Claude Lyons Logo

Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time.

A term used for power when it is necessary to distinguish among Apparent Power, Complex Power and its components, and Active and Reactive Power.

The temperature surrounding an object.

Electrical test instrument used to measure current in a circuit.

A unit of measurement for electrical current or rate of flow of electrons (coulombs per second). If a group of electrons whose total charge is 1 coulomb passes a point in a conductor in 1 second, the electric current is 1 ampere. Its mathematical symbol is ‘I’ the term is often shortened to ‘amps’.

The number of ampere-hours that can be delivered under specified conditions of temperature, rate of discharge, and final voltage.

The product of voltage and current in a circuit.

Sparking that results when undesirable current flows between two points of differing potential. This may be due to leakage through the intermediate insulation or a leakage path due to contamination.

A winding that develops current output from a generator when its turns cut a magnetic flux.

A nonlinear device to limit the amplitude of voltage on a power line. The term implies that the device stops overvoltage problems (i.e. lightning). In actuality, voltage clamp levels, response times and installation determine how much voltage can be removed by the operation of an arrester.

A transformer used to step voltage up or down. The primary and secondary windings share common turns, and it provides no isolation.

American Wire Gage (AWG). This term refers to the U.S. standard for wire size.


An alternating current power system consisting of more than two current carrying conductors in which these conductors all carry the same current.

A single or group of connected electric cells that produces a direct electric current (DC).

Total loss of electric power from the power distributor.

Operational sequence of a switch or relay where the existing connection is opened prior to making the new connection.

A low voltage condition lasting longer than a few cycles. “Brownouts” differ from “sags” only in duration.

A small, low voltage transformer placed in series with the power line to increase or reduce steady state voltage.

A heavy, rigid conductor used for high voltage feeders.


The ability of a component to store an electrical charge.

An assembly of capacitors and switching equipment, controls, etc., required for a complete operating installation.

A voltage transformer that uses capacitors to obtain a voltage divider effect. It is utilized at EHV voltages instead of an electromagnetic VT for cost and size purposes.

Electricity produced by a surplus or a shortage of electrons in an object.

The path followed by a flow of electric current.

A device that can be used to manually open or close a circuit, and to automatically open a circuit at a predetermined level of over current without damage to itself.

The highest circuit voltage to earth on which a circuit of a transducer may be used and which determines its voltage test.

Circuit-Switchers are multipurpose switching and protection devices. Often used for switching and protection of transformers, single and back-to-back shunt capacitor banks, reactors, lines, and cables. They can close, carry, and interrupt fault currents.

The greatest root-mean-square (effective) difference of potential between any two conductors of the circuit.

The capability of a conductor to carry electricity, usually expressed as a percent of the conductivity of a same sized conductor of soft copper.

A substance or material that allows electrons, or electrical current, to flow through it.

An electrical load in which the maximum current is expected to continue for three hours or more

The constant voltage or current that a device is capable of sustaining. This is a design parameter of the device.

The relationship of the value of the measured to the corresponding value of the output.

Condition when an inductor or transformer core has reached maximum magnetic strength.

Equipment that requires an uninterrupted power input to prevent damage or injury to personnel, facilities, or itself.

A current transformer (CT) which clamps around a current-carrying conductor so the conductor does not have to be opened for insertion of the transformer primary. Particularly suited for monitoring where current must be sensed at many points for relatively short periods.

The movement of electrons through a conductor. Measured in amperes and its symbol is “I”


Direct current (DC) is one-directional flow of electric charge.

DC offsets are instances where direct current (DC) overlaps an alternating current (AC) distribution system. This overlapping of two different types of current can cause overheating in the equipment.

Calculations that reduce standard tabulated ratings based, generally based on ambient temperature or proximity to a heat source.

The actual, expected load or loads that a device or structure will support in service.

1) Any electrical insulating medium between two conductors. 2) The medium used to provide electrical isolation or separation.

A test that is used to verify an insulation system. A voltage is applied of a specific magnitude for a specific period of time.

Electric current flowing in only one direction

A protection relay in which the tripping decision is dependent in part upon the direction in which the measured quantity is flowing.

The surge current that is dissipated through a surge arrester.

The rate of current flow from a cell or battery.

Overhead or underground power lines that carry electricity through cities and neighborhoods to your home or business.

A transformer that reduces voltage from the supply lines to a lower voltage needed for direct connection to operate consumer devices.

A nominal operating voltage of up to 38kV.

A proprietary communication protocol used on secondary networks between HMI, substation computers or bay computers and protective devices.

The process by which the electrodes are formed and assembled in a charged state. The cell or battery is activated when electrolyte is added.

Transformers that use only dry-type materials for insulation. These have no oils or cooling fluids and rely on the circulation of air about the coils to provide necessary cooling. Such units are usually limited in size to a few hundred kVA.

