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Audio/Video System Power Management -Design Techniques
Audio/Video SystemPower Management
-Design Techniques
CinemaSource Technical Bulletins. Copyright 2002 by CinemaSource, Inc.All rights reserved. Printed in the United States of America.
No part of this bulletin may be used or reproduced in any manner whatsoever without written permission,except in brief quotations embodied in critical reviews.
CinemaSource is a registered federal trademark.
For information contact: The CinemaSource Press, 18 Denbow Rd. Durham, NH 03824
CinemaSource ,18 Denbow Rd., Durham, NH 03824
cinemasource.com800-483-9778
Chapter 1: Circuit Level Solutions for Transients• Blocking Transients --------------------------------------------------------------- Page 4• Diverting Transients --------------------------------------------------------------- Page 4• A new transient device for telecom -------------------------------------------- Page 7
Chapter 2: Eliminating EMI/RFI Interference• Understanding EMI/RFI ---------------------------------------------------------- Page 8
Chapter 3: Eliminating Voltage Fluctuations• Voltage Regulation Techniques ------------------------------------------------- Page 9
Chapter 4: Eliminating Power Dropouts• Uninterruptable Power Supplies ------------------------------------------------- Page 11• Calculating KVA Load ------------------------------------------------------------- Page 12
Glossary: • Power management terms ------------------------------------------------------ Page 13
ACKNOWLEDGEMENTS: The author wishes to thank the following people and companies for helping supplying material for this design guide
• Leviton Corpor ation (diagrams and text)• TrippLite Corpor ation (diagrams and text)
• PC Power Protection, HWSams and company (diagrams and text)• Bob Whitehead, Whitehead Triangle Foundation (research assistance)
• Television Engineering Handbook by Blair Benson, McG raw-Hill, Inc .• George Gavutis (research assistance)
• Jamen Towle (research assistance)
Audio/Video System Power Management - Design Techniques
4Chapter One: Circuit Level Solutions
CHAPTER ONE:Circuit LevelSolutions For
Transients
The first and foremost duty of any good piece ofpower management device is to protect theequipment connected to it from damaging transients.
The object of transient protection is to keep incoming highvoltage impulses away from semiconductors and othervoltage sensitive devices. Electrical engineers typicallyuse two standard techniques to accomplish this protection.
Blocking
The first is to electrically block the incoming transient.Because transients are very fast in nature, they can bedescribed as high frequency phenomena and the standardmethod to block high frequencies is to use an inductor.Inductors are coils of wire wound around permeable coresthat exhibit the curious phenomena of impeding highfrequencies but allowing low frequencies (such as 60 hzpower) to pass straight through. Engineers generallycombine inductors with capacitors to form "low pass
filters". These are even more adept at reducing highfrequency signals because they include a low impedancepath to ground. If you look at our schematic of amultistage surge (transient) suppressor below, you will seea typical low pass filter construction.
Diverting
The second technique used to dispose of transients, is todump them to directly to ground (or neutral) before they goany further. Engineers often refer to this category oftechniques as "diverters" and have many components attheir disposal to accomplish the task. Our table on thenext page shows the four most popular devices. What isinteresting is that all four have characteristics that areuseful for diverting transients, but none alone is ideal forthe job. In order to see why, lets take a quick look at each.
The first diverter to consider is the popular Metal Oxide
Multistage T ransient Suppression Circuits Employ Several Varieties OfTransient Suppressors
120 VAC Wave Form with T ransients
5Chapter One: Circuit Level Solutions
Varistor (MOV). MOVs find their way into virtually all surgesuppressors because they are effective and inexpensive.The problem is: many of the inexpensive "strip-type" surgesuppressors use just one or two MOVs and no otherdevices. This, of course, is better than no transientprotection at all, but constitutes a pretty minimal level ofprotection.
MOVs are voltage sensitive devices that are capable ofdumping lots of current (up to 6500 amps) but suffer fromseveral drawbacks. First, they are relatively slow torespond to high voltage transients and require impulses inexcess of 300 volts to trigger. Another thing: they candegrade over time and use. In fact, with just a few strongtransients, some MOVs can short and then burn
completely open. This will not damage the equipmentconnected to the surge suppressor (the MOV has takenhit, just like it's supposed to), but the protection capabilitydisappears completely. The result is that you have a surgesuppressor strip that still works fine, but offers NOprotection capability. (Chances are you have one of thosestrip-type surge suppressors in you house right now withan MOV that is non-functional.)
