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Glossary of Terms

Below is a comprehensive glossary of terms that relate to power generation.


  • A
  • AC: Alternating Current (AC) is an electric current that alternates its direction at regular intervals.
  • ANSI: American National Standards Institute. An industry group that establishes and publishes standards.

    Active Power: Active power is the real power (kW) supplied by the generator set to the electrical load. Active power creates a load on the generator set’s engine and is limited by the horsepower of the engine. Active power does the work of heating, turning motor shafts, etc.

    Alternator: An electric generator for producing alternating current.

    Ambient Temperature: The temperature of the still air surrounding a power supply, measured a minimum of 4 inches from the supply.

    Ampacity: Ampacity is the safe current-carrying capacity of an electrical conductor in amperes as defined by code.

    Ampere: A unit of electric current flow.

    Amp Hours: Number of amps used/produced in a given hour.

    Apparent Power: Apparent power is the product of current and voltage, expressed as kVA. It is real power (kW) divided by the power factor (PF).

    Armature: The armature of an AC generator is the assembly of windings and metal core laminations in which the output voltage is induced. It is the stationary part (stator) in a revolving-field generator.



    Battery Bank: A group of batteries wired in a fashion that allows more power storage.

    Blackout: A total loss of the AC utility (commercial power). It may be caused by the tripping of a circuit breaker, power distribution failure or utility power failure.

    Brownout: The condition created during peak usage periods when electric utility companies intentionally reduce their line voltage by approximately 10 – 15% to counter excessive demand.

    BTU: British Thermal Unit. The quantity of heat required to raise the temperature of 1 pound of water by 1 °F starting from 60 °F. 3.7 BTUs per hour is equivalent to 1 Watt.

    Buss: Buss can refer to the current-carrying copper bars that connect the AC generators and loads in a paralleling system, to the paralleled output of the AC generators in a system or to a feeder in an electrical distribution system.

    Buss Capacity: Buss capacity is the maximum load that can be carried on a system without causing degradation of the generator frequency to less than a prescribed level (usually 59 Hz in a 60 Hz system).

    Bypass: A circuit used to change the path of the electrical power so that it goes around (or bypasses) its normal path.



    Capacitor: A device that stores electrostatic energy in a manner similar to the way an inductor stores electromagnetic energy. Often used for filtering or DC blocking.

    CFM: Cubic feet per minute, which is a measurement of the volume of air flowing in a system.

    Circuit: A circuit is a path for an electric current across a potential (voltage).

    Circuit Breaker: A circuit breaker is a protective device that automatically interrupts the current flowing through it when that current exceeds a certain value for a specified period of time.

    Clean Power: Electrical power which has been conditioned and/or regulated to remove electrical noise from the output power.

    Conduit:  A flexible or rigid tubular raceway for data or power cables.

    Contactor: A contactor is a device for opening and closing an electric power circuit.

    Continuous Load: A continuous load is a load where the maximum current is expected to continue for three hours or more.

    Cross Current Transformer (CCT): Cross Current Transformers are used to step down the higher line current to a lower current that the control system was designed for.

    CT (Current Transformer): Current transformers are instrument transformers used in conjunction with ammeters, control circuits and protective relaying.

    Current: Current is the flow of electric charge. Its unit of measure is the ampere.

    Current Limiting: A feature that protects the electrical equipment from damage under overload conditions such as a short circuit. The maximum output current is automatically limited to a predetermined safe value. If the equipment is specified for auto restart, normal operation is automatically restored when overload condition is removed.

    Current Rating: The maximum current that a piece of electrical equipment is designed to carry or produce.



    Digital Master Control (DMC): This device is designed to control the power systems in a facility.

    Direct Current (DC): Electric energy of constant value and flowing in one direction.

    Distribution Circuit Breaker: A device used for overload and short current protection of loads connected to a main distribution device.

    Dropout Voltage: The voltage at which a device fails to operate properly or safely



    Earth Fault Protection: A copper bar that electrically joins all the metal sections of the switchgear. This bar is connected to the earth or ground connection when the system is installed. The grounding or earthing protects personnel from stray currents that could leak to the metallic enclosures.

    Efficiency (EFF): Efficiency is the ratio of total output power to input power expressed as a percentage.

    EIA: Electronic Industries Alliance. An organization that helps set standards in the electronics industry.

    Electrostatic: A potential difference (electric charge) measurable between two points which is caused by the distribution if dissimilar static charge along the points.

    Emergency System: An emergency system is independent power generation equipment that is legally required to feed equipment or systems whose failure may present a life safety hazard to persons or property.

