NEMA Surge Protection Institute

A

ac - See alternating current

ac filter - A power electronic device that eliminates harmonics, attenuates distortions and/or EMI/RFI noise.

Available interrupt current (AIC) - The highest current at rated voltage that a device is intended to interrupt under standard test conditions.

Alternating current (ac) - Electrical current which reverses direction periodically, expressed in hertz or cycles per second. Abbreviated "ac". The number of such cycles per second equals the frequency in hertz, i.e. 60 cycles per second = 60 hertz (Hz), in the United States.

Ampere - The quantitative unit measurement of electrical current.

Ampacity - A term used to describe current-carrying capacity, expressed in amperes, of a wire or cable.

Amperage - The strength of an electric current measured in amperes.

Amplifier - A device used to increase the power and voltage level of a signal.

Antenna - A device to receive radio signals. It may be either broadband or single channel.

Arrester - See surge arrester.

ATS - See autotransfer switch

Attenuation - The reduction of a signal or electrical surge from one point to another. Wire resistance, arresters, and power conditioners attenuate surges to varying degrees.

Attenuator - A passive device used to reduce signal strength.

Autotransfer switch (ATS) - A device that automatically transfers power from one source to another.

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B

Back Filter - A filter inserted in the power line feeding an equipment to be surge tested; this filter has a dual purpose: (1) To prevent the applied surge from being fed back to the power source where it may (might according to the word usage in this guide) cause damage. (2) To eliminate loading effects of the power source on the surge generator. (IEEE 100)

Balanced load - An alternating current (ac) power system that has more than two current-carrying conductors that carry equal, i.e., balanced, currents.

Battery disconnect switch - A switch that disconnects a battery reservoir from an uninterruptible power supply (UPS) and protects personnel when batteries or UPSs require service.

BIL - The basic impulse level or basic insulation level. To meet a given BIL rating a device must be able to withstand a simulated lightning wave that has a wave shape of 1.2 by 50 microseconds and in both positive and negative polarities. The wave reaches its peak in 1.2 microseconds and decays to ½ of the peak value in 50 microseconds. BIL ratings are expressed in kV (kilovolts).

Blackout - The unplanned, total loss of commercial electrical power typically caused by nature, an animal, tree limb, car hitting a pole, or similar event. Sometimes called an “outage.” 

Bonding - A complete and permanent electrical interconnection between two or more points (usually grounding systems) that reduces any voltage difference.

Branch circuit - An electrical circuit individually protected by a fuse or circuit breaker that starts at the service panel and ends at the electrical outlets.

Breaker - Short for circuit breaker.

Brownout -- A long-duration under-voltage condition, usually hours or days in length. Brownouts can be caused by heavy usage during peak hours, or they may be planned as an energy conservation strategy.

Building service entry - The point where commercial power enters the building.

Bus bar (bus) - A heavy, rigid conductor that serves as a common connection for two or more circuits.

Bypass - A circuit that provides an alternative path for electrical power to go around (or bypass) its normal path and allows maintenance personnel to service equipment, like UPSs, without interrupting service.

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C

Capacitance - A term referring to the electrical properties of a capacitor or to a circuit that displays capacitor-like behavior.

Capacitor - A discrete electrical device which has two electrodes and an intervening insulator, which is called the dielectric.

Capacity - The rated load of a machine, apparatus, or appliance. Maximum capacity is the maximum load of a machine, apparatus, or appliance of which it is capable under existing service conditions. 

Category A, B, C - Categories of location for transient suppression within a facility. Class A refers to outlets and long branch circuits. Class B refers to major feeders and short branch circuits near the distribution panel. Class C refers to the commercial power service entrance and outside the facility (refer to IEEE C62.41).

CATV (Community Antenna Television) - An RF distribution system which distributes television broadcast programs, original programs, premium programming and other services using a network of coaxial cable.

CBEMA - The former Computer and Business Equipment Manufacturers Association; replaced by the Information Technology Industry Council (ITIC).

