Electrical Glossary: A–Z of Electrical Terms

Electrical terminology can be difficult to understand, particularly when similar terms are used across cables, circuit protection, lighting, heating, control equipment and electrical testing.

This A–Z glossary explains commonly encountered electrical terms in clear, practical language. It is intended for electricians, electrical contractors, apprentices, facilities teams and customers purchasing electrical products.

This glossary is provided for general information only. It should not be treated as electrical design, installation, testing or repair advice. Electrical work should always be completed by a suitably qualified and competent person.

A

AC

AC stands for alternating current.

In an AC circuit, the direction and magnitude of the current change repeatedly. The electricity supplied to most UK homes and commercial premises is alternating current operating at a nominal frequency of 50Hz.

AFDD

AFDD stands for Arc Fault Detection Device.

An AFDD monitors a circuit for electrical characteristics associated with dangerous arc faults. When a potentially hazardous arc is detected, the device disconnects the circuit to help reduce the risk of fire.

AFDDs may be incorporated into combined protective devices that also provide overload, short-circuit and residual-current protection.

Alternator

An alternator converts mechanical energy into alternating electrical energy.

Alternators are used in generating equipment, vehicles and standby power systems. They operate by creating relative movement between a magnetic field and electrical conductors.

Ammeter

An ammeter is an instrument used to measure electrical current.

Current is measured in amperes. An ammeter is connected in series with a circuit, although clamp meters can measure current without disconnecting the conductor.

Ampere

The ampere is the standard unit used to measure electrical current.

It indicates the rate at which electrical charge flows through a conductor. The symbol for amperes is A.

Apparent Power

Apparent power is the combined effect of voltage and current in an AC circuit.

It is measured in volt-amperes, or VA. Apparent power includes both the power that performs useful work and the power associated with reactive components such as motors and transformers.

Armoured Cable

Armoured cable includes a protective metallic layer that provides additional mechanical protection.

Steel wire armoured cable, commonly known as SWA, is widely used for external, underground, industrial and distribution applications.

The cable, glands and termination equipment must be selected to suit its size, construction and installation environment.

B

Back Box

A back box provides a mounting enclosure for electrical accessories such as switches, socket-outlets and fused connection units.

Back boxes may be manufactured from metal or plastic and can be designed for surface-mounted, flush-mounted, dry-lining or floor installations.

The box depth must be sufficient for the accessory, terminals and conductors being installed.

Ballast

A ballast controls the electrical current supplied to certain discharge and fluorescent lamps.

Traditional ballasts use magnetic components, while electronic ballasts operate at a higher frequency and may provide improved starting and control.

Many modern LED luminaires use electronic drivers rather than conventional ballasts.

Battery

A battery stores chemical energy and converts it into direct-current electrical energy.

Batteries are used in emergency lighting, alarms, test equipment, uninterruptible power supplies, vehicles and portable electrical products.

Battery chemistry, voltage, capacity and charging requirements must match the intended equipment.

Bonding

Protective bonding connects conductive parts together to reduce dangerous differences in voltage during an electrical fault.

Main protective bonding commonly connects incoming metallic services, where required, to the installation’s main earthing terminal.

Bonding should not be confused with earthing, although both form part of the protective arrangements of an electrical installation.

Busbar

A busbar is a conductive bar or strip used to distribute electrical power.

Busbars are commonly installed inside consumer units, distribution boards, panelboards, switchgear and busbar trunking systems.

They are normally manufactured from copper or aluminium and must be rated for the expected current and fault conditions.

C

Cable Basket

Cable basket is an open cable-management system manufactured from welded steel wire.

It is commonly used to support power, data and control cables in commercial and industrial installations.

Cable basket is available in different widths, depths and finishes, with bends, tees, couplers and support brackets available to complete the system.

Cable Cleat

A cable cleat secures one or more cables to a supporting surface.

Cleats help maintain cable position and may be designed to withstand the mechanical forces produced during a short circuit.

The cleat type, spacing and material should suit the cable arrangement and installation conditions.

Cable Gland

A cable gland secures and terminates a cable where it enters an enclosure or item of equipment.

Depending on its design, a gland may provide strain relief, environmental sealing, armour termination and electrical continuity.

The gland must match the cable’s size, construction and required ingress protection.

Cable Tray

Cable tray is a cable-support system commonly used in commercial and industrial installations.

It is generally manufactured from perforated steel, although other materials and finishes are available. Bends, risers, tees, couplers and brackets are used to create a complete cable route.