    EPO: Emergency Power Off.

    Exciter: An exciter is a device that supplies direct current (DC) to the field coils of a synchronous generator, producing the magnetic flux required for inducing output voltage in the armature coils (stator).

    ETL: Electrical Testing Laboratories. A US independent safety agency that sets standards for product safety.



    Fault: A fault is any unintended flow of current outside its intended circuit path in an electrical system.

    Fault Current: The level of current that can flow if a short circuit is applied to a voltage source.

    First Start Sensor: A first start sensor is an electronic device within some paralleling equipment that senses generator set and bus voltage and frequency, and determines whether or not a generator set is the first unit ready to close to the bus following a call to start under “black start” conditions.

    Forward Converter: A power supply switching circuit that transfers energy to the transformer secondary when the switching transistor is on. Forward converter circuits store minimal energy in the transformer.

    Frequency: Frequency is the number of complete cycles per unit of time of any periodically varying quantity, such as alternating voltage or current. It is usually expressed as (Hz) Hertz or CPS (cycles per second).

    Frequency Adjust Potentiometer: A frequency adjust potentiometer is used to manually bring the frequency (speed) of the incoming set to that of the bus for synchronizing purposes.

    Frequency Regulation: Frequency regulation is a measure that states the difference between no-load and full-load frequency as a percentage of full-load frequency.

    Full Load: The greatest load that a circuit is designed to carry under specific conditions; any additional load is considered an overload.

    Fuse: A protective device employing a fusible link that melts (blows) after a certain current level is exceeded for a certain length of time.

    Fusible Switch: A fusible switch is an isolating switch and overcurrent protective device used for feeder or transfer switch isolation and protection. It is typically a manually operated, stored energy opening and closing, bolted compression blade switch, with provisions for installing current limited fuses.



    Generator: A generator is a machine which converts rotating mechanical energy into electrical energy.

    Grid: The utility-owned power distribution system.

    Ground: A ground is a connection, either intentional or accidental, between an electrical circuit and the earth or some conducting body serving in place of the earth.

    Ground Fault Protection: This function trips (opens) a circuit breaker or sounds an alarm in the event that there is an electrical fault between one or more of the phase conductors and ground (earth). This ground fault protection function may be incorporated into a circuit breaker.

    Ground Loop: A condition that causes unwanted feedback when two or more circuits share a common electrical return or ground lines.

    Ground Return: Ground return is a method of ground fault detection that employs a single sensor (CT) encircling the main bonding jumper between the power system neutral and ground. This device in itself is not capable of locating the faulted circuit but when used in conjunction with ground fault sensors on all feeders and source connections, can provide bus fault protection when properly coordinated (delayed).



    Hertz (Hz): The term Hertz is the preferred designation for cycles per second (CPS) and is used to describe frequency. Appliances in the US use 60 Hz, elsewhere it is 50 Hz.

    Hydroelectric Power: The use of flowing water to produce electrical energy.



    Inrush Current: The peak instantaneous input current drawn by a power supply when it is initially turned on.

    Insulated Case Circuit Breaker: An insulated case circuit breaker is a power circuit breaker that is provided in a preformed case, similar to a molded case breaker.

    Insulation: Insulation is non-conductive material used to prevent leakage of electric current from a conductor. There are several classes of insulation in use for generator construction, each recognized for a maximum continuous-duty temperature.

    Internal Voltage: The voltage a generator would develop at no load if it were not connected in a parallel operation.

    Interrupting Capacity: Interrupting capacity is the magnitude of an electrical current that a device can safely interrupt without failure of the component.

    Inverter: A device that changes DC power at its input into AC power at its output. Also called a power converter.


    kVA (kilo-Volt-Amperes):
    kVA is a term for rating electrical devices. A device’s kVA rating is equal to its rated output in amperes multiplied by its rated operating voltage.

    kVAR: (kilo-Volt-Amperes Reactive) is the product of the voltage and the amperage required to excite inductive circuits. It is associated with the reactive power which flows between paralleled generator windings and between generators and load windings that supply the magnetizing currents necessary in the operation of transformers, motors and other electromagnetic loads.

    kW: This is an abbreviation for kilowatt, an alternate term for rating electrical devices. One thousand watts.

    kW Load Sensor: The kW load sensor is an electronic device provided to sense kW level at various points in a system, for use in control functions within the system, such as kW load alarms, or load demand.



    Lagging Power Factor: Lagging power factor in AC circuits (a power factor of less than 1.0) is caused by inductive loads, such as motors and transformers, which cause the current to lag behind the voltage.

