NEMA Surge Protection Institute

M

Magnetic field - The lines of force that exist around an energized electrical conductor, magnet, or inductor.

Main service entrance -- The enclosure that contains connection panels along with fuses or breakers, and located at the point where the utility power lines enter a building. 

Mains - The ac power source available at the point of use in a facility. It consists of the set of electrical conductors (referred to by terms including “service entrance”, “feeder”, or “branch circuit”) for delivering power to connected loads at the utilization voltage level. Ref. IEEE 100.

Maintenance bypass - A circuit that allows maintenance personnel to repair or service equipment without affecting supply of electricity to other equipment.

Make-before-break - Operational sequence of a switch or relay where the new connection occurs prior to disconnection of the existing connection.

Manual bypass switch (MBS) - A manually operated transfer switch that allows maintenance personal to bypass the major electronics in the UPS so the UPS can be safely serviced.

Master antenna television (MATV) -- 

Matching transformer - A device, often called a balun, which converts between a 75 ohm impedance and a 300 ohm impedance.

MATV - see master antenna television

Maximum continuous operating voltage (MCOV) - The maximum designated root-mean-square (rms) value of power-frequency voltage that may be applied continuously between the terminals of the arrester.  Ref. IEEE 100.

MBS - See manual bypass switch

MCOV - See maximum continuous operating voltage

Mean Time Between Failure (MTBF) - A statistical estimate of the time a component, subassembly, or operating unit will operate before failure will occur. 

Measured limiting voltage - The maximum magnitude of voltage that appears across the terminals of the SPD during the application of an impulse of specified waveshape and amplitude. (IEC TC 37)

Megahertz (MHz) -- A term for one million hertz (cycles per second).

Metal oxide varistor (MOV) - A solid state device which becomes conductive when the voltage across it exceeds a certain level. When the voltage exceeds the MOV's threshold, current flows through the MOV.

Meter board - The board on which the main switch and associated equipment mount.

Meter constant - The ratio of the output of an instrument transformer (current transformer, power transformer) to the input of the meter to determine the difference between meter readings to determine the kilowatt-hours used.

Meter inspection - An examination of the meter to determine its accuracy.
Meter loop (meter socket) - The necessary equipment and wiring that the customer’s electrician install prior to the meter installation.

Meter test - An instrumental test of meter accuracy under all load conditions.

Micro (µ) - A metric prefix that means one-millionth of a unit, or 10^-6.

Microwaves - Ratio frequencies of 1000 megahertz or greater that have very short wavelengths and exhibits some of the properties of light and allow communication signals sent point-to-point in a concentrated beam.

Minimum ground clearance - The least distance between a conductor and ground level under selected design-loading conditions.

Mitigate (power quality) - The reduction of power quality disturbances by the use of power conditioning equipment.

Modem - A hardware device that converts digital computer data into analog tones that can be transmitted over dial-up telephone circuits.

Module - A sub-unit of an electronic system that can be plugged in or otherwise easily replaced.

Momentary over-voltage (voltage swell) - A temporary increase in the rms value of the voltage or current of more than 10 percent of the nominal voltage, at the power frequency, for any duration from 0.5 cycle to 1 minute.

Moore’s Law - The principle by which, in the computer industry, each new chip contains roughly twice as much capacity as its predecessor released 10 to 24 months previously. Named after an Intel founder, Gordon Moore, who made this observation in a 1965 speech.

Motor alternator - A device that consists of an ac generator mechanically linked to an electric motor, which is driven by utility power or by batteries. An alternator is an ac generator.

Motor generator - A motor generator consists of an ac motor coupled to a generator. The utility power energizes the motor to drive the generator, which powers the critical load. Motor generators provide protection against noise and spikes, and, if equipped with a heavy flywheel, they may also protect against sags and swells.

MOV - See metal oxide varistor 

MTBF - See mean time between failure

Multi-Port SPD - Pending a resolution of different definitions among standard-writing bodies, the terms ‘multi-port’ has not been used in this protocol.