CBEMA curve - A set of curves developed by the Computer Business Equipment Manufacturers Association (CBEMA) that represents the withstand capabilities of computers in terms of the magnitude and duration of a voltage disturbance. It was a standard for measuring the performance of all types of equipment and power systems until replaced by the Information Technology Industry Council (ITIC) curve.

Channel - In television, a portion of the RF spectrum 6 Mhz wide which carries the audio and video carriers of the television signal.

Charger - An ac-to-dc converter that provides power to an UPS inverter and maintains the battery reservoir charge.

Choke - A form of inductor which is constructed to allow desirable frequency signals to pass while acting with high impedance to other signals at some undesirable frequency.

Circuit breaker - A resettable device that responds to a preset level of excess current flow by opening the circuit, thereby preventing damage to circuit elements.

Clamping voltage - The peak voltage that SPDs allow into an electric circuit based on a specific test waveform. See Measured Limiting Voltage

Clean ground - An undefined term that describes an earth connection that does not cause electrical equipment to malfunction. See computer ground; or isolated ground.

Coax - A cable constructed by using two concentric conductors separated by an insulator.

Coaxial cable - A popular transmission medium usually consisting of one central wire (two for twinaxial) surrounded by a delicate insulator and encased in either a wire mesh or an extruded metal sheathing. Also, a concentric cable consisting of a center conductor, a dielectric, and a shield. Coax used for most MATV and CATV work has a characteristic impedance of 75 ohms.

Common mode (CM) - Refers to electrical interference measured as a ground reference signal common to both of two current-carrying conductors.

Common mode noise (voltage) - Electrical noise between the power conductors and ground, i.e., between line and ground or between neutral and ground. 

Composite video signal - the complete video signal including the picture (luminance) signal, the blanking and sync pulses, and the color (chrominance).

Conductor - An object or substance, usually a wire, bus bar, rod, or tube, that conducts, i.e., provides a path for, electric current.

Conduit - A tube, duct, or pipe that provides a metallic or nonmetallic tubular raceway for wires and cables that carry data or power.

Cord connected - Any SPD provided with a power-supply cord terminating in an attachment plug for connection of the device to a receptacle in the ac power circuit. (UL 1449-1996).

Coupling - The means by which power or signals transfer from one circuit element or network to another. Also, the effect of a power or signal source interfering with a signal transmission system.

CPU - An acronym used for the central processing unit of a computer.

Critical load - That portion of electrical/electronic equipment for which power quality is a vital consideration. The term "load" applies to all current-carrying devices on a given circuit or feeder. The term "critical load" refines the language to include only those current-using devices whose operation is considered essential.

Cross talk - The unwanted transfer of energy from one communication circuit to another by means of mutual inductive, capacitive, or conductive coupling.

Crowbar - A circuit that protects sensitive equipment by becoming a “crowbar” (or short circuit) between power conductors when conditions require it and temporarily shunting all current to ground and clamping the voltage to zero.

CSA - The abbreviation for Canadian Standards Association, a Nationally Recognized Testing Laboratory headquartered in Canada.

Current - The flow of electricity in a circuit as expressed in amperes. Current refers to the quantity or intensity of electrical flow. Voltage, on the other hand, refers to the pressure or force causing the electrical flow.

Current balance - The equal flow of current in each phase of a three-phase power system.

Current carrying - A circuit component carrying current.

Current distortion - Distortion in the ac waveform. See distortion.

Current transformer (CT) - A device that provides a means for measuring current beyond the range of a meter and uses the strength of the field around the conductor to induce a current in its secondary.

Cutout - A device used manually or automatically to interrupt current through any particular apparatus or equipment.

Cycles per second - A term that describes the frequency of alternating current (ac). Frequency is more properly described using the term "hertz", which is synonymous with cycles per second.

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D

dB - See decibel

dc - See direct current

dc standby current (varistor) - Varistor current measured at rated dc voltage (IEEE 100).

Decibel (dB) - A logarithmic function used to simplify MATV calculations. Decibels may be added or subtracted. 0 dB is the standard reference level for all MATV calculations.

Delta - A standard three-phase circuit connection configured such that the ends of each phase winding connected in series form a closed loop with each phase 120 electrical degrees from the other. It appears as a triangle and looks like the Greek letter delta (r). 