Another component sometimes used in surge suppressorsare gas discharge tubes. These components are smallglass vials filled with inert gas (Helium or Argon) and arecapable of discharging massive amounts of current(20,000 + amps). These devices would be ideal fortransient voltage suppression except that they turn on
6Chapter One: Circuit Level Solutions
A New Transient Protection Devicefor Telecom Equipment
Historically Metal Oxide Varistors (MOVs) have beenused to protect the “tip” and “ring” lines of telephoneequipment. The problem is that MOVs they actrelatively slowly (10-20 Nanoseconds), exhibit highcapacitance and tend to deteriorate with use.
A new transient protection device called Sidactorshave been developed. Sidactors ion implant devicesthat use thyristor technology in an ultrafast, crowbarcircuit topology. They are virtually immune toovervoltage transients and can divert over 500Amps.They also exhibit ultralow capacitance and provideyears of use wuth no degradation. Sidactors are nowmaking their way into premium power conditioningproducts. Tripplite, for example, uses sidactors intheir Isobar models.
relatively slowly (1usec or so). This means a highvoltage impulse may be fast enough to rip rightby before any diverting action gets taken.
Specially-built semiconductor devices are alsoavailable for transient suppression. AvalancheZener Diodes and Thyristors are the two mostpopular. These devices offer ultra-fast switchingtimes (in the Picosecond range, 1000 timesfaster than MOVs) and very sensitive to appliedvoltages, some can turn on with a transient ofjust a few dozen volts). The drawback to thesedevices is that they aren't capable of dumpingthe massive currents that the MOV and gas-tubedevices are. Electrical designers typically use thesemiconductor devices physically downstreamfrom the MOV and gas-tube components as anextra measure of protection.
Take a look again at our multistage surgesuppressor schematic. You will see many of thecomponents we just discussed employed in amultilayered design that calls upon the strengthsof each protective device.
Images Courtesy of MacMillan, Inc.
7Chapter One: Circuit Level Solutions
One important problem that rises whenaddressing transient suppression is the point atwhich transients occur along the sine
waveform. Because of the random nature oftransients, they can occur at any time and can showup at any point on the AC power waveform. As youcan see in the top right diagram, surges that occur atthe lowest point on the waveform (270°) have moreroom to build up voltage. Although the devices tosuppress transients are working as they should, if thetransient occurs at just the wrong time excessivevoltage can be allowed to build and this can destroysensitive components if it is allowed into electronicequipment.
In order to eliminate this effect, electrical engineersuse a technique called “Sine Wave Tracking”. Sinewave tracking effectively means that the surgesuppression devices operate along the ACwaveform.This keeps damaging transient voltages tomuch lower levels than simple clamping techniques.
One of the popular methods designers use to trackthe sine waveform involves Ferrite Rod-CoreInductors. This allows standard transient suppressiondevices (MOVs, Gas tubes, Zener diodes, etc.) tooperate regardless of where the surge occurs on thewaveform. In effect, the transient protection devicesride the AC waveform operating as if they wereferenced to ground. The diagram on the bottom rightillustrates this effect. More sophisticated transientsupressors use hybrid circuits. These supressors aregenerally much more expensive but are very effective.
Tripplite Isobar series of surge suppressors offer sinewave tracking technology.
UnderstandingSine WaveTracking
Images Courtesy of TrippLite, Inc.
Hybrid Component SurgeSuppressor Circuit
8Chapter Two: Eliminating EMI/RFI Inter ference
EMI/RFI signals are low-amplitude high frequencysignals that are generated by noise sources alongthe distribution path to your home theater room. This
interference can come from local television and radiostation broadcasts, local industrial manufacturingprocesses, atmospheric phenomena and many, manyother sources. Because EMI/RFI usually consists of highfrequency signals, the general way to eliminate them iswith a high frequency filter. This is usually included in thebank of transient suppression devices located in the front-end of a power management device.