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N

Nameplate rating - The full-load continuous rating of a specific electrical apparatus, like a transformer or generator, under specified conditions set by the manufacturer.

National Electrical Code® - A set of rules and regulations, plus recommended electrical practices, published by the National Fire Protection Association. Abbreviated NEC.^®

NEC^® - See National Electrical Code®

Negative sequenced - Occurs when a three-phase electrical quantity, such as voltage, current, or power, crosses the zero line in the order of A-C-B rather than A-B-C.

NEMA  - The National Electrical Manufacturers Association (NEMA) is a standards-setting body for electrical equipment.

Network - A system of transmission and distribution lines cross connected and operated to permit multiple power supply to any principal point on it. It provides a high level of reliability by restoring power quickly to customers during an outage by switching them to another circuit.

Neutral - The grounded junction point of the legs of a wye circuit or the grounded center point of one coil of a delta transformer secondary.

Neutral conductor - One of the conductors of a three-phase wye system. Sometimes called the return conductor, it carries the entire current of a single-phase circuit and the resultant current in a three-phase system that is unbalanced. The neutral is bonded to ground on the output of a three-phase delta-wye transformer.

Node - A point of connection to a network. Also referred to as a device, workstation, printer, computer, or any other point of connection to a network.

Noise - An undesirable signal. A distortion of the normal sine wave. Can be caused by radar and radio transmitters, fluorescent lights, power control circuits, acing utility and industrial equipment.

Noise-conducted - Any signal of greater than 1000 times the fundamental power frequency (60Hz) with a magnitude of less than twice the RMS voltage of the distribution system. It is superimposed on the power waveform and is in the range from 100kHz to 100MHz. 

Nominal rate of rise (of an impulse wave front) - The slope of the line that determines the virtual zero. It is usually expressed in volts or amperes per microsecond. Ref. IEEE 100.

Nominal system voltage - A nominal value assigned to designate a system of a given voltage class. Note: See ANSI C84.1. Ref. IEEE 100.

Nominal varistor voltage - The voltage across the varistor measured at a specified pulsed dc current, IN(dc), of specific duration. IN(dc) is specified by the varistor manufacturer. Ref. IEEE 100.

Nominal voltage - The normal or specified voltage level. For three-phase wye systems, nominal voltages are 480/277 V (600/346 V in Canada) and 208/120 V, where the first numbers expresses phase-to-phase (or line-to-line) voltages and the second numbers equals the phase-to-neutral voltage. The nominal voltage for most single-phase systems is 240/120 V.

Nonlinear load - A load in which the current varies with the voltage in a nonlinear fashion. For example, in a switched-mode power supply or almost any other electronic power supply. The current does not vary in direct proportion to the voltage because it uses power in pulses or other waveforms that to not track the sine wave.

Nonlinear load current - Current drawn by a load that does not have a direct relationship to the voltage waveforms.

Normal mode (NM) - Refers to electrical interference measured between line and neutral (current-carrying conductors). The operation of lights, switches and motors generates normal-mode interference.

Normal-mode noise - A noise signal which appears between a set of phase conductors, irrespective of their associated ground conductor.

Notching - A negative or positive change in the wave-shape that repeats cycle to cycle, caused by light peak currents of variable-speed drives or other phase loads.

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O

OCV - See open-circuit voltage

Off-line - Describes the operating of a standard utility power system that supplies power directly to the load and then transfers to battery power supplied through an inverter when the voltage drops below a specified level. The time required between the loss of utility power and inverter start-up can disrupt sensitive loads.

Ohm (Ώ) - The unit of measurement for resistance (symbol R), impedance (symbol Z), and reactance (symbol X).

Ohm’s law – The relationship between voltage (pressure), current (electron flow), and resistance. The current in an electrical circuit is directly proportional to the voltage and inversely proportional to the resistance: E = IR, I = E/R, and R = E/I, where E = voltage, I = current, and R = resistance.

On-line - Describes the operation of an uninterruptible power system which supplies conditioned power through an inverter or converter to the load and supplies the backup power to the load without delay when a utility power outage occurs.