Capacitor

A capacitor stores electrical energy in an electric field.

Capacitors are used in motor circuits, power-factor correction equipment, electronic power supplies, lighting controls and interference-suppression circuits.

Capacitance is measured in farads.

Circuit Breaker

A circuit breaker automatically disconnects a circuit when excessive current flows.

Depending on the design, it may provide protection against overloads, short circuits or both. Circuit breakers can normally be reset after the cause of the fault has been identified and corrected.

Circuit Protective Conductor

A circuit protective conductor, commonly abbreviated to CPC, provides a path for fault current from exposed conductive parts back towards the source.

The CPC forms part of the protective earthing arrangement and allows the protective device to disconnect the supply when a fault occurs.

Class I Equipment

Class I electrical equipment uses basic insulation and a protective earth connection.

Accessible conductive parts are connected to earth so that a fault can cause the protective device to disconnect the supply.

Class II Equipment

Class II equipment uses double or reinforced insulation and does not rely on a protective earth connection.

It is often identified by a square-within-a-square symbol.

Conduit

Conduit is a tube used to route and protect electrical conductors and cables.

It may be manufactured from PVC, steel, flexible metal or other materials. Conduit systems include fittings such as bends, couplers, bushes, boxes and inspection fittings.

Consumer Unit

A consumer unit is a type of distribution board commonly used in domestic and similar installations.

It contains the main switch and protective devices for the installation’s final circuits. Depending on the design, it may include MCBs, RCBOs, RCDs, AFDDs and surge protective devices.

Contactor

A contactor is an electrically operated switching device used to control electrical loads.

Contactors are commonly used for motors, heating, lighting and industrial equipment. A relatively small control current energises a coil, causing the main contacts to open or close.

Current

Electrical current is the flow of electrical charge through a conductor.

It is measured in amperes. The current flowing in a circuit depends on the applied voltage and the circuit’s impedance.

D

DC

DC stands for direct current.

In a DC circuit, current normally flows in one direction. Batteries, solar photovoltaic systems and many electronic power supplies produce or use direct current.

Delta Connection

A delta connection is a method of connecting the three windings of a three-phase motor or transformer.

The end of each winding is connected to the beginning of another, forming a closed triangular arrangement.

Delta connections are often used when a three-phase motor is operating at its normal rated voltage.

DIN Rail

A DIN rail is a standardised metal rail used to mount electrical and electronic equipment.

Circuit breakers, contactors, relays, terminal blocks, timers and control devices are commonly designed to clip onto DIN rail.

Direct-on-Line Starter

A direct-on-line starter, commonly abbreviated to DOL starter, connects a motor directly to the electrical supply.

It is a straightforward motor-starting method but can result in a high starting current. DOL starters commonly include a contactor and overload protection.

Disconnector

A disconnector provides a means of isolating electrical equipment or a circuit from the supply.

It is intended to provide a safe separation for maintenance or other work. A disconnector may not be designed to switch normal load current unless it is specifically rated for that purpose.

Distribution Board

A distribution board divides an incoming electrical supply into a number of separate circuits.

It contains protective and switching devices such as circuit breakers, RCBOs, RCDs, fuses and surge protective devices.

Distribution boards are available for single-phase and three-phase applications.

Diversity

Diversity is an allowance used in electrical design to recognise that not every connected load will operate at full power at the same time.

Correct application of diversity can help determine the expected maximum demand of an installation.

Double-Pole Switching

A double-pole device switches two conductors at the same time.

In a single-phase circuit, this normally means switching both the line and neutral conductors.

E

Earth

Earth is the conductive mass of the ground, whose electrical potential is conventionally taken as zero.

Electrical installations use earthing arrangements to help protect people, property and equipment during faults.

Earth Electrode

An earth electrode is a conductive part installed in contact with the ground to provide an electrical connection to earth.

Earth rods, plates and foundation electrodes are examples. Earth electrodes are commonly associated with TT earthing systems.

Earth Fault

An earth fault occurs when a live conductor makes unintended contact with earth or an earthed conductive part.

The resulting fault current should cause the appropriate protective device to disconnect the supply within the required time.

Earth Fault Loop Impedance

Earth fault loop impedance is the total impedance of the path taken by earth fault current.

It includes the supply transformer, line conductor, fault path, protective conductor and the return path to the source.

It is commonly represented by the symbol Zs at a point within the installation.

Earthing

Earthing connects exposed conductive parts and other required parts of an electrical installation to the main earthing terminal.