A transformer that has switched windings allowing its use on two different primary voltages.


A protection system which is designed to excite during faults to earth.

A three-phase transformer intended essentially to provide a neutral point to a power system for the purpose of grounding.

The current that is generated in a transformer core due to the induced voltage in each lamination. It is proportional to the square of the lamination thickness and to the square of the frequency.

The apparent opposition to current within a battery that manifests itself as a drop in battery voltage proportional to discharge current. Its value is dependent on battery design, state-of-charge, temperature and age.

Intentionally connected conductors or electric equipment to earth, where the connection and conductors are of sufficiently low impedance to allow the conducting of an intended current.

The flow of electrons.

In a lead-acid battery, the electrolyte is sulfuric acid diluted with water. It is a conductor and also a supplier of hydrogen and sulphate ions for the reaction.

A negatively charged particle that orbits the nucleus of an atom. The flow of electrons produces electricity.

The voltage of a battery at the termination of a discharge but before the discharge is stopped.

The Cell or Battery voltage at which point the rated discharge capacity has been delivered at a specific Rate-of-Discharge. It is also used to specify the cell or battery voltage below which the connected equipment will not operate.

The ability to do work. Energy = Power x Time

A system designed to ensure safety, security, and reliability to an electrical network.

The magnetizing current of a device such as a transformer. Also known a field current.

An electrical system or cable designed to operate at 345kv (nominal) or higher.


The ability, in amps, of a switching device to “close” into a fault of specific magnitude, without excessive arcing.

The current that flows as a result of a short-circuit condition.

The magnetizing current of a device such as a transformer. Also known as exciting current.

The ability of insulation or jacketing material to resist the support and conveyance of fire.

An unintended electrical discharge to ground or another phase. Flashovers can occur between two conductors, across insulators to ground or equipment bushings to ground.

In ac systems, the rate at which the current changes direction, expressed in hertz (cycles per second); A measure of the number of complete cycles of a wave-form per unit of time.

A device installed in the conductive path with a predetermined melting point coordinated to load current. Fuses are used to protect equipment from over current conditions and damage.

The amount of time required to extinguish the arc and clear the circuit.

The time needed for a fuse element to melt, thereby initiating operation of the fuse. Also known as Melt Time.


A machine which converts mechanical energy into electrical energy.

A power system’s layout of its substations and power lines.

1. An electrical term meaning to connect to the earth. 2. A conducting connection, whether intentional or accidental by which an electric circuit, or equipment, is connected to the earth or some conducting body that serves in place of the earth.

An undesired current path between ground and an electrical potential.


A sinusoidal component of the voltage that is a multiple of the fundamental wave frequency.

The presence of harmonics that change an AC waveform from sinusoidal to complex. They can cause unacceptable disturbance to electronic equipment.

Harmonics occur when some loads affect the main waveform of voltage. In this situation, the new loads prevent the waveform from reaching its highest and lowest levels. Harmonics can cause circuit breakers to trip and transformers to overheat.

1) A unit of frequency equal to one cycle per second. 2) In alternating current, the changing of the negative and positive poles.

An electrical system or cable designed to operate between 46kv and 230kv.

An electrical system or cable designed to operate between 46kv and 230kv.

High Voltage

High Voltage Alternating Current

High Voltage Direct Current

Electricity generated by flowing water making a turbine spin.

Hertz (Frequency)


1) The total opposing force to the flow of current in an ac circuit. 2) The combination of resistance and reactance affecting the flow of an alternating current generally expressed in ohms.

Current in a conductor resulting from a nearby electromagnetic field.

A voltage produced in a circuit from a nearby electric field.

1) The property of a circuit in which a change in current induces an electro motive force. 2) Magnetic component of impedance.

The initial surge of current experienced before the load resistance of impedance increases to its normal operating value.

A transformer that is only designed to reduce current or voltage from a primary value to a lower value secondary that can be applied to a meter or instrument, at a proportional safer level.

1) A non-conductive material used on a conductor to separate conducting materials in a circuit. 2) The non-conductive material used in the manufacture of insulated cables.

Any material that will not allow electricity to easily flow through.

Interharmonics is a condition where a signal affects the main voltage waveform. It can cause display monitors to flicker and equipment to overheat. Interharmonics can also cause communication issues.

Surge arresters with a high energy handling capability. These are generally voltage classed at 3-120kV.


A unit for measuring electrical energy. (demand)

One kilowatt of electrical energy produced or used in one hour. (energy)

KVAR is the measure of additional reactive current flow which occurs when the voltage and current flow are not perfectly in phase.


A flash of light caused by an atmospheric electrical discharge between two clouds, or between a cloud and the earth.

A device used to protect an electrical component from over-voltage.

An electrical device or devices that use(s) electric power.