One-port SPD - A surge protective device (SPD) having provisions (terminals, leads, plug) for connection to the ac power circuit but no provisions (terminal, leads, receptacles) for supplying current to the ac power loads (UL 1449-1998)

Open circuit - Describes the condition of a disconnect in a circuit caused intentionally or by a fault.

Open-circuit voltage (OCV) - The voltage available from the test set up (surge generator, coupling circuit, back filter, connecting leads) at the terminals where the surge protective device (SPD) under test will be connected. (SC definition)

Orderly Shutdown - The turning off of power to computer devices in such a way that data is not lost or corrupted.

Oscillation - The change with time of a quantity’s value from the maximum and minimum values of one cycle.

Oscillatory transient - A power quality term that describes a voltage or current transient that rises suddenly and sharply to some level and then degrades over time to a waveform that decreases in frequency and amplitude.

Oscilloscope - An electronic instrument that produces a visible graphical display on a cathode-ray tube of the instantaneous value of one or more rapidly varying electrical quantities as a function of time or other electrical or mechanical quantity.

Out of phase - A condition that occurs when two waves have the same frequency, but their maximum values occur at different times.

Outage - See blackout.

Output - The current, voltage, power, or driving force that a circuit or a device delivers.

Output impedance - A measure of a source’s ability to supply current to a load.

Overload - A condition when the flow of electricity exceeds the rated capacity of a device or system or when the load wants more from the power source, utility, or UPS than the power source can supply.

Overload capacity - The maximum load that a machine, apparatus, or device can carry beyond its normal nameplate rating without damaging itself.

Overvoltage - An increase in the normal voltage level lasting for seconds or minutes greater than the rating of a device or component. The term can also apply to transients and surges. When applied to a long-duration variation it refers to a voltage with a value at least 10 percent above the nominal voltage for a period of time greater than 1 minute.

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P

Pad-mount transformer - A transformer inside a large metal cabinet on a pad outside a house or building that converts the utility voltage supplied via an underground line to a level that a customer can use (120/240 V for a home), similar to a pole-mounted transformer. The cabinet has a lock and a “DANGER” sign on it.

Panelboard - A single panel or group of panel units assembled into a single panel, containing buses and, over-current protection devices (with or without switches) to control power circuits.

Parallel operation - The connection of two or more components side by side to the same pair of terminals to increase the capacity of the power system or provide power redundancy. For example, paralleling for capacity two transformers means that two 50-kVA units in parallel have the sum of their individual ratings, i.e., the power of a single 100-kVA transformer. Paralleling two UPS units for redundancy means that if one fails, the other unit will provide backup power.

Passive filter - A combination of inductors, capacitors, and resistors that eliminates one or more harmonics. The most common type simply uses an inductor in series with a shunt capacitor, which diverts the major distorting harmonic component from the circuit.

Patch Panel - A device which consolidates cables in a central location, allowing for easy changes.

PDU - See power distribution unit

Peak - The maximum instantaneous measurement of an electrical event.

Permanently connected SPD - Any SPD provided with terminals or leads for field connection to wiring systems in accordance with the National Electrical Code^® - ANSI/NFPA-70). This includes receptacle/type SPDs intended for installation in outlet boxes. (UL 1449-1998)

Periodic waveform - A waveform that repeats itself after a period of time.

Phase - The stage or progress of a cycle movement, such as a current or voltage wave. Also a conductor that carries one of three separate phases (designated, A, B, and C) of power in al alternating current system. Almost all residential customers use either two-phase or three-phase service.

Phase angle - In a power system, the displacement, in time, of the phase of one quantity from the phase of another, at power system frequency.

Phase balancing - Connecting loads in a three-phase power system so that all three phases carry the same current.

Phase compensation - Switching capacitors into or out of a power distribution system to reduce the phase difference between the current and voltage to keep to power factor close to unity.

Phase conductor - The wire cable in each phase of a transmission or distribution line.

Polarity - An electrical condition that determines the direction in which current tends to flow.