    Leg: A leg is a phase winding of a generator, or a phase conductor of a distribution system.

    Line Loss: A voltage drop over the length of electric line wire.

    Line-To-Line Voltage: Line-to-line voltage is the voltage between any two phases of an AC generator.

    Line-To-Neutral Voltage: In a 3-phase, 4-wire, Y-connected generator, line-to-neutral voltage is the voltage between a phase and the common neutral where the three phases are tied together.

    Load: The amount of lighting and number of electric appliances needed to be supported by your local power supply system.

    Load Demand: Load Demand is a paralleling system operating mode in which the system monitors the total kW output of the generator sets, and controls the number of operating sets as a function of the total load on the system.

    Load Factor: The load factor is the ratio of the average load to the generator set power rating.

    Load Management: Load management is the overall control of load connected to match available generator capacity. Priority control and load shedding are the two features required for load management.

    Load Shedding: Load shedding is the process by which the total load on a paralleling system is reduced, on overload of the system bus, so that the most critical loads continue to be provided with reliable electrical service.

    Low Voltage: AC system operating voltages from 120 to 600 VAC.



    Main Breaker: A main breaker is a circuit breaker at the input or output of the bus, through which all of the bus power must flow. Main breakers provide overcurrent protection and a single disconnect point for all power in a switchboard or device.

    Mains: A term used extensively outside of the United States to describe the normal power service (utility).

    Manual Bypass Switch (MBS): A manually operated transfer switch used to bypass the major electronics in the UPS, so the UPS can be serviced without power interruption.

    Medium Voltage: AC system operating voltages from 601 to 15000 VAC.

    Megawatt (MW): One million watts of electricity.



    NEC (National Electrical Code): This document is the most commonly referenced general electrical standard in the United States.

    NEMA: National Electrical Manufacturers Association

    NFPA: National Fire Protection Association

    NFPA 110: National Fire Protection Agency Section 110 (NFPA 110) deals with the regulations concerning Emergency Power Systems (EPS). This section deals with regulations on installation, operation, and monitoring of EPS.

    Neutral: Neutral refers to the common point of a Y-connected AC generator, a conductor connected to that point or to the mid-winding point of a single-phase AC generator.

    Neutral Current: Neutral current is the current that flows in the neutral leg of a paralleling system. Often, this term is used in reference to circulating currents or cross currents.

    Normal Standby Mode: In the normal standby mode, power to the load is supplied by the utility. The paralleling system is ready to provide power to the load in the event of utility failure.


    Ohm: The ohm is a unit of electrical resistance.

    On-Set Paralleling: On-set paralleling is a manual paralleling system that is built onto the generator set, no additional switchboards are required.

    Out-Of-Phase: Out-Of-Phase refers to alternating currents or voltages of the same frequency which are not passing through their zero points at the same time.

    Overcrank: An alarm function provided with most generator sets that indicate that the generator set has failed to start.

    Overload: A condition in which the load wants more from the power source than the power source has been designed to supply.

    Overload Protection: A protective feature that limits the output current of a power supply under overload conditions so that it will not be damaged.

    Overload Rating: The overload rating of a device is the load in excess of the nominal rating the device can carry for a specified length of time without being damaged.

    Overshoot: Overshoot refers to the amount by which voltage or frequency exceeds the nominal value as the voltage regulator or governor responds to changes in load.

    Over-Temperature Protection: Design feature that protects the silicon die from exceeding its designed operating temperature range. The device will thermally cycle until the abnormal condition is corrected.

    Over-Voltage: A power surge that happens when high-current items are switched off. The suddenly surplus voltage is dissipated across the rest of the circuit and can sometimes far exceed the peak current of your electronic equipment.



    Pass Thru: Refers to a junction box connection where the network bus comes to a connector and then continues straight on through. In most Pass Thru connections, very little input and output is done.

    Peak Load: Peak load is the highest point in the kilowatt demand curve of a facility. This is used as the basis for the utility company’s demand charge.

    Phase: Phase refers to the windings of an AC generator. In a three-phase generator there are three windings, typically designated as A-B-C, R-S-T or U-V-W. A single-phase generator has only one winding.

    Phase Angle: Phase angle refers to the relation between two sine waves which do not pass through zero at the same time. Considering one full cycle to be 360 degrees, the phase angle expresses how far apart the two waves are in relation to each other in degrees.

    Phase Rotation: Phase rotation describes the order (A-B-C, R-S-T, or U-V-W) of the phase voltages at the output terminals of a three-phase generator. The generator phase rotation must match the facility phase rotation.