Delta connection - A method of connecting a three-phase source or load in series to provide a closed circuit (three-wire, plus ground).

Delta-delta - Three-phase transformer circuit created by a delta source and a delta load with both the primary and secondary winding connected in a delta configuration.

Delta-wye - Three-phase transformer circuit created by a delta source and a wye load with the transformer primary connected in a delta configuration and the transformer secondary connected in a wye configuration.

Demand (kW) - The maximum electrical power, in fifteen minute intervals, that a utility customer requires from the utility.

Derating factor - A factor in percent for reducing the capacity of electrical equipment, like transformers.

Differential-mode voltage - The voltage (noise) that appears across two specified sets of active conductors. See transverse mode noise.

Device failure - An irreversible change in characteristic, resulting in an inability to perform as intended. (SC definition)

Digital multimeter (DMM) - An instrument that measures voltage, current, and resistance and displays measurements on a digital readout.

Diode - A two-terminal device that conducts current better in one direction that the other. It uses include rectification (ac to dc conversion) and detection (retrieving an information signal from a modulated carrier wave).

Diode/capacitor input - A power supply that uses a full-wave bridge rectifier connected to a capacitor to produce a pulse current.

Dip - An undefined, imprecise term sometimes incorrectly used as a substitute for sag.

Direct coupling - Two circuits connected to each other through an inductor resistor, or wire.

Direct current (dc) - A unidirectional current whose average value does not equal zero but a constant number.

Direct plug-in - Any SPD incorporating integral blades for direct insertion into a standard wall receptacle. (UL 1449-1996).

Dirty power - An undefined, imprecise term sometimes used to describe the electric power that causes power quality problems, especially to electronic equipment operation.

Disk - A memory device which uses a magnetic media for the storage of information. Disk, as a term, has expanded into other areas often used to describe the shape of the storage media, that is: floppy disk, compact disk, laser disk, or hard disk. It sometimes refers to the way in which the storage media mimics a disk media; that is, RAM disk or bubble memory.

Distortion - The wave shape of a signal that is not normal is distorted. Distortion is a term that describes abnormal wave shapes.

Distortion factor - The ratio of the root means square (rms) of the harmonic content of a waveform to the rms of the fundamental quantity, expressed in percent of the fundamental. See total harmonic distortion (THD).

Distribution - The way in which power is routed to various current-using sites or devices. Outside the building, distribution refers to the process of routing power from the power plant to the users. Inside the building, distribution is the process of using feeders and circuits to provide power to devices.

Disturbance - Any event that adversely affects the normal power flow in a system, such as lightning or a short circuit.

Dropout - Loss of equipment operation due to noise, sag, or interruption. A discrete voltage loss. Or a voltage sag (complete or partial) for a very short period of time (milliseconds).

Dropout voltage - The voltage at which a device fails to operate.

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E

Earth electrode - A ground electrode, water pipe, building steel, or some combination of these, that establishes a building’s earth ground.

Earth ground - A low-impedance path to earth that discharges lightning, static, and radiated energy, and keeps the main service entrance at earth potential.

Eddy currents - Induced currents in transformer winding and core caused by the magnetic field from the normal alternating current and harmonics.

Effective value - For any time-variant voltage or current waveform, the constant value that gives the same average power. For sinusoidal waveforms, the effective value equals 0.707 times the peak value.

Electric field - Describes forces associated with electrical charges.

Electrode - Generally, a conducting medium through which an electric current enters or leaves a different medium, such as an electrolyte, gas, or vacuum.

Electromagnetic - A magnetic field caused by an electric current. For example, the electric current in energized power lines causes electromagnetic fields that can interfere with nearby data cables.

Electromagnetic compatibility - The ability of a device, equipment, or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment.

Electromagnetic interference (EMI) - A term that describes electrically-induced noise or transients. 

Electromagnetic susceptibility - The inability of a device, equipment, or system to function without degradation in the presence of an electromagnetic disturbance.

Electrostatic discharge (ESD) - The effects of static discharge can range from simple skin irritation for an individual to degraded or destroyed semiconductor junctions for an electronic device.