Polyphase - An alternating current (ac) supply with two or more hot conductors. Voltages between the conductors and the voltage waveforms for each conductor are usually displaced 120 degrees. The voltage from each hot conductor to neutral is equal.

Power - A general term which means the capacity for doing work. In the electrical environment, this is usually measured in watts.

Power conditioner - See line conditioner.

Power distribution unit (PDU) - A portable electrical distribution device that provides an easily expandable and flexible electrical environment for a computer and its associated peripherals. 

Power disturbance - A disturbance, like a surge or sag, that originates from the utility’s power system.

Power quality - A general term describing the powering and grounding of electronic equipment in a manner suitable to the operation of the equipment and compatible with the premise wiring system and other connected equipment.

Power quality problem – The difference between the quality of electricity at an electrical outlet and the quality of the electricity required to reliably operate an appliance, resulting in a malfunction or damage.

Power warning fault (PWF) - An option in a UPS that supplies a warning signal to some computer systems that the UPS may shut down. Some computers can take advantage of this signal to automatically back up and shut down before the UPS shuts down.

Power-line monitor - An instrument that monitors the condition of the power supplied to a given load.

Propagation - The travel of an electrical waveform along a medium, as in a surge passing along a power cord to a system.

Protective scheme - A group of interrelated devices that prevent damage to equipment caused by very high voltages and currents.

Pulse life - The number of surges of specified voltage, current amplitudes, and waveshapes that may be applied to a device without causing degradation beyond specified limits. The pulse life applies to a device connected to an ac line of specified characteristics and for pulses sufficiently spaced in time to preclude the effects of cumulative heating. Ref. IEEE 100.

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R

Radial array - A group of earthing electrodes or conductors of equal length and ampacity, connected at a central point and extending outward at equal angles, spoke fashion, to provide a low-earth-impedance reference.

Radiation -- RF energy which is emitted or leaks from a distribution system and travels through space. These signals often cause interference with other communication services.

Radio-Frequency Interference (RFI) - Electromagnetic signals of a frequency associated with electromagnetic radiation which are coupled to a conductor either directly or as with an antenna.

Rated Temporary Overvoltage (TOV) Withstand Level - Combination of magnitude and duration of a temporary overvoltage that the SPD can withstand without changes in characteristics or functionality, as declared by the manufacturer.

Reactance - A physical property of a circuit component that tends to hinder the flow of alternating current.

Reactive power - The out-of-phase component of the total volt-amperes in an electric circuit that represents the power required to provide a magnetic field to drive reactive loads, like motors, in a circuit. Expressed in VARs (volt amperes reactive).

Real power - The in-phase component of volt-amperes in an electric circuit that does useful work, expressed in watts or kilowatts.

Receiver - the part of a communications system which converts electrical waves into visible or audible form.

Recovery voltage - The voltage that occurs across the terminals of a circuit interrupting device when it interrupts the flow of electricity.

Redundancy - The addition of extra components in an electrical power system that provide backup in the event of loss of any of those components.

Reflection - The return wave created when a traveling wave encounters load, a source, or a junction that has a change in line impedance.

Regulation - A term used to describe the action of holding a constant electrical value in the face of fluctuations.

Relay - An electromagnetic device that interprets input conditions (which reflect the operation of another piece of equipment) in a prescribed manner, and after specified conditions occur, responds to cause contact operation or similar abrupt change in a circuit controlling the equipment.

Remote Emergency Power Off  (REPO) - A device that permits the total shutdown of electrical power devices from an exit, in the event of an emergency. 

Resistance - A term describing the opposition of elements of a circuit to alternating or direct current. Symbol is R.

REPO (Remote Emergency Power Off) - A device that permits the total shutdown of electrical power devices from an exit, in the event of an emergency. 

Resistor - A discrete electronic component designed to produce a dc voltage drop when dc current passes through it.

Resonance - The condition that occurs when the capacitive reactance equals the inductive reactance of a circuit.

Response time - The time required, after the initiation of a specified disturbance to a device or system, for an output to reach a specified value.

