UK Electrical Glossary

Alternating current reverses direction repeatedly, unlike direct current. An AC voltage also changes polarity over its cycle. Frequency, measured in hertz (Hz), tells you how many complete cycles occur each second. In a three-phase system, the three AC waveforms are offset in time; three-phase does not mean three times the frequency.

Curriculum: Electrical Science

The standard protective measure in UK installations: earthing, protective bonding and a protective device working together so that a fault to earth causes disconnection within a prescribed time. If any leg fails — poor earth path, missing bond, wrong device — the fault can sit live on touchable metalwork instead of clearing.

Curriculum: Installation

A protective device that recognises the electrical signature of dangerous arcing — the sputtering current of a damaged cable or a loose terminal — and disconnects the circuit before the arc starts a fire. Ordinary MCBs and RCDs cannot see these faults, because a series arc neither leaks current to earth nor draws obvious overcurrent.

Curriculum: Installation

A practical assessment that many electricians take near the end of their training, at an approved assessment centre. Over a few days they install, test and find faults on a realistic set-up while an assessor watches. You might hear of it when reading how people get into the trade: it is one step on the usual route to being recognised as a fully qualified electrician. Nothing in this app is part of it or stands in for it.

Curriculum: Career Taster

Protection against touching live parts during normal, fault-free service. It is delivered by insulation around conductors, barriers and enclosures around terminals, and placing live parts out of reach. It is the first layer of shock protection; fault protection is the second, and an installation needs both.

Curriculum: Installation

A college course that teaches the knowledge side of electrical installation work, first at Level 2 and then at Level 3. Many adults take it to learn the theory, usually alongside or before working on site. On its own it does not make someone a qualified electrician: the usual route also needs an NVQ assessed on real work and the AM2. This app covers similar topics in a similar order, but it is not that course and cannot award it.

Curriculum: Career Taster

The distribution board of a domestic installation. A main switch, residual-current protection and a row of circuit devices on a DIN rail split the incoming supply into individually protected final circuits. Modern UK consumer units sit in non-combustible enclosures and increasingly give every circuit its own RCBO instead of sharing RCDs across banks.

Curriculum: Installation

Low-resistance measurement proving that conductors which must be complete actually are. That means protective conductors back to the earthing terminal, bonding conductors to services, and the legs of ring final circuits. Performed dead with a low-ohm meter, it is the first step of the test sequence because everything after it assumes the earth path exists.

Curriculum: Testing & Inspection

Multipliers applied to a cable's tabulated current rating to reflect how it is actually installed. High ambient temperature, grouping with other loaded circuits, contact with thermal insulation and the type of protective device all de-rate it. The cable size that survives every applicable factor — and still beats the design current — is the one you install.

Curriculum: Installation

The earth wire of a circuit. It links the metal parts of the equipment the circuit feeds back to the main earthing terminal. Normally it carries no current; if a fault makes that metalwork live, it carries the fault current so the fuse or breaker switches off. In UK wiring it is green and yellow, or bare inside twin-and-earth cable, with green-and-yellow sleeving wherever it is exposed.

Curriculum: Installation

Current is the rate at which electric charge flows, measured in amperes (A), often called amps. In a simple closed circuit, the supply drives current through the load and back to the source. Current is not used up by a lamp: energy is transferred there. A cable current rating describes how much current it can carry in stated conditions, not how much it always carries.

Curriculum: Career Taster

The current a cable can carry continuously in its installed conditions without its insulation exceeding the temperature it was designed for. Tabulated values assume reference conditions; the real, installed capacity is what remains after correction factors for temperature, grouping and insulation have taken their share.

Curriculum: Installation

Direct current flows in one direction, although its size can vary. A battery supplies DC, while an inverter can convert DC from a battery into AC for suitable loads. DC does not mean low voltage or harmless: direction of flow and voltage level are separate ideas. Insulation resistance instruments use a DC test voltage, but that is not the normal supply voltage of the circuit.

Curriculum: Electrical Science

The design allowance for the fact that connected loads never all run flat-out at once. A kitchen's sockets, oven and kettle sum to a huge figure on paper, but the realistic simultaneous demand is far lower. Applying diversity turns the connected load into a sensible maximum demand, keeping supplies, switchgear and cables from being absurdly oversized.