EMF - Electromotive force, or voltage.

EMI -- See electromagnetic interference

EMP - Electromagnetic pulse.

Energy - The capability of doing work over time, expressed in kilowatt-hours for electrical energy.

Equipment event log - A record of equipment problems and activities that provides diagnostic information along with power monitor data to correlate equipment problems with power events.

Equipment grounding conductor - The conductor that connects the non-current-carrying parts of conduits, raceways, and equipment enclosures to the grounded conductor (neutral) and the grounding electrode at the service equipment entrance (main panel) or secondary of a separately derived system (e.g., isolation transformer). See Section 100 in ANSI/NFPA 70-1990.

ESD - See electrostatic discharge

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F

Failure Mode - The observed effects of failure.

Farad - Unit of measurement for capacitance.

Fast tripping (fuse saving) - The common utility protective relaying practice in which the circuit breaker or line recloser operates faster than a fuse can blow. It clears transient faults without a sustained interruption but subject’s industrial loads to a momentary or temporary interruption.

Fault - An unintentional short circuit that causes a failure or interruption in an electrical circuit or a power system.

Fault transient - A short circuit on the power system, usually caused by lightning, tree branches, or animals and cleared by momentarily interrupting the current.

Feeder - An electrical supply line, either overhead or underground, that connects from a generating plant or an interchange point to a load or distribution system.

FERC - Federal Energy Regulatory Commission

Field effects - The effects of electric and magnetic fields on objects near current-carrying conductors.

Filter - An electronic device which opposes the passage of a certain frequency band while allowing other frequencies to pass. Filters are designed to produce four different results. A high-pass filter allows all signals above a given frequency to pass. A low-pass filter allows only frequencies below a given frequency to pass. A bandpass filter allows a given band of frequencies to pass while attenuating all others. A trap filter allows all frequencies to pass but acts as a high-impedance device to the tuned frequency of the filter.

FIPS Pub. 94 (Federal Information Processing Standards Publication 94) (September 21, 1983), -- Produced as an official publication of the National Bureau of Standards (since renamed National Institute for Standards and Technology). It gives guidelines for federal agencies in the use of automatic data processing (ADP) facilities in an electrical environment.

Fixed peak current - A desired (specified) test current level which has taken into consideration the additional impedance of the specimen under test, requiring adjustment of the pre-set open-circuit voltage or short-circuit current of the surge generator. (SC definition)

Flashover - Arcing that is caused by the breakdown of insulation between two conductors where a high current flow exists, with a high potential difference between the conductors.

Flicker - Slow variations in voltage that cause the light intensity in a fluorescent light to vary and give the impression of unsteadiness of visual perception.

Fluctuation - A change, i.e., surge or sag, in voltage amplitude, often caused by load switching or fault clearing.

Flywheel energy storage system (FESS) - a mechanical battery that converts energy to mechanical motion and, when needed, converts that motion back to energy.

Follow current - Current supplied by the electrical power system and flowing through the SPD during and following the passage of discharge current.

Forced outage - A power system interruption caused by the improper operation of equipment or human error.

Forward transfer impedance - The amount of impedance between the source and load that affects the transfer of power, including harmonics and inrush current, to the load.

FPN - Fine print note; National Electrical Code^® (NEC^®) explanatory material.

Frequency - The number of times is a specific period (how frequently, usually in cycles per second) alternating current (ac) reverses its direction, measured in hertz (Hz). Each reversal from one direction to another and back again constitutes a cycle. In North America, utilities provide power with a frequency of 60 cycles per second, or 60 hertz. In ac circuits, designates number or times per second that the current completes a full cycle in positive and negative directions. See also alternating current.

Frequency deviation - A variation, i.e., increase or decrease, in the nominal power frequency that can occur from several cycles to several hours.

Frequency modulation (FM) - A method to change a carrier frequency so that it varies above and below a center or resting frequency in step with the signal it transmits.

Frequency response - A measurement of how well a device or circuit transmits various frequencies.

Fuse - An overcurrent protective device with a fusible link that operates and opens the circuit on an overcurrent condition.

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G

Gain - A measure of amplification of a device, usually expressed in dB and at the highest frequency of operation.