Another place to look for EMI/RFI filtering is in the back-end of power conditioners. The best ones have separatefilters connected in series with each component outletjack. Here the filters perform the same function as in the
front-end (blocking high-frequency noise) but with adifferent intent. Instead of blocking noise from travelingprimarily TO each A/V component, they block noise fromtraveling BETWEEN components. The reason this isnecessary: many A/V components are very noisythemselves. Anything that has a motor in it, for example,can put inductive spikes out on the line, and digitalequipment can radiate all sorts of digital clock-noise out.As a result noisy components should be isolated fromeach other and the only way to do that is with a powerconditioner with EMI/RFI filtering on the accessory outlets.Below we show a graphic borrowed from Tripplite thatillustrates the EMI/RFI isolation between the outputreceptacles.
CHAPTER TWO:Eliminating EMI/RFI
Interference
120 VAC Wave Form with EMI/RFI
Images Courtesy of TrippLite, Inc.
9Chapter Three: Eliminating Voltage
CHAPTER THREE:Eliminating Voltage
Fluctuations
Another power anomaly to consider is voltage levelfluctuations. Depending on the load, either in yourhouse or on the external power grid, AC power can
vary in voltage. You've undoubtedly heard about "brownouts", these are times when the load is so great that thevoltages sink, sometimes to less than 100 Volts. There aremany devices designed to regulate the incoming powerand output a constant 120VAC. Manufacturers generallycall these devices "voltage regulators" but you may findsome that refer to them as power conditioners or lineregulators.
Do you need one in your home theater? Well, it's up toyou. Most power supplies in consumer electronicsequipment will not be damaged by fluctuations in voltagelevel. In fact, most component power supplies aredesigned to expect it and deal with it. However, thesefluctuations in voltage can affect the performance of thedevice and a voltage regulator might be useful from thatstandpoint. The problem is that there is a wide spectrum of
devices that regulate AC power. For the standpoint of thisdocument there are two popular methods and one is farsuperior to the other:
The first used a method called “Tap-Switching”. Basicallythis is a transformer with a large number of secondarytaps that are selectively switched when the input voltagechanges. The output is a constant 120VAC. This type ofregulator is expensive to make but outputs nice clean ACsine waves.
The other type of regulator uses some called aFerroresonant transformer. This type of regulator is veryreliable and inexpensive but has the side effect ofoutputting a somewhat distorted waveform. (See theillustrations on the next page.) This output waveformwould work just fine for computer systems and otherdevices but would be ill advised in a home theater systembecause of the excessive harmonic noise introduced.
120 VAC Wave Form with Voltage
Tap-Switching Voltage Regulation Circuit
10Chapter Four: Eliminating Power Dropouts
AC Unregulated Input Wav eform AC Regulated Output Wav eform
Ferroresonant Voltage Regulator
11Chapter Four: Eliminating Power Dropouts
Any discussion of power management devices wouldbe remiss without a discussion about UninterruptiblePower Supplies (UPSs). These devices are simple
in philosophy; when power dropouts occur, a backupsupply kicks in to keep devices powered up. Like thevoltage regulators we discussed before, one could arguethat UPSs might be unnecessary for most home theatersbecause most A/V equipment can be powered down safelywith out losing data or memory presets. However, it shouldbe noted that some UPSs are more than just backupsources of power. Some also perform industrial strengthtransient protection, voltage regulation and have extensivewaveform shaping capabilities. Combined with backuppower capabilities, these functions guarantee a constantsource of very high grade power for any electrical system,home theater or not.
Below we show a diagram of how a standard UPS works.Basically the standby power is supplied by a batterywhose DC voltage is inverted to AC when the line power
drops out. The key to systems like this is to have a veryfast transfer switch so that the connected equipment iswithout power for a fraction of a second. Themanufacturers of UPSs boast quite a bit about their“Transfer Times”. Expect most to be in the 4 to 10 mSrange or lower.
Another type of UPS can be described as “Always On”. Asthe block diagram on the next page illustrates, there is notransfer switch in this type of UPS because the batterycontinuously feeds to the load. In effect, the load has twopower supplies, one from the incoming line and one fromthe UPS battery. If the line power drops out, the the UPSpower is still there and the connected equipment is thusnever starved. If you do decide to look into one of these“Always On” UPSs for your home theater, be prepared tospend a few thousand dollars. They generally have hugebatteries and are built for rugged use.