It provides a path for fault current and supports automatic disconnection of the supply during a fault.

Electrical Installation Certificate

An Electrical Installation Certificate, commonly abbreviated to EIC, records the design, construction, inspection and testing of a new electrical installation or major alteration.

The certificate provides evidence that the completed work has been assessed against the applicable requirements.

Electrical Installation Condition Report

An Electrical Installation Condition Report, commonly abbreviated to EICR, records the condition of an existing electrical installation.

It identifies damage, deterioration, defects and other observations that may affect safety or require further investigation.

Electromagnetic Compatibility

Electromagnetic compatibility, or EMC, describes the ability of electrical equipment to operate correctly without causing unacceptable electromagnetic interference or being excessively affected by it.

Emergency Lighting

Emergency lighting operates when the normal lighting supply fails.

It may provide illumination for escape routes, exits, open areas and high-risk task areas.

Emergency luminaires can be maintained, non-maintained or combined.

Extra-Low Voltage

Extra-low voltage describes voltage levels below conventional low-voltage supply levels.

Extra-low-voltage systems include SELV and PELV arrangements, which are used in applications such as controls, doorbells, landscape lighting and certain bathroom installations.

F

Fault Current

Fault current is the current that flows because of an electrical fault.

Its magnitude depends on the supply characteristics and the impedance of the fault path. Protective devices must be capable of safely interrupting the expected fault current.

Flexible Conduit

Flexible conduit provides mechanical protection while allowing movement or easier routing around equipment and obstacles.

It is available in metallic and non-metallic forms, with different fittings and ingress-protection ratings.

Frequency

Frequency is the number of complete electrical cycles that occur each second.

It is measured in hertz. The nominal frequency of the UK public electricity supply is 50Hz.

Fused Connection Unit

A fused connection unit, or FCU, provides a fused connection between a fixed electrical circuit and an item of equipment.

It may be switched or unswitched and can include indicators, flex outlets, timers or additional control functions.

Fuse

A fuse is an overcurrent protective device containing an element designed to melt when excessive current flows.

When the element melts, the circuit is broken. The fuse must be replaced after operation.

Fuse Switch

A fuse switch combines switching or isolation with fuse protection.

It is commonly used for distribution circuits, machinery and industrial equipment.

G

Galvanised Steel

Galvanised steel has a protective zinc coating that helps resist corrosion.

It is widely used for cable tray, trunking, conduit, channel and support systems.

The type of galvanised finish should suit the intended indoor or outdoor environment.

Generator

A generator converts mechanical energy into electrical energy.

Generators may provide temporary power, standby power or a primary electricity supply. Output can be single-phase or three-phase depending on the equipment.

Gland Plate

A gland plate is a removable section of an enclosure through which cables can enter.

Cable glands or other entry devices are installed in the plate, allowing equipment to be prepared and maintained more easily.

Grid Switch

A grid switch system uses separate switch modules mounted within a common frame and front plate.

Grid systems allow different combinations of switches, indicators, fuse modules and control devices to be assembled in one accessory.

H

Harmonics

Harmonics are voltage or current waveforms with frequencies that are multiples of the supply’s fundamental frequency.

They can be produced by electronic equipment, variable-speed drives, LED drivers and switch-mode power supplies.

Excessive harmonics can cause overheating, interference and reduced equipment performance.

Henley Block

Henley block is an informal term commonly used for a heavy-duty service connector block.

It allows larger supply conductors to be joined or divided and is frequently used around electricity meters, isolators and consumer units.

Hertz

The hertz is the standard unit of frequency.

One hertz represents one complete cycle per second. The symbol is Hz.

High-Bay Lighting

High-bay luminaires are designed for buildings with high ceilings.

They are commonly installed in warehouses, factories, sports halls and large commercial spaces.

Light output, beam angle, mounting height and control requirements should be considered when selecting a high-bay fitting.

HRC Fuse

HRC stands for high rupturing capacity.

An HRC fuse is designed to interrupt high fault currents safely. It commonly contains a fuse element enclosed within a ceramic body filled with an arc-quenching material.

I

Impedance

Impedance is the total opposition a circuit presents to alternating current.

It includes both resistance and reactance and is measured in ohms.

Inductance

Inductance is the property of a conductor or coil that opposes changes in current by producing an induced voltage.

It is measured in henrys.

Motors, transformers, relays and contactor coils all use inductive effects.

Ingress Protection Rating

An Ingress Protection rating, usually shown as an IP code, indicates the level of protection an enclosure provides against access, solid objects and water.