Generate -- To produce electrical energy.

Generation -- The act or process of converting other forms, e.g., mechanical, chemical, solar, or nuclear of energy, into electric energy.

GFI (Ground Fault Interrupter) - A device that interrupts the flow of electric current in an electric circuit when the fault current to ground exceeds some predetermined value.

Ghosting - A signal interference condition producing positive or negative pictures displaced in time from the desired picture, caused by multi-path signal reception. Ghost pictures also result from cable ringing.

Grid - An interconnected system of electric transmission lines and associated equipment that moves or transfers bulk electric energy from the generators to the loads.

Ground - A general term that refers to the point at which other portions of a circuit are referenced when making measurements. Power systems grounding is that point to which the neutral conductor, safety ground, and building ground are connected. This grounding electrode may be a water pipe, driven ground rod, or the steel frame of the building.

Ground block - Connector which passes the RF signal through with minimum loss while providing a means of attaching a ground wire to the drop cable typically near the entry to the home. Grounding is accomplished by either a special ground rod or connection to the water system (See your local code for required grounding methods)

Ground electrode - A conductor or group of conductors in contact with the earth that provides a low-impedance connection to the ground. 

Ground fault - Any undesirable current path from a current-carrying conductor to ground.

Ground grid - A system of interconnected bare conductors arranged in a pattern over a specified area, on a buried below the surface of the earth, that provides safety for workers by limiting potential differences within it perimeter to safe levels. It does not act as a signal reference grid.

Ground impedance tester - An instrument that measures the impedance of a circuit from the point of test to the bond between the neutral and ground bond. Some can handle 120 V ac single-phase voltage while others can handle 600 V ac three-phase voltage. It also measures voltage and determines the presence of neutral-to-ground connections, isolated ground shorts, reversed polarity, and an open equipment grounding conductor.

Ground loop - The condition of having two or more ground references in a common system. When two or more grounds have a potential difference between them, current can flow. This flow of current is a new circuit or loop which can interfere with the normal operation of the system.

Ground noise - An undefined, imprecise term that describes unwanted electrical signals appearing between the earth conductor and any other conductor.

Ground potential rise (GPR) - When a large amount of energy is rapidly deposited into the ground by a cloud-to-ground lightning strike or by an electrical fault on a utility power system, the ground potential at this injection point rises to a higher level with respect to the more distant ground. This has the effect of creating a voltage potential gradient in the earth, which can cause dangerous touch and step potentials to personnel exist. By creating an equi-potential ground plane beneath a facility by electrically bonding all separate “grounds” into a “system” or by burying ground mats and meshes, this danger to personnel and equipment can be reduced. It is also important to note that GPR is not only dangerous to personnel, it can also cause damage to equipment.

Ground window - The opening through which all grounding conductors, including metallic raceways, enter a specific area. Provides the building grounding system a connection to an area that would otherwise have a grounding connection.

Grounded - Connected to earth or to some conducting body that substitutes for earth.

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H

Hard-wired - Applied to equipment that connects its power source with wiring (generally customer or contractor-supplied) attached directly to terminal blocks or distribution panels rather than via an input line cord and output receptacle. 

Harmonic - A frequency that is a multiple of the fundamental frequency. For example, 120 Hz is the second harmonic of 60 Hz, 180 Hz is the third harmonic, and so forth.

Harmonic content - The waveform that remains after subtracting the fundamental component from a harmonic waveform. 

Harmonic distortion - Excessive harmonic content that distorts the normal sinusoidal waveform is harmonic distortion. This can cause overheating of circuit elements and might appear to a device as data-corrupting noise.

Harmonic resonance - The power quality term that describes the condition that sometimes occurs in electrical systems in which high currents flow through and damage capacitors or clear fuses in connecting circuits. A condition in which the power system resonates at one of the major harmonics produced by nonlinear elements in the system and increases the harmonic distortion.

Henry - Unit of measurement for inductance.

Hertz - A term describing the frequency of alternating current. The term hertz is synonymous with cycles per second. Abbreviated Hz.