Response Time (Varistor) - The time between the point at which the wave exceeds the clamping voltage level and the peak of the voltage overshoot. For the purpose of this definition, clamping voltage is defined with an 8/20 m current waveform of the same peak current amplitude as the waveform used for this response time. Ref. IEEE 100.

RFI (Radio-Frequency Interference) - Electromagnetic signals of a frequency associated with electromagnetic radiation which are coupled to a conductor either directly or with an antenna. 

Ride-through - The ability of a power conditioner, specifically a UPS, to continue to supply power to critical loads when the utility has discontinued power.

Ripple - An alternating current (ac) component on a direct current (dc) voltage resulting from incomplete filtering, usually associated with the ac component that appears on the output of the dc power supply.

RMS - See root means square

Rolling blackout - An unplanned outage by the utility that could be caused by nature or demand exceeding generation where the utility’s system systematically collapses. 

Root Means Square (RMS) - Used for ac voltage and current values that equal the square root of the average of the squares of all the instantaneous amplitudes occurring during one cycle. Referred to as the effective value of ac because it equals the value of ac voltage or current that causes the same amount of heat produced in a circuit containing only resistance from a dc voltage or current of the same value. In a pure sine wave the RMS value equals 0.707 times the peak value and the peak value equals 1.414 times the RMS value. The normal home wall outlet, which supplies 120 V RMS, has a peak voltage of 169.7 V.

Rotating field - The electrical field that develops in a multiphase generator caused by the varying currents flowing through parts of a stator winding.

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S

Safety Ground - A conductive path that bonds all cabinets and conductor shields to the power-source ground.

Sag (also called dip or voltage sag) - An RMS reduction in the ac voltage, at the power frequency, for durations from a ½ cycle to a few seconds. Ref. IEEE 1100.

SCC - See Short Circuit Current 

Scheduled outage - An outage that results when the utility (or facility) deliberately removes from service at a selected time a component of the power system, usually to allow construction, maintenance, or test.

SCR - See Silicon Controlled Rectifier

Secondary circuit - The distribution circuit on the low-voltage side of a transformer (usually 120/240 V).

Semiconductor - An electronic conductor (e.g., silicon, selenium, or germanium) with a resistivity between metals and insulators that allows current to flow through it normally via holes or electrons.

Series capacitor - In a power system, a capacitor that compensates for voltage drop along a transmission line and reduces the impedance of the transmission line to allow more power to flow in it.

Service drop - The lines running to the customer’s house that usually includes two 120-V lines and a neutral line, from which the customer can obtain either 120- or 240-V power. When these lines are insulated and twisted together, they become triplex cable.

Service entrance equipment - The main control and means of disconnection for the supply of electricity to a building that usually contains circuit breakers, switches, and fuses. 

Shielding - Protective coating that helps eliminate electromagnetic and radio frequency interference.

Short Circuit - An accidentally established connection between two points in an electric circuit, as when a tree limb or an animal bridges the gap between two conductors. This will cause an overload of current on the line, causing damaged lines, blown fuses, and the operation of protective devices such as reclosers and circuit breakers.

Short Circuit Current (SCC) - The current which the test set up (surge generator, coupling circuit, back filter, connecting leads) can deliver at the terminals where the SPD under test will be connected, with the SPD replaced by bonding the two lead terminals. (Also sometimes abbreviated as “SCI”) (SC definition)

Short Circuit Current Rating (SCCR) - The capacity of current with which the tested SPD can withstand at the terminals where connected, without breaching the enclosure in any way. Part of a UL 1449 Listing Classification (Section 37.3), as required by NFPA 70, Section 285.6. (NEC 2002)

Short-duration variation - A variation of the RMS value of the voltage from nominal voltage for a time greater then ½ cycle of the power frequency but less than or equal to 1 minute. Usually further described by a modifier indicating the magnitude of the voltage variation. (e.g., sag, swell, or interruption) and possibly a modifier indicating the duration of the variation (e.g., instantaneous, momentary, or temporary).

Shunt - A device having appreciable resistance or impedance connected in parallel across other devices or apparatus and diverting some (but not all) of the current from it. Ref. IEEE 100.