Curriculum: Installation

Connecting an installation's exposed metalwork to earth and back to the supply source. That gives fault current a deliberate low-impedance path home, turning a live casing into a large, brief fault current that the protective device can detect and disconnect — without it, the casing simply stays live.

Curriculum: Installation

The card many electricians and electrical workers show to get on to UK building sites. Its colour and title reflect the holder's qualifications and role, from trainee up to fully qualified electrician, and most cards also need a recent health and safety assessment. People apply to the scheme themselves once they have the qualifications it asks for; no online course, including this one, can issue one.

Curriculum: Career Taster

The report produced by a periodic inspection of an existing installation, recording its condition against the current edition of BS 7671. Observations are classified by urgency — C1 for danger present, C2 for potentially dangerous, C3 for improvement recommended, and FI where further investigation is needed — and a C1 or C2 makes the report unsatisfactory.

Curriculum: Testing & Inspection

Electrical energy is the amount transferred over time, measured in joules (J) or, on electricity bills, kilowatt-hours (kWh). A load drawing a steady 1 kW for two hours uses 2 kWh. Power describes the rate of transfer; energy describes the accumulated amount. A battery capacity quoted in kWh is therefore different from its power rating in kW.

Curriculum: Electrical Science

Electrical power is the rate at which electrical energy is transferred, measured in watts (W) or kilowatts (kW). A 1 kW load transfers energy twice as fast as a 500 W load. In a DC circuit, power equals voltage multiplied by current. For AC loads, real power also depends on power factor, so multiplying voltage and current alone does not always give watts.

Curriculum: Electrical Science

Protection that operates when basic protection has failed and a live conductor has reached touchable metalwork. In UK practice it usually means automatic disconnection of supply: earthing and bonding hold the touch voltage down while the protective device removes the supply within its required time.

Curriculum: Installation

An earth connection made because equipment needs it to work — a clean signal reference for ICT and electronic systems — rather than for shock protection. It matters now because Amendment 4:2026 of BS 7671 updated the requirements around functional earthing for information and communication technology installations.

Curriculum: Installation

The HSE's guidance note on electrical test equipment used by electricians. It describes what safe voltage indicators, probes and leads look like: fused or current-limited leads, finger barriers, minimal exposed probe metal, and a proving unit to verify the indicator before and after use. 'GS38-compliant' is shorthand for test kit you can trust your life to.

Curriculum: Sparks Basics

Impedance is the combined opposition that resistance and reactance present to alternating current, measured in ohms (Ω). Reactance comes from inductance and capacitance, so impedance is not always the same as a resistance reading. Earth fault loop impedance, Zs, describes the whole fault-current path; R1+R2 describes the resistance of two circuit conductors.

Curriculum: Electrical Science

The resistance between conductors that should be completely separated from each other. It is measured in megohms, line to neutral and each to earth, with a test instrument applying an elevated DC voltage. Healthy insulation reads very high; damaged cables, moisture or pinched conductors show up as a low reading long before they show up as a shock or a fire.

Curriculum: Testing & Inspection

A two-digit code describing how well an enclosure keeps things out: the first digit rates protection against solid objects, from hands down to dust, and the second rates protection against water, from vertical drips to sustained immersion. The digits are independent, and an X in either position simply means that kind of protection hasn't been tested.

Curriculum: Installation

Conductors connecting incoming metallic services — water, gas, oil, structural steel — to the main earthing terminal. Bonding does not clear faults; it holds the touchable metalwork of a building at roughly the same potential while a fault lasts, so a person touching a pipe and an appliance at the same moment isn't bridging a dangerous voltage difference.

Curriculum: Installation

The realistic peak current an installation will draw, worked out from the connected load with diversity applied. It is the figure the supply arrangements are sized against — tails, main switch, distribution equipment — and the first number a designer needs before any circuit-level decisions make sense.

Curriculum: Installation

An automatic switch that turns a circuit off when too much current flows, whether a slow overload or a sudden short circuit. It has replaced the rewirable fuse in modern boards. Each one is chosen by its current rating, such as 6 A for lights or 32 A for sockets, and by its type: B for most home circuits, C or D where equipment draws a big surge as it starts.