High-pass filter - A filter that passes all frequencies above a certain level, and stops all lower frequencies.

Hub - A wiring concentrator used in local area networks.

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I

IEC  - The International Electrotechnical Commission (IEC) is an international standards-setting body for electrical and electronic technologies.

IEEE - The Institute of Electrical and Electronic Engineers (IEEE) is an international society of engineers that develops its own standards.

Impedance - Measured in ohms, impedance is the total opposition to current flow in a circuit where alternating current is flowing. This includes inductive reactance, capacitive reactance, and resistance. Symbol is Z.

Impulse - A surge of unidirectional polarity. Ref: IEEE.

Impulsive transient - A sudden non-power frequency change in the steady-state condition of voltage or current that occurs in unidirectional polarity (primarily either positive or negative.)

Inductance - This term describes the electrical properties of a conductor and its resultant magnetic field when an alternating current is passed through it. This interaction offers an impedance to current flow, thereby causing the current waveform to lag behind the voltage waveform. This results in what is known as a lagging power factor.

Inductor - A discrete circuit element which has the property of inductance. It should be noted that at very high radio frequencies, a straight wire or a path on a printed-circuit board can act as an inductor.

Inductive reactance - The impedance to alternating current produced by inductors.

Insertion loss - Also called "feed through loss". This is the loss that occurs as signals pass through a passive device. Insertion loss occurs in all devices which do not amplify the signal.

Input line cord - The power cord connected to the input terminals of the uninterruptible power supply (UPS) that plugs into an ac utility outlet to supply power to the UPS.

Inrush current - The initial large current demand needed to start up certain types of electrical equipment, like motors, before their resistance or impedance increases to their normal operating value.

Instantaneous - A qualifying term applied to a circuit breaker or other device indicating that no delay is purposely introduced into its action.

Insulation (electrical) - A dielectric substance or air space permanently offering a high resistance to the passage of current and to disruptive discharge through the substance or space. For example, a  non-electrical-conducting material, like rubber or polyethylene, is used to resist current flow.

Insulation (thermal) - A non-heat-conducting material, like fiberglass, that resists heat flow and provides a barrier to heat loss.

Insulator - A device made of porcelain, glass, rubber, or wood that prevents current from flowing, like a porcelain support that insulates conductors from a pole or tower.

Isolation - The degree to which a device can separate the electrical environment of its input from its output, while allowing the desired transmission to pass across the separation. 

Interconnection system - A connection between two electrical systems that allows the transfer of electric energy in either direction. 

Internal impedance - The inherent impedance inside a device or circuit.

Interruptible power - Power that the supplier and the customer have agreed can be stopped by the supplier.

Interruption, sustained (power quality) - The complete loss of voltage (<0.1 pu) on one of more phase conductors for a long time period greater than 1 minute.

Inverter - An inverter converts dc power into ac power.

Isolated ground - An insulated equipment grounding conductor that runs in the same conduit or raceway as the supply conductors and is insulated from the metallic raceway and all ground points throughout its length. It originates at an isolated ground-type receptacle or equipment input terminal block and terminates at the point where neutral and ground bond at the power source. See NFPA 70-1990, Section 250-74, Exception 4, and Section 250-75, Exception.

Isolation - Separation between electrical input and output, such as an isolation transformer or optical coupler or separation of one section of a system from the undesired influences of other sections.

Isolation transformer - A multiple-winding transformer with physically separate primary and secondary windings that allows the magnetic field in the winding of the primary to create (induce) electrical power in the secondary winding but minimizes electrostatic transfer to the secondary windings. This way the electrical power available at the input transfers to the output, but some of the unwanted electrical effects in the input power do not reach the transformer’s output. 

ITIC (Information Technology Industry Council) - replaced the former Computer Business Equipment Manufacturers Association.

ITIC Curve - Replaces the former CBMA curve and provides a set of curves developed by the Information Technology Industry Council (ITIC) that represents the withstand capabilities of computers in terms of the magnitude and duration of the voltage disturbance. 

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J

Joule - The surge energy (watt seconds) that the device is capable of absorbing.

Jumper - A short length of conductor that connects two points in a circuit.