Shunt filter - A filter connected in parallel across a device or circuit to filter out undesirable signals.

Signal - A visual, audible, electrical, or other representation that conveys confirmations, or an electronic wave that embodies information.

Signal-To-Noise Ratio (SIN) - The ratio of desired signal level to the undesired noise level, expressed in dB.

Silicon avalanche diode - A semiconductor device that normally acts as an open circuit but changes to a short circuit when the trigger voltage exceeds a certain amount.

Silicon Controlled Rectifier (SCR) - A device that acts as an electronic dc switch when triggered to conduct by a pulse or a gate signal, and cuts off the flow of electricity by reducing the main current bellow a predetermined level (usually zero).

SIN - See Signal-To-Noise Ratio

Sine Wave - A fundamental waveform produced by periodic oscillation that expresses the sine or cosine of a linear function of time or space, or both.  More on sine wave tracking technology.

Single Phase - A line that carries electrical loads capable of serving the needs of residential customers, small commercial customers, and streetlights. It carries a relatively light load compared to heavy-duty three-phase constructs. With a three-phase source: one or two phase conductors. With a single-phase source: a single output that may be center-tapped for dual voltage levels.

Single-phase condition - An unusual condition where one phase of a three-phase system is lost, causing unusual effects on lighting and other loads.

Single-Point Ground - The practice of tying the power neutral ground and safety ground together at the same point, thus avoiding a differential ground potential between points in a system. More information>>

Sinusoidal - A waveform that can be represented by a sine function.

Skin effect - The tendency of a high frequency current flowing in a conductor to flow near the surface of the conductor. The current may be a radio frequency or a transient surge current.

SPD disconnect - A device for disconnecting a surge protective device (SPD) from the system in the event of SPD failure. It is to prevent a persistent fault on the system and to give visible indication of the SPD failure. Note: At least three functions are needed for SPD disconnectors: protection against thermal problems (such as thermal runaway on varistors), protection against internal short circuits and protections against indirect contact. These functions may be achieved by one or several devices. They may be used in the SPD circuit or in the mains. (IEC TC37)

Soft-start circuit - Circuitry that limits the initial power demand when an UPS is operated in emergency mode and commercial power comes back on. It also controls the rate at which the UPS output increases to normal.

Solid-state - Describes an electronic device whose electrical functions are performed by semiconductors (as opposed to components that conduct in a vacuum or gas, such as tubes) and otherwise completely static components, such as resistors and capacitors.

Spark gap - Any short air space between two conductors electrically insulated from each other; or a device that depends on a spark gap for its operation.

SPD - See Surge-Protective Device

Spike (or impulse, switching surge, lightning surge) - These terms refer to a voltage increase of very short duration (microsecond to millisecond). Spikes may be caused by lightning, switching of heavy loads, and/or short circuits or power system faults.

Standby current - The current flowing in any specific conductor when the SPD is connected as intended to the energized power system at the rated frequency with no connected load. Ref. IEEE 100.

Standby power supply (SPS) - See uninterruptible power supply.

Static switch - A solid-state device that opens and closes circuits without the use of moving mechanical parts.

Suppressed voltage rating (UL) - A discreet rating per UL 1449 standard, signifying the rounded up average clamping voltage of an SPD when subjected to the Measured Limited Voltage test. Said test consists of one 6kV 500A, 8/20µs waveform, followed by ten positive, ten negative, 6kV 3kA 8/20µs waveform duty cycle impulses, and one final 6kV 500A, 8/20µs waveform (60 second intervals). Any device with a deviation of 10% or greater from the first to last impulse is considered failed, per the standard.

Surge - A surge is a transient voltage or current with a duration of a few microseconds.

Surge arrester - A protective device for limiting surge voltages by discharging or bypassing surge current, and it also prevents continued flow of follow current while remaining capable of repeating these functions. Ref. IEEE 100.