Block Diagram of astandard “Standby”
UPS system
CHAPTER FOUR:Eliminating Power
Dropouts120 VAC Wave Form
with Dropouts
12Chapter Four: Eliminating Power Dropouts
Calculating KVA Load for Your HomeTheater System
1) Decide which pieces of equipment you want to support with continuous power
2) List the nameplate ratings for all the supported equipment
3) If the power draw is in AMPs multiply this number by the nominal line voltage for total VA load.
4) If the power draw is in WATTs multiply this number by 1.4 for total VA load.
Example:
Home Theater system with 27” television = 300 watts, 300 x 1.4 = 420VA, TIVO = 150 watts x1.4 = 210 KVA and Receiver 1.2 A x 120 = 240 KVA. Total system draw is: 420 +210 + 240 = 870KVA
Block Diagram of ahigher end “AlwaysOn” UPS system
13Glossar y
Alternating Current – Electrical current which reversesdirection periodically, expressed in hertz or cycles persecond. Abbreviated AC.
Ampere – The quantitative unit measurement of electricalcurrent. Abbreviated Amp or A.
Amperage – A term synonymous with current; used indescribing electrical current.
Apparent Power – The load power as expressed in VA orkVA. This value is usually greater than real power orwatts, due to circuit reactance. This reactance causes thetiming between the voltage and current to vary. Devicesizing must be in accordance with Volts times Amps ratherthan in Watts, since Voltamperes is the apparent loadseen by the power-handling device.
Arc – Sparking generated when current flows betweentwo points of different potential, due to leakage throughthe intervening insulator.
Arrester – A device placed from phase to ground whosenonlinear impedance characteristics provide a path forhigh-amplitude transients.
Autotransformer – A transformer that uses common turnsfor both the primary and secondary windings, thusproviding no isolation for the input from the output.
Balance – A term used to describe the even distribution ofloads on the legs of a three-phase system.
Battery – A group of cells connected in such a way thatmore current and/or voltage is delivered than from onesingle cell.
Blackout – The total loss of commercial electrical power.Sometimes refers to the length of time that the powermust be off to bring the computer down. Usedsynonymously with Outage.
Branch Circuit – A discrete division of a load that isprotected by one fuse or breaker.
Breaker – Short for circuit breaker.
Brownout – A long-duration under-voltage condition,usually hours or days in length. Brownouts can be causedby heavy usage during peak hours, or they may beplanned as an energy conservation strategy.
Building Service Entry – That point where commercialpower enters the building.
Bus – A heavy, rigid conductor. Often equipped withscrews or some other means by which a number ofsmaller conductors can be connected to it. Also called abus bar.
Capacitance – A term referring to the electrical propertiesof a capacitor or to a circuit that displays capacitor-likebehavior.
Capacitor – A discrete electrical device which has twoelectrodes and an intervening insulator, which is called thedielectric.
Choke – A form of inductor which is constructed to allowdesirable frequency signals to pass while acting with highimpedance to other signals at some undesirablefrequency.
Circuit Breaker – A resettable device that responds to apreset level of excess current flow by opening the circuit,thereby preventing damage to circuit elements.
Clamping Voltage - The (peak) voltage occurring on aconductor, measured at the output of a Surge ProtectionDevice (the point connected to the load/device beingprotected), to either the ground or neutral conductor.
Coax – A cable constructed by using two concentricconductors separated by an insulator.
Core – The iron structure of a transformer around whichthe windings are wound. A choke also has a core butdoes not act as a transformer.
Core Saturation – That point at which the iron material,the core, of an inductor or transformer will no longerproduce more lines of flux when current flow through thewindings is increased.
CSA – An abbreviation for the Canadian StandardsAssociation. This is a Canadian safety assurance agencysimilar to the Underwriter ’s Laboratories.
Current – The flow of electricity in a circuit as express inamperes. Current refers to the quantity or intensity of
Power Management
Glossary
14Glossar y
electrical flow. Voltage, on the other hand, refers to thepressure or force causing the electrical flow.
Cycles Per Second – This term describes the frequencyof alternating current. Frequency is more properlydescribed using the term "hertz", which is synonymouswith cycles per second.