    Pitch: Pitch is a mechanical design characteristic of a generator that indicates the ratio of the number of winding slots per generator pole to the number of slots enclosed by each coil.

    Power: Power refers to the rate of performing work or of expending energy. Typically, mechanical power is expressed in terms of horsepower and electrical power in terms of kilowatts.

    Power Circuit Breaker: A power circuit breaker is a circuit breaker whose contacts are forced closed via a spring-charged, over-center mechanism to achieve fast closing (5-cycle) and high withstand and interrupting ratings. A power circuit breaker can be an insulated case or power air circuit breaker.

    Power Factor: Power factor is the ratio between the active power (kW) and apparent power (kVA) in a circuit.


    Prime Power: Prime Power describes an application where the generator set(s) must supply power on a continuous basis and for long periods of time between shutdowns. No utility service is present in typical prime power applications.

    Pulse Alarms: Pulse alarms are alarm logic systems that allow all alarms to be annunciated, even if a previous alarm has been silenced but is still present in the system.



    Radio Frequency (RF): Any frequency within the electromagnetic spectrum associated with radio wave propagation.

    Radio Interference: Radio interference refers to the interference with radio reception caused by a generator set.

    Reactance: Reactance is the opposition to the flow of current in AC circuits caused by inductances and capacitances. It is expressed in terms of ohms and its symbol is X.

    Reactive Power: Reactive power is power that flows back and forth between the inductive windings of the generator and the inductive windings of motors, transformers, etc., which are part of the electrical load. This power does no useful work in the electrical load nor does it present load to the engine. It does apply load to the generator and limits the capacity of the generator.

    Reactor: A reactor is an electrical device that applies only reactive load to a system.

    Real Power: Real power is the product of current, voltage and power factor (the cosine of the angle by which current leads or lags voltage) and is expressed as W (watts).

    Rectifier: An electronic device that converts AC power to DC power (AC/DC).

    Regulation: A method of limiting voltage to a narrow range.

    Resistance: Resistance is the opposition to the flow of current in DC and AC circuits. It is expressed in ohms and its symbol is R.

    Rotor: A rotor is the rotating element of a motor or generator.

    RPM: Revolutions Per Minute.



    Service Entrance: The service entrance is the point where the utility service enters the facility. In low voltage systems the neutral is grounded at the service entrance.

    Short Circuit: A short circuit is generally an unintended electrical connection between current carrying parts.

    Short-Circuit Protection: A protective feature that limits the output current of a power supply to prevent damage to the supply caused by short circuits.

    Shunt Trip: Shunt trip is a feature added to a circuit breaker or fusible switch to permit the remote opening of the breaker or switch by an electrical signal.

    Standby System: An independent power system that allows operation of a facility in the event of normal power failure.

    Starting Current: The initial value of current drawn by a motor when it is started from standstill.

    Surge: Surge is the sudden rise in voltage in a system, usually caused by load disconnect.

    Surge Rating: Surge rating is the rating of a machine, usually in excess of its normal operating level, for which it can provide power for a very short time.

    Surge Suppressor: Surge suppressors are devices capable of conducting high transient voltages. They are used for protecting other devices that could be destroyed by the transient voltages.



    Transfer Switch: A transfer switch is an electrical device for switching loads between alternate power sources. An automatic transfer switch monitors the condition of the sources and connects the load to the alternate source if the preferred source fails.

    Transformer: An electric device consisting of two coils that can be used to raise or reduce voltage through the use of induction between the coils.



    UL listed: UL stands for Underwriters Laboratories Inc., an independent, non-profit, product safety testing and certification organization. If a product is UL listed, it has been tested and approved by the laboratory.

    Utility: The utility is a commercial power source that supplies electrical power to specific facilities from a large central power plant.



    Volt: The volt is a unit of electrical potential. A potential of one volt will cause a current of one ampere to flow through a resistance of one ohm.

    Voltage: The amount of electrical pressure that forces electricity to flow in power lines.

    Voltage Dip: Voltage dip is the dip in voltage that results when a load is added, occurring before the regulator can correct it, or resulting from the functioning of the voltage regulator to unload an overloaded engine-generator.

    Voltage Regulation: Voltage regulation is a measurement that states the difference between maximum and minimum steady-state voltage as a percentage of nominal voltage.

    Voltage Spike: Typically caused by lightning or when the power suddenly comes back on after an outage. Power spikes like this often result in serious damage to electronic components.



    Watt: A unit for measuring total electric power. The unit of measure for true power.

    Wattmeter: A wattmeter records power being delivered from a source to the load. Wattmeters for paralleling systems are calibrated in kilowatts (kW).