Junction box (J box) - A box with a blank cover that provides space to connect and branch enclosed conductors.

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K

k-rated transformer - A transformer specially designed to handle harmonics.

Kilovolt-Ampere (kVA) - An electrical unit related to the power rating of a piece of equipment. It is calculated by multiplying the rated voltage of equipment by the current required (or produced). For resistive loads 1 kilovolt-ampere equals 1 kilowatt.

Kilohertz (kHz) - A term meaning 1000 cycles per second.  

Kilowatt (kW) - The product of the root-mean-square (RMS) current, the root-mean-square (RMS) voltage, and the power factor divided by 1000. 

Kilowatt-hour (kWh) - The basic unit of electric energy that equals 1000 watts of power used for 1 hour. The amount of power the customer uses is measured in kilowatt-hours (kWh). A measurement of power and time used by utilities for billing purposes. For example, a 75-watt light bulb that burns for 10 hours consumes 0.75 kWh (75 watts x 10 hours) or 750 watt-hours.

Kirchhoff’s laws of electric networks - The sum of the electrical currents flowing to a point in a network equals the sum of the currents flowing away from that point.

KVA (1000 VA) - The product of the root-mean-square (RMS) current and the root-mean-square (RMS) voltage divided by 1000; provides the actual measured apparent power and is used for circuit sizing. 

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L

Lag - The time delay between two events (usually electrical quantities), such as delay of current behind voltage.

Lagging load - An inductive load that resists changes in current in which current lags voltage. The time lag between current and voltage is measured in electrical degrees and is known as the phase angle. The cosine of this angle equals the power factor (linear loads only).

LAN (Local Area Network) - A data communications system confined to a certain area. The area served may consist of a single building, or a cluster of buildings.

LC circuit - An electric network that contains both inductive and capacitive elements.

Leading load - A capacitive load that resists changes in voltage with current leading the voltage.

Leakage current - Any current, including capacitively coupled currents, that may be conveyed from accessible parts of a product to ground or to other accessible parts of the product. (UL 1449)

Leased line - A dedicated circuit, typically supplied by the telephone company, that permanently connects two or more user locations. These lines are used to transmit data.  See T1.

LED (light emitting diode) - A semiconductor that emits light when current passes through it.

Lifetime rated pulse currents (varistor) - Derated values of Itm for impulse duration’s exceeding that of an 8/20 us wave shape, and for multiple pulses which may be applied over device's rated lifetime. (IEEE 100)

Lightning arrester - See surge arrester.

Line - A designation of one or more power-carrying conductors for power distribution. The black (or red or blue) wire is the line conductor, the white wire is the neutral, and the green wire is ground. The voltage difference between the line conductor and the neutral is the supply voltage, i.e., 120 volts.

Line conditioner - A line conditioner contains multiple protection devices in one package to provide, for example, electrical noise isolation and voltage regulation.

Line filter - A filter in series with a transmission line that removes unwanted electrical signals.

Line imbalance - Unequal loads on the phase lines of a multiphase feeder.

Line loss - The heat loss in a line caused by the flow of the current through the resistance in the line, usually called I²R loss. 

Line to Line (L-L) - A term used to describe a given condition between conductors of a multiphase feeder.

Line to Neutral (L-N) - A term used to describe a given condition between a phase conductor and a neutral conductor.

Linear load - An electrical load device, which, in steady-state operation, provides an essentially constant load impedance to the power source throughout the applied voltage cycle and in which the current relationship to voltage remains constant for a relatively constant load impedance. 

Load - Any electrical device connected to a power source may be called by the general term of "load".

Load balancing - Switching the various loads on a multiphase feeder to equalize the current in each line.

Load fault - A malfunction, like a tree touching a power line, that causes the lead to demand abnormally high amounts of current from the source.

Load unbalance - Unequal loads on the phase lines of a multiphase system.

Loss - The power dissipated in a power system circuit expressed in watts. In communications, the ratio of the signal power delivered by a device under ideal conditions to the signal actually delivered expressed in decibels (dB).

Low-pass filter - A filter that passes all frequencies below a certain designated cutoff point and blocks all frequencies above that point.

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