Decibel (dB) - One-tenth of a bel, the number of decibelsdenoting the ration of the two amounts of power being tentimes the logarithm to the base 10 of this ratio. (Power dB=10 Log 10 (Power out /Power in ), Voltage dB =20 Log10 (Voltage out /Voltage in )
Direct Current – Electrical current which flowsconsistently in one direction. Abbreviated DC.
Distortion – The waveshape of a signal that is not normalis distorted. Distortion is a term that describes abnormalwaveshapes.
Distribution – The way in which power is routed tovarious current-using sites or devices. Outside thebuilding, distribution refers to the process of routing powerfrom the power plant to the users. Inside the building,distribution is the process of using feeders and circuits toprovide power to devices.
Dropout – A total loss of voltage for a short period of time.
Electromagnetic Interference – A term that describeselectrically induced noise or transients. Abbreviated EMI.Ferroresonance – When an iron-core inductor is part of anLC circuit and it is driven into saturation, causing itsinductive reactance to increase to equal the capacitivereactance of the circuit, this action is calledferroresonance.
Ferroresonant Transformer –A transformer that uses theprinciple of ferroresonance to regulate the output voltage.
Filter – An electronic device which opposes the passageof a certain frequency band while allowing otherfrequencies to pass. Filters are designed to produce fourdifferent results. A high-pass filter allows all signals abovea given frequency to pass. A low-pass filter allows onlyfrequencies below a given frequency to pass. A bandpassfilter allows a given band of frequencies to pass whileattenuating all others. A trap filter allows all frequencies topass but acts as a high-impedance device to the tunedfrequency of the filter.
Flashover – Arcing that is caused by the breakdown ofinsulation between two conductors where a high currentflow exists, with a high potential difference between theconductors.
Fuse – A device that automatically self-destructs when the
current passing through it exceeds the rated value of thefuse.
Ground – A general term that refers to the point at whichother portions of a circuit are referenced when makingmeasurements. Power-systems grounding is that point towhich the neutral conductor, safety ground, and buildingground are connected. This grounding electrode may bea water pipe, driven ground rod, or the steel frame of thebuilding.
Grounded Conductor: Another name for the neutralconductor. A conductor which is intentionally ground-ed,either solidly or through a non-interrupting current limitingdevice.
Grounded Conductor–(NEC): The conductor that is usedto connect the equipment or the wiring systemwith a grounding circuit to a grounding electrode orelectrodes.
Ground Fault – An undesired path that allows current toflow from a line to ground.
Ground Loop – The condition of having two or moreground references in a common system. When two ormore grounds have a potential difference between them,current can flow. This flow of current is a new circuit orloop which can interfere with the normal operation of thesystem.
Harmonic – A frequency that is a multiple of thefundamental frequency. For example, 120 Hz is thesecond harmonic of 60 Hz, 180 Hz is the third harmonic,and so forth.
Harmonic Distortion – Excessive harmonic content thatdistorts the normal sinusoidal waveform is harmonicdistortion. This can cause overheating of circuit elementsand might appear to a device as data-corrupting noise.
Hertz – A term describing the frequency of alternatingcurrent. The term, hertz, is synonymous with cycles persecond. Abbreviated Hz.
Impedance – Measured in ohms, impedance is the totalopposition to current flow in a circuit where alternatingcurrent is flowing. This includes inductive reactance,capacitive reactance, and resistance. Symbol is Z.Impulse – A disturbance of the voltage waveform that isless than about one millisecond. Voltages can rise tohundreds or even thousands of volts in a very short periodof time. An impulse may be additive or subtractive.(Sometimes called a notch.)
Inductance – This term describes the electrical propertiesof a coil of wire and its resultant magnetic field when analternating current is passed through it. This interaction
15Glossar y
offers an impedance to current flow, thereby causing thecurrent waveform to lag behind the voltage waveform.This results in what’s known as a lagging power factor.
Inductor – A discrete circuit element which has theproperty of inductance. It should be noted that at veryhigh radio frequencies, a straight wire or a path on aprinted-circuit board can ct as an inductor.
Induced Current: (General) — Current in a conductordue to the application of a time-varying electromagneticfield.
Induced Voltage: (General) — A voltage producedaround a closed path or circuit by change in magnetic fluxlinking that path.
Induced Voltage: (Lightning Strokes) —The voltageinduced on a network or electric installation by an indirectstroke.
