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Glossary of Electrical Terms 

This glossary is intended to cover the meaning of common terms used in the Critical Power Industry.

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Alternating Current (AC) — An electric current that reverses its direction many times a second at regular intervals.

Ammeter — An instrument for measuring the flow of electrical current in amperes. Ammeters are always connected in series with the circuit to be tested.


Ampacity — The maximum amount of electric current a conductor or device can carry before sustaining immediate or progressive deterioration.


Ampere-Hour (Ah) — A unit of measure for battery capacity. It is obtained by multiplying the current (in amperes) by the time (in hours) during which current flows. For example, a battery that provides 5 amperes for 20 hours is said to deliver 100 ampere-hours.


Ampere (A) — A unit of measure for the intensity of an electric current flowing in a circuit. One ampere is equal to a current flow of one coulomb per second.


Apparent Power — Measured in volt-ampers (VA). Apparent power is the product of the RMS voltage and the RMS current.


Armature — The movable part of a generator or motor. It is made up of conductors which rotate through a magnetic field to provide voltage or force by electromagnetic induction. The pivoted points in generator regulators are also called armatures.



Breaker — Short for Circuit Breaker. Is a device for interrupting an electric circuit to prevent excessive current, such as that caused by a short circuit, from damaging the apparatus in the circuit or from causing a fire.

Buss — Separate, metallic strips that extend through the center of the service panel that the breakers snap onto.



Capacitance — The ability of a body to store an electrical charge. Measured in farads is the ratio of the electric charge of the object (Q, measured in coulombs) to the voltage across the object (V, measured in volts).

Capacitor — A device used to store an electric charge, consisting of one or more pairs of conductors separated by an insulator. Commonly used for filtering out voltage spikes.

Circuit — A closed path in which electrons from a voltage or current source flow. Circuits can be in series, parallel, or in any combination of the two.

Circuit Breaker — An automatic device for stopping the flow of current in an electric circuit. To restore service, the circuit breaker must be reset (closed) after correcting the cause of the overload or failure. Circuit breakers are used in conjunction with protective relays to protect circuits from faults.

Conductor — Any material where electric current can flow freely. Conductive materials, such as metals, have relatively low resistance. Copper and aluminum wire are the most common conductors.

Corona — A corona discharge is an electrical discharge brought on by the ionization of a fluid such as air surrounding a conductor that is electrically charged. Spontaneous corona discharges occur naturally in high-voltage systems unless care is taken to limit the electric field strength.

Current (I) — The flow of an electric charge through a conductor. An electric current can be compared to the flow of water in a pipe. Measured in amperes.

Cycle — The change in an alternating electrical sine wave from zero to a positive peak to zero to a negative peak and back to zero. See Frequency.



Demand — The average value of power or related quantity over a specified period of time.

Dielectric constant — A quantity measuring the ability of a substance to store electrical energy in an electric field.


Dielectric strength — The maximum electric field that a pure material can withstand under ideal conditions without breaking down (i.e., without experiencing failure of its insulating properties).


Diode — A semiconductor device with two terminals, typically allowing the flow of current in one direction only. Diodes allow current to flow when the anode is positive in relation to the cathode.



Electrolyte — Any substance which, in solution, is dissociated into ions and is thus made capable of conducting an electrical current. The sulfuric acid-water solution in a storage battery is an electrolyte.


Electromotive Force (EMF) —  A difference in potential that tends to give rise to an electric current. Measured in volts.


Electron — A tiny particle that rotates around the nucleus of an atom. It has a negative charge of electricity.


Electron theory — The theory which explains the nature of electricity and the exchange of "free" electrons between atoms of a conductor. It is also used as one theory to explain the direction of current flow in a circuit.



Farad — A unit of measure for capacitance. One farad is equal to one coulomb per volt.

Ferroresonance — (nonlinear resonance) is a type of resonance in electric circuits which occurs when a circuit containing a nonlinear inductance is fed from a source that has series capacitance, and the circuit is subjected to a disturbance such as the opening of a switch. It can cause overvoltages and overcurrents in an electrical power system and can pose a risk to transmission and distribution equipment and to operational personnel.

Frequency — The number of cycles per second. Measured in Hertz. If a current completes one cycle per second, then the frequency is 1 Hz; 60 cycles per second equals 60 Hz.

Fuse — A circuit interrupting device consisting of a strip of wire that melts and breaks an electric circuit if the current exceeds a safe level. To restore service, the fuse must be replaced using a similar fuse with the same size and rating after correcting the cause of failure.



Generator — A device that converts mechanical energy into electrical energy.


Ground — The reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth.


Ground Fault Circuit Interrupters (GFCI) — A device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds some predetermined value that is less than that required to operate the overcurrent protective device of the supply circuit.



Henry — A unit of measure for inductance. If the rate of change of current in a circuit is one ampere per second and the resulting electromotive force is one volt, then the inductance of the circuit is one henry.

Hertz — A unit of measure for frequency. Replacing the earlier term of cycle per second (cps).



Impedance — The measure of the opposition that a circuit presents to a current when a voltage is applied. Impedance extends the concept of resistance to AC circuits and possesses both magnitude and phase, unlike resistance, which has only magnitude.


Inductance — The property of a conductor by which a change in current flowing through it induces (creates) a voltage (electromotive force) in both the conductor itself (self-inductance) and in any nearby conductors (mutual inductance). Measured in henry (H).


Inductor — A coil of wire wrapped around an iron core. The inductance is directly proportional to the number of turns in the coil.


Insulator — Any material where the electric current does not flow freely. Insulative materials, such as glass, rubber, air, and many plastics have relatively high resistance. Insulators protect equipment and life from electric shock.