The first numeral relates primarily to solid-object protection, while the second relates to water protection.

The complete rating and manufacturer’s instructions should be checked when selecting equipment for a particular environment.

Insulation

Electrical insulation is a material that restricts the flow of current.

PVC, XLPE, rubber, glass and ceramic are examples of materials used to insulate conductors and electrical equipment.

Insulation Resistance

Insulation resistance testing assesses the condition of insulation between conductors and between conductors and earth.

A suitable test instrument applies a DC test voltage and measures the resulting resistance.

Inverter

An inverter converts direct current into alternating current.

Inverters are used in solar photovoltaic systems, battery-storage systems, uninterruptible power supplies and motor-control equipment.

Isolator

An isolator is a device used to disconnect equipment or a circuit from every relevant source of electrical energy.

It allows maintenance or work to be carried out without the equipment remaining energised.

J

Junction Box

A junction box provides an enclosure in which electrical conductors can be joined, branched or terminated.

The enclosure and connectors must be suitable for the cable type, current, voltage and environmental conditions.

Connections may need to remain accessible unless an approved maintenance-free arrangement is used.

Joule

The joule is the standard unit of energy.

One joule is equal to one watt of power used for one second.

 K

kA

kA stands for kiloampere.

One kiloampere equals 1,000 amperes. The term is commonly used when stating the short-circuit breaking capacity of protective devices.

Kilowatt

A kilowatt is a unit of power equal to 1,000 watts.

The symbol is kW. Heating equipment, motors, EV chargers and generators are commonly rated in kilowatts.

Kilowatt-Hour

A kilowatt-hour is a unit of electrical energy.

One kilowatt-hour represents one kilowatt of power used for one hour. Electricity meters and energy bills commonly record consumption in kWh.

L

LED

LED stands for light-emitting diode.

An LED produces light when current passes through a semiconductor junction.

LED lighting is available in lamps, strips, panels, downlights, floodlights, battens and many other formats.

Line Conductor

A line conductor is an energised conductor used to carry electrical power from the source to the load.

It was traditionally referred to as the live or phase conductor.

Load

A load is any electrical equipment or part of a circuit that consumes electrical energy.

Examples include lamps, heaters, motors, appliances and electronic equipment.

Low Smoke Zero Halogen

Low Smoke Zero Halogen, commonly abbreviated to LSZH or LSOH, describes cable materials designed to produce reduced smoke and halogen emissions when exposed to fire.

These cables are commonly specified for public buildings, transport systems and escape routes.

Low Voltage

Within electrical installation terminology, low voltage covers defined voltage ranges above extra-low voltage.

Everyday 230V and 400V supplies are classified as low voltage, despite the potentially serious risk they present.

Lumen

A lumen is a unit used to measure luminous flux, or the total quantity of visible light emitted by a source.

The symbol is lm.

Lux

Lux is the unit used to measure illuminance, which is the quantity of light reaching a surface.

One lux is equal to one lumen per square metre.

M

Main Earthing Terminal

The main earthing terminal is the point at which the installation’s protective conductors are connected to the earthing arrangement.

It may connect the earthing conductor, main protective bonding conductors and circuit protective conductors.

Main Protective Bonding

Main protective bonding connects relevant incoming conductive services to the installation’s main earthing terminal.

It helps limit differences in potential during a fault.

MCB

MCB stands for miniature circuit breaker.

An MCB provides protection against overload and short-circuit current. It can normally be reset after operation once the fault has been corrected.

MCCB

MCCB stands for moulded case circuit breaker.

MCCBs are commonly used for higher-current commercial and industrial circuits. They may offer adjustable protection settings and additional control or monitoring accessories.

MICC Cable

MICC stands for mineral-insulated copper-clad cable.

It uses copper conductors insulated by compressed mineral material within a metal sheath.

MICC cable offers strong fire and temperature performance but requires specialist termination accessories and installation techniques.

Motor Starter

A motor starter switches and protects an electric motor.

It may include contactors, overload relays, control devices and short-circuit protection.

Common arrangements include direct-on-line, reversing and star-delta starters.

Multimeter

A multimeter is a test instrument that combines several measurement functions.

Depending on the model, it may measure voltage, current, resistance, continuity, capacitance, frequency and temperature.

N

Neutral Conductor

The neutral conductor carries current back towards the source during normal operation.

It is connected to the neutral point of the supply system and is normally close to earth potential, although it should never automatically be assumed to be safe to touch.