Refers to a group of rubber insulating products used to electrically connect apparatus with which load can be separated manually. Loadbreak products are manufactured by T&B Elastimold.


An identified force that exists around a magnet or electrical field.

The protection system which is normally expected to operate in response to a fault in the protected zone.

An electrical system or cable designed to operate between 1kv and 38kv.

One million watts.

A testing device that applies a DC voltage and measures the resistance (in millions of ohms) offered by conductor’s or equipment insulation.

An expression used by some manufactures to describe a category of medium voltage switchgear equipment where the circuit breakers are all enclosed in grounded, sheet-steel enclosures.

An expression used by some manufacturers to describe a category of low voltage, 600 volt class switchgear equipment, where the circuit breakers are all enclosed in grounded, sheet-steel enclosures.

An instrument that records the amount of something passing through it, such as electricity.

A substation located at the electrical interface of two sections of electrified railway. It contains provision for the coupling of the sections electrically in the event of loss of supply to one section.

A transformer that often is mounted on a leak proof base and can be installed and operated in a semi-trailer, box truck or sea freight container.

The rating of a device to withstand momentary, very high current, without incurring damage.

A device that converts electrical energy into mechanical energy.

Metal Oxide Varistor


In multiphase circuits, the conductor used to carry unbalanced current. In single-phase systems, the conductor used for a return current path.

A device that connects the neutral point of a three phase system to ground. Neutral Grounding Resistors are used to limit ground fault current on Neutral Grounded (WYE) systems.

A basic particle in an atom’s nucleus that has a neutral electrical charge.

Noise is any unnecessary current or voltage affecting the waveform of the main power supply. This waveform distortion can cause data issues and equipment to malfunction.

A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class.

Notching is an intermittent disturbance that can affect voltage. It normally happens when light dimmers or arc welders are being used. It results in data loss and issues with the transmission of data.

Negative Phase Sequence

Energy produced by splitting atoms in a nuclear reactor.


In multiphase circuits, the conductor used to carry unbalanced current. In single-phase systems, the conductor used for a return current path.

A tap changer that is not designed for operation while the transformer is supplying load.

The unit of measurement of the electrical resistance of a material.

A tap changer that can be operated while the transformer is supplying load.

The current used by a lamp and ballast combination during normal operation.


In the case of DC circuits, a way of joining two or more electrical devices or wires by connecting positive leads and negative leads together.

The amplitude of the ac wave form from its positive peak to its negative peak.

The angular displacement between a current and voltage waveform, measured in degrees or radians.

A round or toroidal core transformer mounted on bushings of power transformers, bulk oil circuit breaker, and other dead tank circuit breakers.

A transformer used to lower the voltage at a set ratio so that the voltage can be measured by instruments and meters at a safe representative level.

The ratio of energy consumed (watts) versus the product of input voltage (volts) times input current (amps). In other words, power factor is the percentage of energy used compared to the energy flowing through the wires.

A large transformer, generally larger than 1,000 kVA in capacity.

Energy used to do work measured in watts.

A relay designed to initiate disconnection of a part of an electrical installation or to a warning signal, in the case of a fault or other abnormal condition in the installation.


The output at standard calibration.

Residual Current Device. A protection device which is actuated by the residual current.

A component of apparent power (volt-amps) which does not produce any real power (watts). It is measured in VARs volt-amps reactive.

The average value of the instantaneous product of volts and amps over a fixed period of time in an AC circuit.

A transformer used to vary the voltage, or phase angle, of an output circuit. It controls the output within specified limits and compensates for fluctuations of load and input voltage.

The algebraic sum, in a multi-phase system, of all the line currents.

The algebraic sum, in a multi-phase system, of all the line-to-earth voltages.

The opposition to current flow, expressed in ohms.

An output current which reverses polarity in response to a change of sign or direction of the measurand.

The magnitude of AC fluctuation in a DC signal, after filtering. Ripple is usually expressed as a percentage of rated output.


Supervisory Control And Data Acquisition.  A Computer system used to remotely monitor and control substation equipment.

For DC circuits, a way of joining batteries, electrical devices and wires in such a way that positive leads are connected to negative leads. This is generally done to increase voltage.

The conductors (electrical cable with multiple wires) that connect and carry the electrical current the service conductors (drop or lateral) above ground to the service equipment of the building. It can also be used as a panel feeder and in branch circuits.

A dangerous electrical condition associated with the possible release of energy caused by contact or approach to energized parts.

1. A load that occurs when at ungrounded conductor comes into contact with another conductor or grounded object. 2. An abnormal connection of relatively low impedance, whether made intentionally or by accident, between two points of different potential.

This implies a power supply or a load that uses only two wires for power.

Energy produced by the sun’s light or heat.

A short duration of increased voltage lasting only one-half of a cycle.

