Electrical Jargon Buster

Detects dangerous series arc faults — sparking inside cables or connections — that could start fires before a conventional MCB or RCD would operate. Required by BS 7671 Amendment 4 in new domestic installations.

The fourth amendment to the 18th Edition IET Wiring Regulations, effective April 2026. Introduces mandatory requirements for AFDDs in new dwellings, updated EV charging rules, prosumer installation requirements (Chapter 82), and other safety updates.

Connecting metallic services (water pipes, gas pipes, structural steel) and exposed conductive parts together and to earth to prevent dangerous potential differences between them. Main bonding is at the incoming services; supplementary bonding applies in special locations like bathrooms.

The main panel in a domestic installation housing the main switch, RCDs or RCBOs, MCBs, and other protective devices. Modern units must have individual RCBOs per circuit (or dual-RCD split-load), a metal-clad enclosure in rented properties, and a Type 2 SPD under Amendment 4.

The ability of the upstream (higher-level) protection device to remain closed while only the device nearest the fault operates. An RCBO-based consumer unit achieves full discrimination — a fault on one circuit trips only that circuit, not a whole RCD bank.

The principle that not all connected loads operate simultaneously, allowing circuits and cable sizes to be designed for less than the sum of all individual connected loads. Applied when sizing meter tails, main switches, and sub-mains.

Connecting the exposed conductive parts of electrical equipment to earth so that, in the event of a fault, dangerous voltages cannot appear on metalwork you can touch. Works together with the protective device (RCD/MCB) to quickly disconnect a fault.

A formal report produced after a thorough inspection and test of an electrical installation. It identifies defects, deterioration, or departures from BS 7671, classifying them as C1 (danger present), C2 (potentially dangerous), C3 (improvement recommended), or FI (further investigation). Legally required every 5 years in rented properties.

The everyday name for a consumer unit — the main distribution board in a property. Older ones contained rewireable fuses; modern ones contain MCBs and RCDs. The term "fuse box" is technically inaccurate for modern installations but remains in common use.

The professional body that publishes BS 7671 — the IET Wiring Regulations — the UK's national standard for electrical installations. All competent electricians work to this standard.

A two-digit code classifying protection against solid particles (first digit, 0–6) and liquids (second digit, 0–9). IP44 is typical for bathroom Zone 1 fittings; IP65 for outdoor use; IP66 for heavy spray or hosing.

The unit of energy used on electricity bills. 1 kWh = consuming 1,000 watts for one hour. The average UK home uses approximately 2,700–3,100 kWh per year. A 3 kW electric kettle running for 20 minutes uses 1 kWh.

The total impedance of the path taken by fault current — from the supply, through the circuit conductors, through the fault, and back via the earth path. A low Zs ensures the protective device operates within the required disconnection time. Tested during an EICR.

An automatic, resettable protective device that trips when current exceeds its rated value (e.g. 6A, 10A, 16A, 20A, 32A). Protects against short circuits and overloads. Does not protect against electric shock — that requires an RCD or RCBO.

A UK government-approved membership scheme and certification body for electrical contractors. NICEIC-registered contractors are regularly assessed for competence and compliance with BS 7671. Analogous to NAPIT for self-certifying work under Part P.

The SI unit of electrical resistance. Ohm's Law: V = I × R — voltage equals current multiplied by resistance. 1 Ω allows 1 ampere to flow under 1 volt of potential difference. Resistance matters in cable sizing and loop impedance calculations.

The part of the Building Regulations (England) covering electrical safety in dwellings. Most electrical work in kitchens, bathrooms, and new circuits must be certified by a registered competent person (e.g. NICEIC or NAPIT member) or notified to the local authority. Failure to comply can prevent property sales.

The most common UK supply earthing arrangement. The neutral and protective conductor are combined in the distribution network (PEN conductor) but separated within the installation. PME is generally unsuitable as the means of earthing for electric vehicle outdoor charging points without additional measures.

A consumer who also produces energy — typically via solar PV panels or battery storage. BS 7671 Amendment 4 introduces a new Chapter 82 with specific requirements for prosumer installations, including bi-directional metering, isolation, and protection.

A life-saving device that detects residual current (a current difference between live and neutral — indicating a fault to earth or through a person) and disconnects power in milliseconds. A 30 mA RCD operating in <40 ms is the standard for shock protection in dwellings. Does not protect against overloads — that's the MCB's job.

Combines an RCD and an MCB in a single device — provides both earth-fault protection and overcurrent/short-circuit protection in one module. One RCBO per circuit means a fault only trips that circuit (discrimination), not a whole bank of circuits like a shared RCD would.

Protects sensitive electrical equipment against transient overvoltages — brief but extreme voltage spikes caused by lightning strikes, grid switching, or large motor starts. A Type 1+2 or Type 2 SPD is now required in new domestic installations under Amendment 4. Particularly important if you have solar PV, EV chargers, or smart home systems.

The most common UK domestic earthing system — also called PME. The supply's neutral and protective conductors are combined outside the property and separated at the cut-out. Your supplier's earth terminal is provided at the meter. See also: TT.

An earthing arrangement where the installation's protective conductor connects to an independent local earth electrode (a copper rod driven into the ground), separate from the supply authority's earth. Common in rural properties or older installations without a PME supply. Requires an RCD as the primary protective device due to higher fault loop impedances.

The SI unit of electrical potential difference. UK mains supply is nominally 230 V AC (tolerance +10% / −6%), giving a range of 207–253 V. Three-phase commercial supply is 400 V line-to-line. The driving force that pushes current through a circuit.