Nominal Voltage

Nominal voltage is the stated voltage used to identify an electrical system or product.

The actual measured voltage may vary within permitted limits.

Non-Maintained Emergency Lighting

A non-maintained emergency luminaire normally remains off while the mains supply is healthy.

It illuminates automatically when the normal supply fails.

Nuisance Tripping

Nuisance tripping is the informal term used when a protective device disconnects a circuit even though no dangerous fault appears to be present.

Possible causes include starting currents, accumulated leakage current, unsuitable device selection, transient overvoltage or an intermittent fault.

The cause should be investigated rather than repeatedly resetting or replacing the protective device.

O

Ohm

The ohm is the standard unit of electrical resistance and impedance.

The symbol is Ω.

Ohm’s Law

Ohm’s Law describes the relationship between voltage, current and resistance.

Voltage equals current multiplied by resistance:

V = I × R

The formula can be rearranged to calculate any of the three values when the other two are known.

Overcurrent

Overcurrent is current that exceeds the intended or rated value of a conductor or item of equipment.

It may be caused by an overload or a fault such as a short circuit.

Overload

An overload occurs when a circuit or item of equipment carries more current than it is designed to carry during otherwise normal operation.

A sustained overload can cause overheating and damage.

Overvoltage

Overvoltage occurs when the voltage rises above the normal or rated level.

It may be temporary, sustained or transient. Surge protective devices are used to limit certain transient overvoltages.

P

Parallel Circuit

A parallel circuit provides more than one path for current to flow.

Components connected in parallel have the same voltage across them, while the total current is shared between the branches.

PAT

PAT is commonly used to describe the inspection and testing of electrical appliances.

The process can include visual inspection, earth-continuity testing, insulation testing and functional checks, depending on the equipment.

PELV

PELV stands for Protective Extra-Low Voltage.

Like SELV, a PELV system uses electrical separation from higher voltages. Unlike SELV, one point of the PELV circuit may be connected to earth.

PME

PME stands for Protective Multiple Earthing.

It is associated with TN-C-S supply systems in which the combined neutral and protective conductor is connected to earth at multiple points in the distribution network.

Power

Electrical power is the rate at which electrical energy is transferred or converted.

Power is measured in watts.

In a basic DC or resistive circuit, power can be calculated by multiplying voltage by current.

Power Factor

Power factor is the ratio between true power and apparent power in an AC circuit.

A power factor closer to one generally indicates that the electrical supply is being used more effectively.

Motors, transformers and electronic equipment can affect power factor.

Prospective Fault Current

Prospective fault current is the highest current expected to flow at a point in an installation if a short circuit or earth fault occurs.

Protective devices and switchgear must have suitable short-circuit ratings for the prospective fault current.

Protective Device

A protective device disconnects or limits the electrical supply when an unsafe condition occurs.

Examples include fuses, circuit breakers, RCDs, RCBOs, AFDDs and surge protective devices.

PVC

PVC stands for polyvinyl chloride.

It is widely used as an insulation and sheathing material for cables and as a material for conduit, trunking, enclosures and wiring accessories.

Q

Quadrature

Quadrature describes two alternating quantities that are displaced by 90 electrical degrees.

The term is used when discussing AC waveforms, motors, control systems and reactive power.

Quiescent Current

Quiescent current is the current consumed by electronic equipment while it is powered but not actively operating a load.

It may be relevant when assessing standby consumption, battery life and control-system power requirements.

R

RCBO

RCBO stands for residual-current circuit breaker with overcurrent protection.

An RCBO combines residual-current protection with overload and short-circuit protection for an individual circuit.

RCD

RCD stands for residual current device.

An RCD monitors the difference between the current flowing through the live conductors of a circuit.

If an imbalance indicates current is flowing through an unintended path, the RCD disconnects the supply.

Reactive Power

Reactive power is power that moves back and forth between the supply and reactive components such as inductors and capacitors.

It is measured in volt-amperes reactive, or var.

Although it does not perform useful mechanical or heating work, it affects current and the capacity of the electrical system.

Relay

A relay is an electrically operated switching device.

Energising its coil causes one or more contacts to change state. Relays are used for control, isolation, signalling and protection.

Resistance

Resistance is the opposition a material or circuit presents to the flow of direct current.

It is measured in ohms.

Resistance depends on factors including the material, conductor length, cross-sectional area and temperature.

Ring Final Circuit

A ring final circuit begins and ends at the same protective device, forming a complete loop.

It is commonly used to supply socket-outlets in UK installations.