Inverter — An apparatus that converts direct current into alternating current.



Kilowatt-hour (kWh) — The product of power in kW and time in hours. Equal to 1000 Watt-hours. For example, if a 100W light bulb is used for 4 hours, 0.4kWhs of energy will be used (100W x 1kW / 1000 Watts x 4 hours). Electrical energy is sold in units of kWh.

Kilowatt-hour Meter — A device used to measure electrical energy use.


Kilowatt (kW) — Equal to 1000 watts.



Load — Anything which consumes electrical energy, such as lights, transformers, heaters, and electric motors.


Load Rejection — The condition in which there is a sudden load loss in the system which causes the generating equipment to be over-frequency. A load rejection test confirms that the system can withstand a sudden loss of load and return to normal operating conditions using its governor. Load banks are normally used for these tests as part of the commissioning process for electrical power systems.



Mutual Induction — Occurs when changing current in one coil induces voltage in a second coil.



Ohm — (Ω) A unit of measure of resistance. One ohm is equivalent to the resistance in a circuit transmitting a current of one ampere when subjected to a potential difference of one volt.


Ohm's Law — The mathematical equation that explains the relationship between current, voltage, and resistance (V=IR).


Ohmmeter — An instrument for measuring the resistance in ohms of an electrical circuit.


Open Circuit — An open or open circuit occurs when a circuit is broken, such as by a broken wire or open switch, interrupting the flow of current through the circuit. It is analogous to a closed valve in a water system.



Parallel Circuit — A circuit in which there are multiple paths for electricity to flow. Each load connected in a separate path receives the full circuit voltage, and the total circuit current is equal to the sum of the individual branch currents.

Piezoelectricity — Electric polarization in a substance (especially certain crystals) resulting from the application of mechanical stress (pressure).


Polarity — A collective term applied to the positive (+) and negative ( - ) ends of a magnet or electrical mechanisms such as a coil or battery.


Power — The rate at which electrical energy is transferred by an electric circuit. Measured in Watts.


Power Factor — The ratio of the actual electrical power dissipated by an AC circuit to the product of the r.m.s. values of current and voltage. The difference between the two is caused by reactance in the circuit and represents a power that does no useful work.


Protective Relay — A relay device designed to trip a circuit breaker when a fault is detected.



Reactive Power — The portion of electricity that establishes and sustains the electric and magnetic fields of AC equipment. Exists in an AC circuit when the current and voltage are not in phase. Measured in VARS.


Rectifier — An electrical device that converts an alternating current into a direct one by allowing a current to flow through it in one direction only.


Relay — An electrical coil switch that uses a small current to control a much larger current.


Reluctance — The resistance that a magnetic circuit offers to lines of force in a magnetic field.


Resistance — The opposition to the passage of an electric current. Electrical resistance can be compared to the friction experienced by water when flowing through a pipe. Measured in ohms.


Resistor — A device usually made of wire or carbon which presents a resistance to current flow.


Rotor — The rotating part of an electrical machine such as a generator, motor, or alternator.



Self Induction — Voltage occurs in a coil when there is a change of current.


Semiconductor — A solid substance that has a conductivity between that of an insulator and that of most metals, either due to the addition of an impurity or because of temperature effects. Devices made of semiconductors, notably silicon, are essential components of most electronic circuits.


Series-Parallel Circuit — A circuit in which some of the circuit components are connected in series and others are connected in parallel.


Series Circuit — A circuit in which there is only one path for electricity to flow. All of the current in the circuit must flow through all of the loads.


Service — The conductors and equipment used to deliver energy from the electrical supply system to the system being served.


Short Circuit — When one part of an electric circuit comes in contact with another part of the same circuit, diverting the flow of current from its desired path.


Solid-State Circuit — Electronic (integrated) circuits which utilize semiconductor devices such as transistors, diodes, and silicon-controlled rectifiers.



Transistor — A semiconductor device with three connections, capable of amplification in addition to rectification.


True Power — Measured in Watts. The power manifested in tangible forms such as electromagnetic radiation, acoustic waves, or mechanical phenomena. In a direct current (DC) circuit, or in an alternating current (AC) circuit whose impedance is a pure resistance, the voltage and current are in phase.



VARS — A unit of measure of reactive power. Vars may be considered as either the imaginary part of apparent power or the power flowing into a reactive load, where voltage and current are specified in volts and amperes.


Variable Resistor — A resistor that can be adjusted to different ranges of value.


Volt-Ampere (VA) — A unit of measure of apparent power. It is the product of the RMS voltage and the RMS current.


Volt (V) — A unit measure of voltage. One volt is equal to the difference of potential that would drive one ampere of current against one-ohm resistance.


Voltage — An electromotive force or "pressure" that causes electrons to flow and can be compared to water pressure which causes water to flow in a pipe. Measured in volts.


Voltmeter — An instrument for measuring the force in volts of an electrical current. This is the difference of potential (voltage) between different points in an electrical circuit. Voltmeters that have a high internal resistance are connected across (parallel to) the points where voltage is to be measured.



Watt-hour (Wh) — A unit of electrical energy equivalent to the power consumption of one watt for one hour.


Watt (W) — A unit of electrical power. One watt is equivalent to one joule per second, corresponding to the power in an electric circuit in which the potential difference is one volt and the current one ampere.


Wattmeter — The wattmeter is an instrument for measuring the electric power (or the supply rate of electrical energy) in watts of any given circuit.


Waveform — A graphical representation of electrical cycles which shows the amount of variation in amplitude over some period of time.

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