Surge-protective device (SPD) – Is the generic term used to cover both Surge Arresters (including secondary surge arresters) and Transient Voltage
Surge Suppressors (TVSS) devices. An SPD is a non-linear protective device
for limiting surge voltages on equipment by discharging, bypassing, or
diverting surge current; it prevents continued flow of follow current and is
capable of repeating these functions as specified.

Surge response voltage - The voltage profile appearing at the output terminals of a protective device and applied to downstream loads, during and after a specified impinging surge, until normal stable conditions are reached. Ref. IEEE 100. (See also, clamping voltage)

Surge suppressor - See SPD.

Sustained - When applied to quantity the duration of a voltage interruption, refers to the time frame associated with a long duration variation (i.e., greater than 1 minute).

Swell - See momentary over-voltage.

Switched-mode power supply - A power supply that uses electronic components to convert ac power into high-frequency dc power. 

Switchgear - A group of switches, relays, circuit breakers, etc., that controls the distribution of power to other distribution equipment and large loads. Also, substation equipment designed and operated to switch electrical circuits and interrupt power flow.

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T

T1 - AT&T term for a digital carrier facility used to transmit a formatted high speed digital signal. See Leased Line.

Temporary Overvoltage (TOV) - An oscillatory over-voltage that is undamped or slightly damped.

Three phase - An electrical system with three different voltage lines or legs, which carry sine-wave waveforms that are 120° out-of-phase from one another.

Thermal runaway - An operational condition when the sustained power loss of an SPD exceeds the dissipation capability of the housing and connections, leading to a cumulative increase in the temperature of the internal elements culminating in failure. (IEC TC37)

Thyristor - A semiconductor bistable switch (with on and off states) that operates uni-directionally or bi-directionally. A three-terminal device (a controlled rectifier) or a two-terminal device (diode may trigger it).

Total demand distortion (TDD) - The ration of the root mean square (rms) of the harmonic current to the root mean square value of the rated of the rated or maximum demand fundamental current, expressed as a percent.

Total harmonic distortion (THD) - The ratio of the root mean square (rms) of the harmonic content to the root mean square value of the fundamental quantity, expressed as a percent of fundamental, that describes a wave-shape change caused by the presence of multiples of the fundamental frequency of the ac power. The square root of the sum of the squares of the rms harmonic voltages or currents divided by the rms fundamental voltage or current.

Transfer switch - A switch that transfers load from one source to another.

Transformer - A device used for changing the voltage of an ac circuit and/or isolating a circuit from its power source.

Transient - An electrical event of a nonrepetitive nature. the term is used interchangeably with the term "impulse"; however, the term relates more to the intermittent occurrence of "surges" and "sags". A short duration, fast-rise-time voltage caused by lightning, large motors starting, utility switching operations, and other appliances switching.

Transient response - The ability of a power conditioner to respond to a change in voltage or power.

Transient step load response - The ability of a power conditioner to maintain a constant output voltage when sudden load (current) changes occur. 

Transient voltage surge suppressor (TVSS) - A surge protective device intended for connection electrically on the load side of the main overcurrent protection in circuits not exceeding 600 volts RMS. Ref. UL 1449.

Transverse mode noise (normal mode) - An undesirable voltage that appears from line to line of a power line.

Trip-out - A disconnection of an electric circuit that occurs when the circuit breaker has opened, putting the line out of service; usually refers to an automatic rather than a manual action.

Triplen harmonics - Odd multiples of the third harmonic, which deserve special attention because of their natural tendency to add to each other.

Two-port SPD - A surge protective device (SPD) with two sets of terminals, input and output. A specific series impedance is inserted between their terminals. Note – The measured limiting voltage may be higher at the input terminals than at the output terminals. Therefore, equipment to be protected must be connected to the output terminals. Ref. IEEE TC37.

Traverse-mode noise - Often used as a synonym for normal-mode noise, it more clearly relates to noise that is the result of the conversion of common-mode noise to normal-mode noise after it passes through a transformer.

TVSS - See transient voltage surge suppressor

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U

UHF (Ultra High Frequency) - Off-air television channels 14-83.

UL - The abbreviation for Underwriter's Laboratories, Inc., a Nationally Recognized Testing Laboratory headquartered in the United States.