The integrity and conductor arrangement of a ring final circuit must be verified during inspection and testing.

RMS

RMS stands for root mean square.

It is a method of expressing an AC voltage or current as an equivalent value that produces the same heating effect as a corresponding DC quantity.

Rotor

The rotor is the rotating part of a motor or generator.

In an electric motor, the rotor turns because of the interaction between magnetic fields within the machine.

S

Selectivity

Selectivity describes the coordination of protective devices so that the device closest to a fault operates before upstream devices.

Good selectivity can prevent a fault on one circuit from disconnecting a larger section of an installation.

Self-Induction

Self-induction occurs when a changing current creates a changing magnetic field that induces a voltage in the same conductor or coil.

The induced voltage opposes the change in current that produced it.

SELV

SELV stands for Separated Extra-Low Voltage.

A SELV circuit is electrically separated from higher-voltage systems and earth. It is used where additional protection against electric shock is required.

Semiconductor

A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator.

Silicon is widely used to manufacture diodes, transistors, LEDs and integrated circuits.

Series Circuit

A series circuit contains components connected one after another to create a single path for current.

The same current passes through each component in the circuit.

Short Circuit

A short circuit is an unintended low-resistance connection between conductors at different electrical potentials.

It can produce a very high current and should cause the appropriate protective device to disconnect the supply rapidly.

Short-Circuit Breaking Capacity

Short-circuit breaking capacity is the maximum fault current that a protective device can safely interrupt.

It is commonly stated in kiloamps on MCBs, RCBOs, MCCBs and other circuit breakers.

Shunt

A shunt is a parallel path that allows some current to bypass another part of a circuit.

Precision shunt resistors are also used to measure high currents by producing a small, measurable voltage drop.

Simmerstat

A simmerstat is an energy regulator used in electric hotplates and similar heating appliances.

It controls heat output by repeatedly switching the element on and off. The selected setting changes the proportion of time the element remains energised.

Sine Wave

A sine wave is the smooth repeating waveform associated with a conventional AC supply.

At 50Hz, one complete cycle lasts approximately 20 milliseconds.

Single-Phase Supply

A single-phase supply normally uses a line conductor and neutral conductor, together with the required protective conductor.

The nominal UK voltage between line and neutral is 230V.

Socket-Outlet

A socket-outlet is an accessory designed to receive a compatible plug.

Socket-outlets are available in switched, unswitched, indoor, weatherproof, industrial and USB-equipped formats.

Solenoid

A solenoid is a coil of wire that creates a magnetic field when current flows through it.

The magnetic field may move a metal plunger or operate a mechanical device.

Solenoids are used in relays, contactors, valves, locks and doorbells.

SPD

SPD stands for surge protective device.

An SPD limits transient overvoltages caused by events such as lightning activity or electrical switching.

Type 1, Type 2 and Type 3 SPDs are available for different positions and protection requirements.

Star Connection

A star connection joins one end of each of three windings at a common point.

It is used in three-phase motors, generators and transformers.

Star-Delta Starter

A star-delta starter reduces the starting current of a suitable three-phase motor.

The motor starts with its windings connected in star and is then changed to a delta connection as it approaches normal operating speed.

Starter

A starter switches and controls an electrical load, most commonly a motor.

Motor starters may also provide overload protection and control functions.

The term starter can also refer to the component used to initiate the discharge in certain fluorescent lamps.

Stator

The stator is the stationary part of an electric motor or generator.

In an AC motor, the stator windings create a magnetic field that acts on the rotor.

Surge Current

Surge current is a temporary increase in current.

It may occur when motors, transformers, power supplies or large capacitive loads are first energised.

SWA Cable

SWA stands for steel wire armoured cable.

Its steel wire armour provides mechanical protection, making it suitable for many industrial, external and underground installations.

SWA Gland

An SWA gland terminates steel wire armoured cable.

It secures the cable, grips the armour and provides a means of maintaining electrical continuity.

Switch Disconnector

A switch disconnector can switch normal load current and provide isolation.

Unless it includes another protective function, it does not normally provide overload or short-circuit protection.

Switch Fuse

A switch fuse combines a manually operated switch with fuse protection.

It is commonly used for distribution circuits, machinery and industrial equipment.

Switchgear

Switchgear is the collective term for equipment used to switch, control, isolate and protect electrical circuits.

It can include switches, fuses, circuit breakers, contactors, relays, busbars and metering equipment.

Synchronous Speed

Synchronous speed is the speed of the rotating magnetic field within an AC motor.

