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Audio/Video System PowerManagement

- Introduction

CinemaSource Technical Bulletins. Copyright 2001 by CinemaSource, Inc.All rights reserved. Printed in the United States of America.

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Audio/Video System PowerManagement - Introduction

Chapter 1: Understanding AC Power

• Volts and amps --------------------------------------------------------------------- Page 4• Typical residential wiring ---------------------------------------------------------- Page 5• Know your electrical outlets ------------------------------------------------------ Page 7• Isolated ground receptacles ----------------------------------------------------- Page 8

Chapter 2: AC Power Disturbances

• AC Power Basics ----------------------------------------------------------------- Page 9• Big Electricity ----------------------------------------------------------------------- Page 10• Understanding transients ------------------------------------------------------ Page 14• Understanding Harmonics ------------------------------------------------------ Page 16

Chapter 3: Power Management Through Sequencing• Understanding Power Sequencing --------------------------------------------- Page 17• Example of Power Sequencing ------------------------------------------------- Page 18

Chapter 4: Eliminating Ground Loops in Home Theaters

• What are Ground Loops ---------------------------------------------------------- Page 19• Ground Loop Elimination Techniques ----------------------------------------- Page 21• Grounding from the engineers perspective ---------------------------------- Page 22

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)

4Chapter One: Understanding AC Power

CHAPTER ONE:Understanding

AC Power

After being generated at regional power plants, ACpower usually makes its way to residential areasthrough a series of tall, girder-type metal towers and

high voltage (50,000+ volt) distribution lines. These highvoltage lines then feed into local utility substations wherethe high voltage is lowered via step-down transformers.The AC power is then distributed throughout residentialneighborhoods via 4800 volt lines and is stepped downonce again by telephone pole-mounted transformers to240/120V levels for household use. The diagram belowshows a schematic for the typical pole mountedtransformer.

It is beyond the scope of this guide to provide a detailedoverview of residential electrical installations and theNational Electrical Code, and for safety's sake, it is best toleave these details to a licensed electrician anyway if workis needed. However, the home theater builder should havea basic understanding of the principles involved in ACdistribution in the home so power in the home theaterroom can be managed properly.

Voltages and Amps

In new homes, most current building codes require that 20Amp circuits be installed to all normal usage outlets butyou may find 15 Amp circuits in some areas of the country.A 20 Amp circuit is capable of providing 2400 watts ofpower but you certainly don’t want to run that much onone circuit. (A circuit is several outlets that are allconnected in parallel to a single distribution feed from thebreaker box.) A good rule of thumb is to run circuits at80% of their peak capacity, or less, for continuous loads.For example, on a single 20 amp circuit you would wantno more than 80% x 2400 watts or 1920 continuous watts.

In retrofit situations, where the building is alreadycompleted, it is often necessary to tap into an existingcircuit for system power. Note that this should be doneonly if the circuit can provide enough amps to power theentire system and any other items connected.

Just a quick note about AC power voltages. You will find

How 4800 VAC Utility Line Power is Converted to120 VAC for Residential Use

120 VAC Wave Form


that electrical equipment is marked with a variety of powervoltages. Sometimes you will see 110V, 117V or 120Vmarked by the power cord or identified in the owner’smanual. In case you didn’t know, all these voltages indicatethe same 120 Volt feeds that are common throughout NorthAmerica. Why all the different voltages? Who knows, butthe utilities in North America are required to supply 120VAC+/- 5% to their customers and this is the number to keep inmind.

Typical Residential Wiring Schematic

Above we illustrate how the wiring circuits in a typicalbuilding are derived from the utility power coming in. In theaverage residential house, circuits will be a mix of 15 and20 amp circuits. In new construction, it is best to put amotorized screen and a projection TV on it’s own 20 Ampcircuit. The reason for this is not that screen and projectorsdraw so much continuous power (as matter of fact, most ofthe time they are drawing little power), it’s that duringstartup, any motorized device has a high “in-rush” currentdraw. In the diagram right, we show the power drawn by asmall AC motor during the first few milliseconds of use. Thisin-rush power draw is better sourced from a dedicatedcircuit than from one that is loaded with lots of otherdevices turning on at the same time.

Typical Residential and Small Business Building Wiring

For the first fewmilliseconds of use, many

devices draw a high amountof in-rush current


Residential/Small Business

Breaker BoxInternal Wiring

Image Courtesy of Leviton Corporation


Know YourElectricalOutlets

Any power management device that isconnected to a wall outlet relies on a solidreliable connection to the house's hot, neutral

and ground power circuits in order to work properly.In many houses, particularly older ones, you may findswitched hot and neutral lines, switched ground andneutral lines, or the ground connection missingentirely. Obviously any kind miswiring seriouslycompromises the safety of your entire system andcan be a safety hazard. We recommend that allhome theater owners test your wall outlets for theseanomalies, as matter of fact, test all the outlets inyour house. The device to do this is cheap and easyto use. Radio Shack sells a good outlet tester, partnumber 22-101 for $6.95. It tells you if there anyproblems with the wiring of your outlets via threediagnostic LEDs (See chart below). If you do findanything odd, call an electrician. Usually problemslike this are simple to fix and won't end up costing aton of money.