Curriculum: Installation

Domestic electrical work that must be notified under Part P of the Building Regulations in England and Wales. Wales covers more kinds of work than England; Scotland and Northern Ireland run their own systems. It includes a new circuit, a replacement consumer unit and some work in bathrooms. There are two lawful routes: an electrician registered with a competent-person scheme certifies and notifies it, or the householder tells local authority building control before starting and pays for it to be inspected.

Curriculum: Installation

Any current above the value a circuit is designed to carry. It comes in two flavours with different time scales: overload, where a healthy circuit is simply asked for too much for too long, and fault current, where a short circuit drives an enormous current that must be broken in milliseconds. Protective devices are chosen to handle both.

Curriculum: Installation

An overload occurs when an otherwise sound circuit carries more current than its intended rating because of the loads connected to it. Several high-power appliances on one extension lead can exceed its rating even without a short circuit. Overload is a kind of overcurrent, and can cause overheating. An RCD alone does not provide overload protection; that is a different protective function.

Curriculum: Installation

A parallel circuit has branches connected across the same two points, so each branch has the same voltage across it. Current divides between branches according to their loads, then combines again. Two identical resistors in parallel have half the resistance of one. Parallel describes the connections; a ring final circuit is a wiring arrangement, not a synonym for any parallel circuit.

Curriculum: Electrical Science

An extra-low-voltage system fed from a safety source like SELV, but with one point deliberately connected to earth. The earthed reference suits control circuits and electronics that need it, at the cost of losing SELV's complete separation — the difference between the two is a reliable exam question.

Curriculum: Installation

The highest current that could flow at a given point if a fault of negligible impedance occurred there — whether between live conductors or from live to earth. Every protective device must be able to break the prospective fault current at its position without failing, which is why PFC is measured or calculated at the origin of every installation.

Curriculum: Testing & Inspection

Verification that conductors are connected the right way round throughout an installation. Single-pole switches and protective devices must sit in the line conductor, the centre contact of screw lampholders on line, and socket terminals wired as marked. A polarity fault can leave equipment live when switched off — which is why it is checked dead and confirmed again live.

Curriculum: Testing & Inspection

The highest current that would flow in a short circuit between live conductors — line to neutral, or line to line on three-phase supplies. Together with the prospective earth fault current it makes up the prospective fault current, and the larger of the two is what switchgear breaking capacity is checked against.

Curriculum: Testing & Inspection

The combined resistance of a circuit's line conductor (R1) and its circuit protective conductor (R2), measured end-to-end during dead testing. It proves the protective conductor is continuous, and added to Ze it predicts the circuit's earth fault loop impedance before the supply is ever switched on.

Curriculum: Testing & Inspection

A circuit that runs outward from the distribution board, feeding each point in turn, and simply ends at the last one — no return leg. Radials are the default worldwide and are simpler to design, test and fault-find than rings; UK practice uses them for lighting, dedicated appliances and increasingly for socket circuits too.

Curriculum: Installation

The paperwork that comes before most jobs on a site. The risk assessment lists what could go wrong and how each risk will be controlled; the method statement sets out the safe order of work, step by step. Electricians read and follow them, often help write them, and a site manager will usually ask to see them before work starts. They belong to one specific job, and they are working documents rather than a qualification.

Curriculum: Sparks Basics

A single DIN-rail device combining an RCD and a circuit breaker, so one module gives a circuit both residual-current protection and overload/short-circuit protection. The practical win is selectivity: a fault on one circuit trips that circuit alone, instead of dropping every circuit that shares a bank RCD.

Curriculum: Installation

A safety switch that compares the current going out on the line conductor with the current coming back on the neutral. If they differ, current is leaking somewhere it should not, possibly through a person, and the RCD cuts the supply in a fraction of a second. Most socket circuits in UK homes need RCD protection, usually rated at 30 mA.

Curriculum: Installation

Resistance describes opposition to current that transfers electrical energy into heat, and is measured in ohms (Ω). For an ohmic component at a fixed temperature, greater resistance means less current at the same voltage. A heating element uses resistance deliberately; cable resistance also causes heating and voltage drop. Resistance is only one part of AC impedance.

Curriculum: Career Taster

The law in Great Britain that says certain workplace accidents, work-related illnesses and dangerous near misses must be reported to the enforcing authority, usually the Health and Safety Executive. Northern Ireland has its own version. On a site the report is normally made by the employer or whoever is in charge, but everyone working there should know it exists and tell that person about any incident straight away.