It is determined by the supply frequency and the number of magnetic poles.

T

Terminal

A terminal is a point at which a conductor connects to electrical equipment, an accessory or another conductor.

Terminal types include screw, clamp, spring, push-in, soldered and crimped connections.

Terminal Block

A terminal block provides an organised method of joining electrical conductors.

It may use screw, spring-clamp, push-in or lever-operated connections and can be designed for enclosure or DIN-rail mounting.

Testing

Electrical testing uses suitable instruments to assess the safety and condition of circuits, installations and equipment.

Testing may include continuity, insulation resistance, polarity, loop impedance and operation of protective devices.

Thermal Overload Relay

A thermal overload relay protects a motor against sustained overload current.

It normally operates the motor starter’s control circuit but must be used with suitable short-circuit protection.

Thermistor

A thermistor is a resistor whose resistance changes with temperature.

PTC thermistors increase in resistance as temperature rises, while NTC thermistors decrease in resistance.

They are used for temperature monitoring, control and equipment protection.

Thermocouple

A thermocouple is a temperature sensor formed from two different metals joined together.

A small voltage is produced as the junction temperature changes.

Thermocouples are used in boilers, gas appliances, furnaces and industrial equipment.

Thermostat

A thermostat controls heating or cooling equipment in response to temperature.

It switches or regulates the connected system to maintain a selected temperature range.

Three-Phase Supply

A three-phase supply uses three alternating voltages separated by 120 electrical degrees.

The nominal UK voltage is normally 400V between phases and 230V between a phase and neutral.

Thyristor

A thyristor is a semiconductor switching device used to control electrical power.

A common example is the silicon-controlled rectifier, or SCR, which begins conducting after a suitable gate signal is applied.

TN-C-S

TN-C-S is an earthing arrangement in which the neutral and protective functions are combined in part of the supply and separated at the installation.

It is commonly associated with Protective Multiple Earthing.

TN-S

TN-S is an earthing arrangement in which separate neutral and protective conductors are provided throughout the supply.

Torque

Torque is a turning force.

For motors, torque describes the rotational force produced at the shaft.

Torque values are also specified for electrical terminals to ensure they are tightened correctly.

Transformer

A transformer transfers electrical energy between circuits through electromagnetic induction.

It may increase or decrease voltage or provide electrical separation.

Transistor

A transistor is a semiconductor device used to switch or amplify electrical signals.

Transistors are fundamental components in electronic controls, power supplies and digital equipment.

Tripping Curve

A circuit breaker’s tripping curve describes how quickly it operates at different levels of overcurrent.

Common MCB curves include B, C and D, which have different instantaneous operating thresholds.

Trunking

Trunking is an enclosed cable-management system that normally has a removable or hinged cover.

Types include mini trunking, dado trunking, multi-compartment trunking, floor trunking and industrial steel trunking.

TT Earthing System

In a TT system, the installation has its own connection to earth through an earth electrode.

The supply neutral is earthed separately at the source.

Tungsten

Tungsten is a metal with a very high melting point.

It was traditionally used for the filaments inside incandescent lamps because it could operate at very high temperatures.

Twin and Earth Cable

Twin and earth is a flat fixed-wiring cable commonly used in domestic and light-commercial installations.

It normally contains insulated line and neutral conductors together with an uninsulated circuit protective conductor.

Two-Way Switching

Two-way switching allows a light or group of lights to be controlled from two positions.

Intermediate switches can be added where control is required from three or more locations.

Type A, AC, F and B RCDs

RCD types indicate the residual-current waveforms a device is designed to detect.

Type AC detects sinusoidal alternating residual current, while Type A can also detect pulsating DC residual current.

Type F and Type B devices provide additional detection capabilities for equipment that may produce more complex residual currents.

U

Uninterruptible Power Supply

An uninterruptible power supply, or UPS, provides temporary power when the normal supply fails or becomes unstable.

UPS systems commonly use batteries and electronic power-conversion equipment to support computers, servers, communications equipment and other critical loads.

Uo/U Cable Rating

Uo/U is a voltage designation shown on many electrical cables.

Uo is the rated voltage between a conductor and earth, while U is the rated voltage between conductors.

Utilisation Category

A utilisation category identifies the type of load and switching duty for which equipment is designed.

For example, contactor ratings can vary depending on whether the device is switching resistive heating or starting and stopping a motor.

V

VA

VA stands for volt-ampere and is a unit of apparent power.

Transformers, UPS systems and control equipment are commonly rated in VA or kVA.