Inrush – A term used to describe the high-current demandof a device when it is initially turned on, due to a low loadimpedance before the device has reached its normaloperating value.
Inverter – The subassembly of a UPS that converts DCpower into AC power.
Isolation – The degree to which a device can separatethe electrical environment of its input from its output, whileallowing the desired transmission to pass across theseparation.
Isolated Equipment Ground - An insulated equipmentgrounding conductor run in the same conduit or racewayas the supply conductors. This conductor is insulated fromthe metallic raceway and all ground points throughout itslength. It originates at an isolated ground type receptacleor equipment input terminal block and terminates at thepoint where neutral and ground are bonded at the powersource. (This term is defined more specifically in the NEC(2), Section 250-74 and 250-75).
Isolation Transformers - Provides a local groundreference point. Attenuates common-mode disturbanceson thepower supply conductors.
Joule - The work done when the point of application of aforce of one newton is displaced a distance of one meterin the direction of the force. (A newton is that force whenapplied to body having a mass of one kilogram, gives it anacceleration of one meter per second squared.)
Leakage Current: (Health Care Facilities) - This is anycurrent, including capacitively coupled current, notintended to be applied to a patient but which may beconveyed from exposed metal parts of an appliance to
ground or to other accessible part of an appliance.
Linear Load - An electrical load device which, in steadystate operation presents an essentially constant loadimpedance to the power source throughout the cycle ofapplied voltage.
Junction Box – A metal box inside which electricalconnections are made. Also called a J-Box.
Kilohertz – A term meaning 1000 cycles per second.Abbreviated kHz.
Kilovoltamperes – Abbreviated kVA. Voltage timesamperage is expressed in kVA. Kilovoltamperes is the"Apparent Power," and can be found by dividing kilowattsby the power factor.
Kilowatts – Term for "Real Power," or the power actuallyused by the load.
LC – An abbreviation for the inductance and capacitancethat is used in the same circuit.
Lightning Arrester – A device used to pass largeimpulses to ground. It is vital that this device be placedupstream from the computer ground.
Line – A term used to describe a given condition betweenconductors of a multiphase feeder.
Line to Line – A term used to describe a given conditionbetween conductors of a multiphase feeder.
Line to Neutral – A term used to describe a givencondition between a phase conductor and a neutralconductor.
Load – Any electrical device connected to a power sourcemay be called the general term of "load."
Megahertz – A term for one million hertz (cycles persecond). Abbreviated MHz.
National Electrical Code – A set of rules and regulations,plus recommended electrical practices, that are put out bythe National Fire Protection Association. AbbreviatedN.E.C.
Neutral – One of the conductors of a three-phase wyesystem is the neutral conductor. Sometimes called thereturn conductor, it carries the entire current of a single-phase circuit and the resultant current in a three-phasesystem that is unbalanced. The neutral is bonded toground on the output of a three-phase delta-wyetransformer.
Noise – An undesirable signal, which is irregular yet
16Glossar y
oscillatory, that is superimposed on the desired signal.See Common-Mode Noise and Normal-Mode Noise.
Ohm – The unit of measurement for resistance (symbolR), impedance (symbol Z), and reactance (symbol X).
Ohm’s Law – The mathematical relationship betweenVolts, Amperes, and Ohms: Volts = Amperes times Ohms.
Oscillation – Generally used to mean an electricalphenomenon that produces a number of occurrencesabove or below a given instantaneous voltage level.
Outage – A long-term loss of voltage resulting from alocalized utility failure.
Overvoltage – Similar to a surge but for a longer period oftime, over 2.5 seconds.
Peak – The maximum instantaneous measurement of anelectrical event.
Phase – A term used to describe the timing between twoor more events tied to the same frequency.
Power – A general term which means the capacity fordoing work. In the electrical environment, this is usuallymeasured in watts.
Radio-Frequency Interference – Electromagnetic signalsof a frequency associated with electromagnetic radiation,which are coupled to a conductor either directly or as withan antenna. Abbreviated RFI.
Rectifier/Charger – A subassembly of a UPS thatperforms the function of converting the incoming AC intoDC for driving the inverter and charging the batteries.
Regulation – A term used to describe the action ofholding a constant electrical value in the face offluctuations.