Curriculum: Sparks Basics

A socket circuit wired as a loop, so load current shares two paths. The cable leaves the consumer unit, visits each socket in turn and returns to the same protective device. A UK signature design, it serves a large floor area from modest cable — and demands its own three-step continuity test, because a broken ring keeps working silently as two overloaded radials.

Curriculum: Installation

The procedure that proves a circuit dead before anyone works on it. Prove the voltage indicator against a known source, isolate the circuit, secure the isolation with your own lock and tag, prove the point of work dead across all conductor combinations, then prove the indicator again. The final step is the one that catches a tester that died mid-job.

Curriculum: Sparks Basics

An extra-low-voltage system supplied from a safety source such as an isolating transformer, kept electrically separate from earth and from every higher-voltage system. Because the voltage is low and there is no path back to earth, touching one conductor cannot drive a dangerous current through you — the classic protective measure for shaver sockets and garden lighting.

Curriculum: Installation

A series circuit has components along one current path, so the same current passes through each. Two resistors in series add to the total resistance, while the supply voltage is shared between them. An opening anywhere in that path interrupts current through both. Series describes the electrical connections, not whether the components look lined up on a drawing.

Curriculum: Electrical Science

A short circuit is an unintended connection of very low impedance between points that should have a voltage difference, such as line and neutral. It bypasses the intended load and can allow a large current to flow, limited by the supply and circuit impedance. It differs from an overload caused by too many loads. PSCC describes the current that could flow in such a fault, rather than a current flowing in normal use.

Curriculum: Installation

A device that diverts brief, high-voltage spikes — from lightning activity or heavy switching — safely to earth before they reach sensitive equipment. Usually fitted at or near the consumer unit, SPDs have moved from a luxury to an expected part of new installations as homes fill with vulnerable electronics.

Curriculum: Installation

Additional local bonding between simultaneously accessible conductive parts, classically in bathrooms. It is used where disconnection times or other conditions for protection can't otherwise be met. In modern installations it can often be omitted when the circuits are RCD-protected and the main bonding is in place, which is itself a favourite exam point.

Curriculum: Installation

The most common modern UK supply arrangement: the distributor combines neutral and earth in a single conductor (the PEN) up to the service head, where it splits into separate neutral and earth for the installation. Its weakness is a broken PEN conductor, which can put the whole installation's earthed metalwork at a dangerous voltage — the reason special rules apply to EV charging and other outdoor equipment on PME supplies.

Curriculum: Installation

An earthing system where the supply itself provides a separate earth conductor back to the source. Traditionally that conductor is the lead sheath or armour of the distributor's cable. Neutral and earth stay separate throughout, which keeps the earth path clean but depends entirely on the integrity of that supply conductor.

Curriculum: Installation

An earthing system where the installation provides its own earth via an electrode in the ground. Common on rural and overhead supplies, it takes no earth from the distributor at all. The soil path makes the earth fault loop impedance high, so an RCD, not the overcurrent device, is what disconnects earth faults on a TT installation.

Curriculum: Installation

Voltage is the electrical potential difference between two points, measured in volts (V). It describes energy transferred per unit of charge, rather than how much charge is flowing. A battery can have voltage across its terminals even when no complete circuit exists and no current flows. Voltage and current are different quantities.

Curriculum: Career Taster

The voltage lost along a cable as current flows through its resistance. It is Ohm's law working against you, growing with both load current and run length. BS 7671 limits how much of the supply voltage may be dropped in the installation's own wiring, so a long run can force a larger conductor than the current alone would need.

Curriculum: Installation

The portion of the earth fault loop that lies outside the installation — the supply transformer winding, the distributor's cables and the earth return path, measured at the origin with the installation's own parallel paths disconnected. Ze depends on the earthing system: a TT installation relying on an electrode in soil sees a far higher value than a TN supply with a metallic return.

Curriculum: Testing & Inspection

The total opposition to current around the loop a fault current would travel. That loop runs from the supply, along the line conductor to the fault, and back through the protective conductor and the earthing. It is the supply's own part (Ze) plus the circuit's R1 + R2, the resistance of its line and protective conductors. A low Zs lets a big fault current flow, which trips the protective device quickly, so electricians measure it and compare it with the maximum allowed for that device.

Curriculum: Testing & Inspection