demand on the grid is lower or more renewable electricity is available. Since 30 June 2022, private charge points sold in Great Britain must meet the smart requirements in the 2021 regulations.
- Tethered vs untethered charger
- A tethered EV charger has a charging cable permanently attached to the unit. An untethered charger has no fixed cable, only a socket, so you plug in your own cable to charge. Tethered is quicker to use day to day. Untethered is easier to adapt if you change cars or need a different cable length.
- Type 2 connector
- A Type 2 connector is the 7-pin plug and socket standard used across Europe for charging electric cars on alternating current. It has a built-in locking mechanism and can carry single or three-phase power. Most UK home chargers and public chargers use a Type 2 socket, and most modern cars have one.
Alarms, lighting and home installs
- Armoured cable (SWA)
- Armoured cable, usually called SWA for steel wire armoured, is a heavy-duty electrical cable with a layer of steel wires around its insulated conductors, under an outer plastic sheath. The armour protects it from damage. It is the cable generally used for outdoor and underground runs, such as the supply to a shed or garden outbuilding.
- Bathroom zones
- Bathroom zones are the areas around a bath or shower, called zone 0, zone 1 and zone 2, that the Wiring Regulations use to decide what electrical equipment is allowed. The closer to the water, the stricter the rules on water resistance and voltage. Areas beyond zone 2 are outside the zones.
- BS 5839-6 (domestic fire detection grades)
- BS 5839-6 is the British Standard code of practice for fire detection and fire alarm systems in homes. It describes the type of system with a Grade, such as D1 or D2, and how much of the home is covered with a Category, LD1, LD2 or LD3. It applies to new-build and existing homes.
- How many smoke alarms you need and where to fit them
- Government fire safety advice is to fit at least one smoke alarm on every level of your home, ideally on the ceiling in the middle of a room and in the hallway and landing. Keep them out of kitchens and bathrooms, where smoke or steam can set them off. English landlords must fit one on every storey lived in.
- IP rating
- An IP rating is a code, such as IPX4 or IP44, that shows how well a piece of electrical equipment resists dust and water. The first digit is protection against solid objects such as dust and the second is protection against water. A higher number means more protection, and an X means that digit is not stated.
- PIR sensor
- A PIR, or passive infrared, sensor detects the infrared radiation that people and other warm objects give off. When something warm moves across its field of view, the sensor triggers a connected device. In homes it is mostly used to switch security lights on at a door, path or driveway when someone approaches.