Resistance – A term describing the opposition ofelements of a circuit to alternating or direct current.Symbol is R.
Resistor – A discrete electronic component designed toproduce a DC voltage drop when current passes throughit.
Residual (voltage) - The amplitude (level) that remainsafter a Surge Protective Device has attenuated the ini-tialtransient.
Root Mean Square – The square root of the arithmeticmean of the squares of a set of electrical amplitudes.Abbreviated RMS.
Safety Ground – A conductive path that bonds allcabinets and conductor shields to the power-sourceground.
Sag – A short-term RMS voltage decrease which exceedsan established upper limit for less than 2.5 seconds.
Secondary – The output winding of a transformer.
Shield – A conductive enclosure or barrier that preventselectrical interference from external sources.
Sine Wave – A fundamental waveform produced byperiodic oscillation that expresses the sine or cosine of alinear function of time or space, or both.
Single Phase – That portion of a power source whichrepresents only a single phase of the three phases thatare available.
Single-Point Ground – The practice of tying the powerneutral ground and safety ground together at the samepoint, thus avoiding a differential ground potential betweenpoints in a system.Shield: As normally applied to instrumentation cables, aconductive sheath (usually metallic) applied over the insu-lation of a conductor or conductors, for the purpose ofproviding means to reduce coupling between theconductors so shielded and other conductors that may besusceptible to, or that may be generating unwantedelectrostatic or electromagnetic fields (noise).
Shielding - Shielding is the use of a conducting barrierbetween a potentially disturbing noise source andsensitive circuitry. Shields are used to protect cables (dataand power) and electronic circuits. They may be in theform of metal barriers enclosures, or wrappings aroundsource circuits and receiving circuits.
Spike (pulse terms) - A distortion in the form of a pulsewaveform of relatively short duration superimposed on anotherwise regular or desired pulse waveform.
Surge – A short-term voltage increase that exceedsestablished upper limits for less than 2.5 seconds.
Surge Impedance - The ratio between voltage andcurrent of a wave that travels on a line of infinite lengthand of the same characteristics as the relevant line.
Static Charge - The electricity generated when twodissimilar substances come into contact. (Conveyor beltsare active producers of static electricity).
Swell - An increase in the AC voltage, at the powerfrequency, for durations from a half-cycle to a fewseconds.
17Glossar y
Tap – A terminal on a transformer winding.
Tap Switching – The action of changing from one terminalon a transformer winding to another, thereby changing theturns ratio of the device to maintain a desire voltagerelationship.
Three Phase – An electrical system with three differentvoltage lines or legs, which carry sine-wave waveformsthat are 120º out of phase from one another.
Total Harmonic Distortion – A term that refers to thealteration of a waveshape by the presence of multiples ofthe fundamental frequency of the signal. AbbreviatedTHD.
Transfer Switch – A device used to transfer the load of apower unit from itself to a bypass line.
Transformer – A device used for changing the voltage ofan AC circuit and/or isolating a circuit from its powersource.
UL – The abbreviation for Underwriter ’s Laboratories, Inc.,an independent United States product-safety assuranceagency.
Undervoltage – Like a sag, but for a longer period oftime; over 2.5 seconds.
Uninterruptible Power System – A power-conditioningand supply system that provides power during outages.Abbreviated UPS.
Volt – The quantitative measurement of electrical force orpotential; also called electromotive force.
Voltage Regulator – A circuit that has a constant outputvoltage when input voltage fluctuates.
Voltampere – The unit of measurement of apparentpower.
Watt – The unit of measurement of actual power.Watt: The unit of power in the International System of units(SI). The watt is the power required to do work at therate of 1 joule per second.
Withstand Current - The crest value attained by a surgeof a given wave shape and polarity that does not causedisruptive discharge on the test specimen.
Withstand Voltage - The specified voltage that, underspecified conditions, can be applied to insulation withoutcausing flashover or puncture.
Waveform – The graphic form of an electrical parameter.
Zero Signal Reference – The result of a properly installedground structure is a constant potential over a broad bandof frequencies between the devices that are part of thestructure. This highly desirable state us called the zerosignal reference, meaning the potential between points onthe ground reference is equal to zero over a broad rangeof frequencies.
18Glossar y
NOTES:
