Electrical Engineering Glossary
Electrical engineering glossary: essential terms explained
This glossary introduces common installation and circuit terms in British English. Definitions describe principles; equipment ratings and installation requirements still need their specific context.
Electric current
The rate of flow of electric charge, measured in amperes (A). Conventional current direction follows positive charge flow, which is opposite to electron drift in a metallic conductor.
Voltage
The electric potential difference between two points, measured in volts (V). Always identify the reference points, such as line-to-neutral or line-to-earth; a voltage is not a property of one isolated terminal alone.
Resistance
The opposition associated with resistive current flow, measured in ohms (Ω). For a component behaving as an ohmic resistance under the stated conditions, U = IR.
Electrical power
The rate of electrical energy transfer, measured in watts (W). P = UI applies to steady DC; in a sinusoidal single-phase AC circuit, active power is UI cos φ using RMS values. The general average depends on voltage and current waveforms.
Ohm's law
For an ohmic resistance, current is proportional to the applied voltage: I = U/R. Resistance can vary with temperature, and non-linear devices do not necessarily obey one constant-resistance relationship.
Kirchhoff's laws
The current law expresses charge conservation at a circuit node. The voltage law states that the algebraic sum of voltages around a closed loop is zero within the applicable circuit model; induced effects must be represented appropriately.
Direct current (DC)
Current that maintains one direction. Its magnitude may be constant or vary; pulsating DC is still distinguished from current that reverses direction.
Alternating current (AC)
Current whose direction reverses with time, commonly periodically. Public supplies normally use approximately sinusoidal AC, while electronic equipment can produce other waveforms.
Frequency
The number of cycles per second of a periodic quantity, measured in hertz (Hz). Many European public supplies operate at a nominal 50 Hz.
Phase difference
The relative position of periodic quantities of the same frequency, often expressed in degrees or radians. For sinusoidal quantities, a time displacement can be related to that phase angle.
Circuit diagram
A drawing representing circuit components and their electrical connections using symbols. It is distinct from a physical layout or a complete installation design.
Capacitor
A component that stores energy in an electric field. Its capacitance relates stored charge to voltage under the stated conditions; a practical capacitor also has voltage, current and other operating limits.
Strappers in lighting circuits
The pair of conductors linking the changeover contacts in conventional two-way lighting switching, including intermediate switching where used. They carry the alternative switching paths and are not transformer windings; identify their actual functions from the circuit.
Coil or winding
A conductor arranged in turns, commonly to produce magnetic effects. Its behaviour includes inductance as well as the resistance and other properties of the actual winding.
Inductance
The relationship between flux linkage and current in the applicable model, measured in henries (H). Changing current produces an induced voltage; for a constant inductance its magnitude relates to L di/dt with the appropriate sign convention.
Capacitance
The relationship between stored charge and voltage, measured in farads (F). For a constant capacitance, changing voltage produces current according to i = C dv/dt.
Transistor
A semiconductor device used for switching, amplification or other control functions. Its operating principle and control quantities depend on whether it is, for example, a bipolar transistor or a field-effect device.
Semiconductor
A material whose electrical behaviour can be controlled through composition, electric fields, light or other conditions. Semiconductor devices use this behaviour for functions such as rectification, switching and amplification.
Diode
A device with a strongly directional current-voltage characteristic. Real diodes have forward losses, reverse leakage and limits; they do not act as perfect one-way valves at every voltage.
Relay
A device that changes an output in response to a control input. An electromechanical relay uses a coil and contacts, while a solid-state relay uses electronic switching elements.
Switch
A device that makes, breaks or changes electrical connections. Its rating, contact arrangement and permitted duty determine the applications for which it is suitable.
Electrical network
An interconnection of electrical components and branches. Series, parallel and more complex arrangements can be analysed with an appropriate circuit model.
Transformer
A device transferring energy between magnetically coupled windings through changing flux. Turns ratio influences voltage and current relationships; losses, limits and any conductive connection depend on its construction.
Electromagnet
A magnetic arrangement energised by electric current, commonly using a coil and magnetic core. Contactors, relays and actuators use electromagnetic force for mechanical operation.
Impedance
The relationship between voltage and current phasors in a sinusoidal steady-state model, measured in ohms. It combines resistance and reactance and can vary with frequency.
Short circuit
An unintended low-impedance connection between points at different potentials. Current is limited by the complete source and fault-path impedance, not literally unrestricted.
Electric circuit
An arrangement of sources, conductors and components. Current flows where a complete conductive path exists, although a circuit can also be described in an open or disconnected state.
Series connection
Components in one unbranched path carry the same current. Their voltages add algebraically; the individual voltage shares depend on their electrical behaviour.
Parallel connection
Branches connected between the same two nodes share the same voltage. The branch currents depend on their loads and add algebraically at the connection points.
Fuse
An overcurrent protective device with an element designed to melt and interrupt the circuit under specified conditions. Its rating, characteristic and breaking capacity must suit the application.
Overcurrent protection
Protection against overload and short-circuit effects, commonly using fuses or circuit-breakers. A sensing relay may require a separate interrupting device; an ordinary RCCB does not provide this protection by itself.
Earthing
A deliberate connection of a system point or conductive part to earth. Protective earthing forms part of an electric-shock protection arrangement; it must not be confused with neutral, functional earth or a guarantee of zero touch voltage.
Frequency converter or variable-speed drive
Electronic equipment that produces a controlled output frequency, commonly with controlled voltage, to operate a suitable motor. Its pulsed output, protection and compatibility requirements differ from a direct mains supply.
Electromagnetic compatibility (EMC)
The ability of equipment to operate satisfactorily in its electromagnetic environment without causing unacceptable disturbance to other equipment. Both emissions and immunity matter.
Power loss
Energy dissipated per unit time, commonly as heat in conductors, cores, insulation or semiconductor devices. Ideal inductors and capacitors exchange stored energy without average loss; practical components have additional loss mechanisms.
Short-circuit current
The current resulting from a short circuit. Prospective fault current is assessed to select protective devices with adequate interrupting capability and to check thermal and mechanical effects.
Power factor
The ratio of active power to apparent power. For sinusoidal voltage and current it equals cos φ; with distortion it also reflects waveform effects. It is not the same as energy-conversion efficiency.
Insulation resistance
Resistance measured between specified conductive parts separated by insulation under stated test conditions. The test voltage, connected equipment and interpretation must suit the circuit; it is not a complete assessment of insulation performance on its own.
Oscillation
A repeated variation of a quantity about a reference value. Electrical oscillation may be periodic, damped or sustained, depending on the circuit and its energy exchange.
Oscillator
A circuit that generates a periodic signal from a supply. Its frequency, waveform and stability depend on its design and operating conditions.
Bridge circuit
An arrangement of branches in a bridge topology, often used for measurement or conversion. A balanced measurement bridge and a bridge rectifier have different functions.
Filter
A circuit or system that changes the relative transmission of frequency components. Its response depends on frequency, loading and the actual design.
Amplifier
A circuit that uses supply energy to increase a signal quantity such as voltage, current or power within specified bandwidth and operating limits.
Resonant circuit
A circuit with inductive and capacitive energy storage that exhibits resonance. Resistance, loading and the selected topology affect the resonant frequency and response.
Frequency response
How a system's gain and phase vary with signal frequency under stated conditions.
High-pass filter
A filter that attenuates lower-frequency components relative to its passband. The cutoff and attenuation slope depend on the design and loading.
Low-pass filter
A filter that attenuates higher-frequency components relative to its passband. It does not provide an ideal abrupt boundary at one frequency.
Three-phase AC
An AC system with three phase quantities nominally displaced by 120 electrical degrees in a balanced sinusoidal arrangement. Supply sequence and actual machine rotation are separate matters.
Power transistor
A transistor designed for specified power-handling tasks. Voltage, current, switching losses, thermal limits and safe operating area must all suit the application.
Protective conductor (PE)
A conductor provided for protective purposes, including fault protection. It is distinct from the neutral conductor and is not intended to carry normal load current.
Operational amplifier
A high-gain differential amplifier commonly used with feedback. Supply range, input conditions, bandwidth and output limits determine its usable behaviour.
Switching amplifier
An amplifier or power stage operated using switching action, such as a class-D stage. Its control and filtering determine the resulting output.
Power control
Adjustment of the power delivered to a load by a suitable control method. The method must suit the load, supply, protection and interference requirements.
Thyristor
A latching semiconductor switching device. A conventional SCR is triggered into conduction and normally remains on until current falls below its holding value; it is not controlled like an ordinary transistor.
Phase-angle control
Control that changes the conduction interval within an AC cycle. The switching device, load compatibility and interference measures depend on the application.
Feedback
Returning information from a system's output to influence its input or control. Its sign and frequency-dependent behaviour affect stability and response.
Digital-to-analogue converter (DAC)
A device or circuit that converts a digital code into an analogue quantity. Resolution does not by itself establish accuracy or usable bandwidth.
Analogue-to-digital converter (ADC)
A device or circuit that converts an analogue input into digital codes. Sampling, input range, resolution and accuracy must be considered together.
Protective switching device
A general term for a switching device used for a specified protective function. A circuit-breaker, RCD and motor-protection device do not provide identical protection.
Protection relay
A device that evaluates measured quantities and initiates a specified protective response. It normally requires suitable associated switching equipment to interrupt the power circuit.
Hysteresis
Dependence of a system's response on its previous state. Different switching-on and switching-off thresholds are one common example.
Thermocouple
A temperature sensor using junctions of dissimilar conductors to produce a temperature-dependent voltage. The measured junction temperature requires appropriate reference-junction compensation.
Noise
Unwanted fluctuations or disturbances in a signal. Their source, spectrum and level affect measurement or communication performance.
Circuit symbol
A graphical symbol representing a component or function in a circuit diagram. Interpret it using the relevant symbol convention and drawing context.
Optical fibre
A light-guiding medium used for optical transmission. Connectors, fibre type, bending limits and the optical system determine its application; do not look into an unknown optical source.
Circuit analysis
Applying appropriate models and calculations to determine circuit quantities and behaviour. Results depend on the validity of the model and assumptions.
Electromagnetic fields
Electric and magnetic fields associated with charges, currents and changing fields. Their behaviour depends on frequency, geometry, materials and the source conditions.