UL 1449 (Underwriters Laboratories Standard Number 1449) - A nationally recognized set of guidelines used by UL to evaluate transient voltage surge suppressors.

Undervoltage - Like a sag, but for a longer period of time: over 2.5 seconds.

Uninterruptible power supply (UPS) (see also motor-generator) - Non-mechanical (static) uninterruptible power supplies can provide protection against all power disturbances. An on-line or "true" UPS converts the utility ac power to dc and uses the dc to charge a battery and to power an inverter that delivers power to the critical load. An off-line UPS, more properly called a standby power supply (SPS), supplies the utility power directly to the critical load and transfers the load to a battery-powered inverter to supply power during outages.

UPS - See uninterruptible power supply

Utility power --Alternating current (ac) supplied to the user by the (usually commercial) electrical utility. May be subject to spikes, sags, swells, electrical noise, and outages.

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V

Varistor - A semiconductor device whose resistance varies with the applied voltage.

VHF (Very High Frequency) - Off-air television channels 2-13.

Voice-grade line - A telephone line that is capable of carrying analog voice-frequency signals.

Volt - Electrical unit of measure (current x resistance).

Voltage - Electrical pressure, or electromotive force (emf). The force that causes current to flow through a conductor, expressed as a difference of potential between two points, since it is a relational term. Connecting both voltmeter leads to the same point will show no voltage present, although the voltage between that point and ground may equal hundreds or thousands of volts. Thus, most nominal voltages are expressed as phase to phase or phase to neutral. The unit of measurement is volts and the electrical symbol is V.

Voltage distortion - Any change from the nominal voltage sine waveform.

Voltage interruption - Disappearance of the supply voltage on one or more phases. Usually qualified by an additional term indicating the duration of the interruption (e.g., momentary, temporary, or sustained).

Voltage-limiting type SPD - A surge protective device (SPD) that has a high impedance when no surge is present, but will reduce it continuously with increased surge current and voltage. Common examples of components used as nonlinear devices are: varistor and suppressor diodes. These SPDs are sometimes called ‘clamping type’. Note – A voltage-limiting device has a continuous V versus I characteristic. Ref. IEC TC37.

Voltage regulator - Voltage regulators control the output voltage, eliminating voltage sags and swells in the input voltage that last from 15 milliseconds to one-half second. They are typically relatively inexpensive feedback-controlled transformers.

Voltage-switching type SPD - An SPD that has a high impedance when no surge is present, but can have a sudden change in impedance to a low value in response to a voltage surge. Common examples of components used as nonlinear devices are: spark-gaps, gas tubes, thyristors and triacs. These SPDs are sometimes called “crowbar type”. Note – A voltage-switching device has a discontinuous V versus I characteristic. Ref. IEC TC37.

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W

Watt (power) - The unit of power that equals 1 joule per second. The mathematical relationship, between watts, volts and amperes is wattage = ampere x voltage. 

Watt-hour - The amount of power used by one watt in one hour.

Watt-hour meter - An electric meter that measures and registers the energy (kilowatt-hours) delivered to a circuit.

Waveform - A graph of a wave that shows its shape and changes in amplitude with time.

Waveform distortion - A steady-state deviation from an ideal sine wave of power frequency principally characterized by the spectral content of the deviation.

Wye - A wye connection refers to a polyphase electrical supply where the source transformer has the conductors connected to the terminals in a physical arrangement resembling a Y. Each point of the Y represents the connection of a hot conductor. The angular displacement between each point of the Y equals 120. The center point provides the common return point for the neutral conductor.

Wye-delta - Transformer connection with a wye primary and delta secondary.

Wye-wye - Transformer connection with a wye primary and wye secondary.

Z

Zero sequenced - All three phases of a power system intersect the zero axis at the same time.

Zero signal reference - A connection point, bus, or conductor used as one side of a signal circuit that may or may not be designated as ground; sometimes referred to as circuit common.

Zigzag transformer - A special type of transformer used to change the phase angle of the transformer primary. 

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