A split phase electric distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial (up to about 100 kVA) applications.

Current required by the ballast during initial arc tube ignition. Current changes as lamp reaches normal operating light level.

A unit relay which responds to abnormal conditions and initiates the operation of other elements of the protection system.

An electrical charge built up due to friction between two dissimilar materials.

An electrical facility where transformers lower high transmission voltages to be distributed to customers.

A high voltage system that takes power from the highest voltage transmission system, reduces it to a lower voltage for more convenient transmission to nearby load centers, delivering power to distribution substations or the largest industrial plants.

A short duration of increased voltage.

An electrical component used for connecting, breaking, or changing the connections in an electrical circuit.

A high voltage spike that occurs when current flowing in a highly inductive circuit, or a long transmission line, is suddenly interrupted.


A mechanism usually fitted to the primary winding of a transformer, to alter the turns ratio of the transformer by small discrete amounts over a defined range.

The greatest longitudinal force that a substance can bear without tearing apart or rupturing; also called ultimate tensile strength.

Multiple phase power supply or load that uses at least three wires where a different voltage phase from a common generator is carried between each pair of wires.

A dangerous electrical condition associated with the possible release of energy caused by contact or approach to energized parts.

A relay having an intentional delaying device.

An electro-magnetic device used to change the voltage in an alternating current electrical circuit.

This is the material that is used to provide electrical insulation between transformer windings at different voltage levels and also between the energized parts and the metal tank of the transformer.

When used in reference to Instrument Transformers, this is simply the ratio of transformation of one or more transformers used in the circuit. If both Cts and VTs are included, the transformer ratio is the product of the CT and the VT.

Mechanisms that use multiple voltage taps on a transformer-like device to adjust voltage on a power line.

1) The effective value of an AC signal. For an amp signal, true RMS is a precise method of stating the amp value regardless of waveform distortion. 2) An AC measurement which is equal in power transfer capability to a corresponding DC current.

1) The effective value of an AC voltage value regardless of the waveform distortion. 2) An AC measurement which is equal power transfer capability to a corresponding DC voltage.

A large fan(s) that is coupled to a generator field. This turbine (fan) is put into motion by the force of water, steam, or hot exhaust gases that rotate the turbine.


Transmission systems in the ac voltage exceeds 800,000 volts.

Refers to an unequal loading of the phases in a three-phase system.

A single relay that can be used alone or in combinations with others.

A protection system that is designed to operate only for abnormal conditions within a clearly defined zone of the power system.

An uninterruptible power supply (UPS) is a device that allows an electrical equipment to keep running for at least a short time when the primary power source is lost. UPS devices also provide protection from power surges.


Volts Alternating Current

Circuit breakers, normally applied at medium voltages, that use vacuum interrupters to extinguish the electrical arc and shut-off flowing current.

In the core-type transformer, the core-and-coil assembly is independent of the tank, so that the assembly is allowed to completely dry.

The unit of measurement of force used to produce an electric current.

The general strength of electrical insulation on a device, determining the maximum continuous voltage that can be applied between the conducting parts and ground potential, without damaging the insulation.

The loss of voltage in a circuit when current flows.

The normal voltage to be applied to an electrical device to provide for proper operation.

The maintenance of a voltage level between two established set points, compensating for transformer and/or line voltage deviation, caused by load current. The voltage change is affected by the magnitude and the power factor of the load current.

Voltage Sags are momentary (typically a few milliseconds to a few seconds duration) under-voltage conditions and can be caused by a large load starting up (such as a air conditioning compressor or large motor load).

The difference between maximum and minimum voltages.

Voltage Swells are momentary (typically a few milliseconds to a few seconds duration) over-voltage conditions which can be caused by such things as a sudden decrease in electrical load or a short circuit occurring on electrical conductors.

The ratio of primary volts divided by secondary volts

Transformer used to accurately scale ac voltages up or down, or to provide isolation.

A transient (sometimes called impulse) is an extremely fast disturbance (millionths of a second to a few milliseconds) evidenced by a sharp change in voltage. Transients can occur on your electric, phone, or even cable TV lines.

A field or factory test in which a conductor or electrical equipment is subjected to a higher than normal AC or DC voltage to test its insulation system.


A unit for measuring electric power.

Waveform distortions are common power problems that cause equipment to malfunction and sources of power to overload. It is an unexpected change in the waveforms of current and voltage as they pass through a device. There are five main types of waveform distortions: DC offset, harmonics, interharmonics, notching and noise.

A machine that uses energy from the wind and transfers the motion to an electric generator.

A three phase, four-wire electrical configuration where each of the individual phases is connected to a common point, the “center” of the Y. This common point normally is connected to an electrical ground.


The point at which a sinusoidal voltage or current waveform crosses the zero reference axis.