Variable Frequency Drive

A variable frequency drive, or VFD, controls the speed and torque of an AC motor by varying the frequency and voltage supplied to it.

VFDs are commonly used with pumps, fans, conveyors and industrial machinery.

Varistor

A varistor is a voltage-dependent resistor.

Its resistance changes as the voltage rises, allowing it to help limit transient overvoltages.

Varistors are commonly found in surge-protection and electronic circuits.

Verification

Verification confirms that an electrical installation complies with the relevant design and safety requirements.

It includes inspection, testing and recording the results.

Volt

The volt is the standard unit used to measure electrical potential difference and electromotive force.

The symbol is V.

Voltage Drop

Voltage drop is the reduction in voltage that occurs as current flows through the impedance of a circuit.

It is affected by conductor size, cable length, load current, material and operating temperature.

Voltage Rating

The voltage rating identifies the maximum system or operating voltage for which a product is designed.

Cables, accessories, protective devices and equipment must be suitable for the voltage of the circuit.

Volt-Free Contact

A volt-free contact is a switch or relay contact that does not supply its own voltage.

It simply opens or closes an externally supplied control circuit.

Volt-free contacts are commonly used in boilers, alarms, timers and building-management systems.

Voltmeter

A voltmeter measures the potential difference between two points in a circuit.

It is connected in parallel with the circuit or component being measured.

W

Watt

The watt is the standard unit of power.

The symbol is W.

Watt-Hour

A watt-hour is a unit of electrical energy.

One watt-hour represents one watt of power used for one hour.

Electricity consumption is normally measured in kilowatt-hours.

Wattmeter

A wattmeter measures the true electrical power being supplied to or consumed by a circuit.

Modern power meters may also display voltage, current, energy and power factor.

Weatherproof

Weatherproof is a general description for equipment intended for exposed or external locations.

The product’s IP rating, operating temperature, materials and installation instructions should be checked rather than relying only on the word weatherproof.

WEEE

WEEE stands for Waste Electrical and Electronic Equipment.

It covers electrical and electronic products that have reached the end of their useful life and may require separate collection, treatment and recycling.

Wiring Accessories

Wiring accessories include switches, socket-outlets, fused connection units, plugs, ceiling accessories, dimmers and data outlets.

They are available in different ratings, finishes, mounting styles and environmental-protection levels.

Wiring Centre

A wiring centre provides a central termination point for heating controls, valves, thermostats, pumps and boilers.

Wiring Diagram

A wiring diagram is a visual representation of the conductors, terminals, accessories and components within a circuit or item of equipment.

It is used during installation, maintenance and fault diagnosis.

X

XLPE

XLPE stands for cross-linked polyethylene.

It is an insulating material used in many power and armoured cables.

XLPE provides good electrical and mechanical performance and generally permits a higher conductor operating temperature than conventional PVC insulation.

X-Rated Capacitor

An X-rated capacitor is a safety capacitor designed for connection across the line and neutral conductors of a mains supply.

It is commonly used for interference suppression and filtering.

Y

Y-Plan Heating System

A Y-plan system is a domestic central-heating control arrangement that uses a three-port motorised valve.

The valve directs heated water to the central-heating circuit, the hot-water cylinder or both.

Yoke

A yoke forms part of the magnetic structure of certain transformers and electrical machines.

In a traditional transformer core, it connects the upright limbs and completes the magnetic path.

Z

Zdb

Zdb is the earth fault loop impedance measured at a distribution board.

It includes the external impedance and the impedance of the distribution circuit supplying the board.

Ze

Ze is the external earth fault loop impedance measured at the origin of an electrical installation.

It represents the part of the fault-current path that is external to the installation.

Zener Diode

A Zener diode is a semiconductor device designed to conduct in reverse bias when a specified voltage is reached.

It is commonly used for voltage regulation, voltage references and protection against excessive voltage.

Zone

A zone is a defined area in which particular electrical installation requirements apply.

Bathroom zones affect the equipment and degrees of protection permitted near baths and showers.

Zones are also used to classify locations where explosive gases, vapours or dust may be present.

Zone Selective Interlocking

Zone selective interlocking is a method of coordinating circuit breakers in larger electrical distribution systems.

The devices communicate so that the breaker nearest a fault can operate rapidly while upstream devices remain closed where possible.

Zs

Zs is the total earth fault loop impedance at a particular point in an electrical installation.

It includes the external earth fault loop impedance and the resistance of the circuit’s line and protective conductors.