Radio Shack 22-101Outlet Tester


There’s a lot of talk about using Isolated groundreceptacles in high-end home theater installationsthese days.Isolated ground outlets are common in

commercial installations where critical measuring ordiagnostic equipment are used. The main difference betweenthem and standard receptacles is that their groundconnections are not bonded to the metallic conduit that theyare installed in. As the diagram below illustrates, acompletely separate ground connection travels through theconduit back to the power system ground. The result of thisconstruction is that the power wires, hot, neutral and groundare shielded back all the way to the power companytransformer. If one has a lot of noise sources in the buildingor locally, using isolated receptacles is a good method tokeep interference to a minimum. For most home applications,though, isolated ground receptacles are overkill. There simplyaren’t the noise sources in a home to warrant their use. Thisdoesn’t stop some installers from putting them in. however.They are becoming more common in full construction high-end home theaters.

About IsolatedGround

Receptacles

Isolated ground receptacleshave grounds that ar e

separated from the body of thereceptacles

9Chapter Two: AC Power Disturbances

If your home theater equipment could talk, it wouldprobably surprise you with a discussion about the poweryou feed it. "Listen, we really like it here," it might say,

"but you've got to do something about the stuff that comesout of your outlets. We want nice clean 120 volt sinewaves. The stuff you feed us is terrible!" Thankfully, A/Vequipment isn't capable of speaking it's mind (at least notyet) but the power situation referred to is probably true.Most AC power, no matter where you live, is far from ideal.Typically it comes into your house embellished with allsorts of junk, some of which can be down right hazardousto your equipment's health. The solution, of course, is toclean it up, but before we jump into "cleaning" part, let’stake a close look at what AC power is all about.

AC Power Basics

Utilities supply electrical power as alternating current,which is usually referred to as AC power. An oscilloscopeis an instrument used to create a graphic image of thegenerated line voltage. The figure above represents theimage an oscilloscope would show of a single 120 Volt ACcycle. Sixty of these cycles occur every second. However,rather than saying this AC frequency is 60 cycles persecond, it is referred to as 60 Hertz, which is usuallywritten in abbreviated form as 60 Hz.

The shape of AC power is called a sine wave. Notice thatin the first half-cycle above the line, the voltage level risesto 120 Volts. In the second half of the cycle below the line,the voltage is -120 Volts. So the voltage and currentalternate polarity each half cycle, hence the termalternating current, or AC.

The 120 Volt AC sine waves shown in the figure aboverepresent an ideal condition where perfect, consistent,stable AC power is shown. In the real world, this is seldomthe case. AC line power is subjected to all sorts ofunwanted disturbances that produce a variety of effects.

Love That Dirty Power

After being generated at regional power plants, AC poweris usually makes its way to residential areas through aseries of tall, girder-type metal towers and high voltagedistribution lines. These high voltage lines then feed intolocal utility substations where the high voltage is loweredvia step-down transformers. The AC power is thendistributed throughout residential neighborhoods via 480volt lines and is stepped down once again by telephonepole-mounted transformers to 240/120V levels forhousehold use.

Because of the long journey, lots of things can get addedto AC power. Local television and radio station broadcastscan be captured as radio frequency interference (RFI),industrial manufacturing plants can add all sorts oftransient spikes and electromagnetic interference (EMI),and lightning storms along the way can add large voltagespikes. As you can see in our "dirty" power diagram below,the resultant waveforms can be a mess arriving with amyriad of noise and glitches.

Because the integrity of AC power is of great concern tomany corporations, particularly those in the dataprocessing industry, a number of power quality studieshave been undertaken over the years. One of the mostcomprehensive was performed in 1972 by two researchersat IBM. They monitored several power-use sites for a totalof 3312 days and characterized typical AC power by"average days between disturbances". No doubt thefrequency of disturbances results will surprise you (seeour table next page). The type of disturbance that is ofgreatest concern to home theater owners is transientimpulses. This term refers to the spike-like glitches thatoccur whenever high energy events are transferred intoAC power lines. Often these events are man-made, suchas large motors turning on and off, or utility company gridswitching, but the real kickers come from naturalphenomena, such as local electrical storms.

CHAPTER TWO:AC Power

Disturbances


A significant AC power quality study was performed in 1972 by George Allen and DonaldSegall, two researchers at IBM. They monitored several sites around the country for power

line disturbances over a 3312 day period. The results are summarized in this table.

FREQUENCY OF POWER LINE DISTURBANCES

Typical Short Dur ation and Long Dur ation Power Line Disturbances


Big Electricity

Lightning, as you probably are aware, is a phenomenaalmost incomprehensible in magnitude. A lightning bolttypically consists of several million volts potential anddumps anywhere from 50,000 to 200,000 amps inapproximately half a second. But does it happen enoughin your locale to be concerned about? After all, it onlyseems like a few powerful thunderstorms a year come by?The National Weather Service collects data onatmospheric phenomena such as this and the frequency oflightning storms in your area can be seen on their annual"Isoceraunic Maps". Take a look at the one we showbelow. Do you remember hearing that manythunderstorms in your area last year? Most people don't,and keep in mind you don't even need to hear the storm tobe concerned, you just need to have one occur near themiles and miles of powerlines that are draped to yourhouse. As the diagram on the next page illustrates,iductive coupling from storms above can still producepowerful transients in power, phone and cable lines.

It Can Be Generated In Your House Too

Now that we have covered external sources of dirty power,let’s look inside the house for internal sources. Believe itor not, some of these can be worse than the externalones. For example, when it comes to transients, one ofthe worst offenders is an oil-fired furnace. If you have one,take a look at the burner assembly. On top of it you willsee a large metal box. This is the ignition transformer thatcreates the spark to ignite the fuel oil. When the furnacecycles on, this device can inject a pulse in the order of2000 volts or so right into the AC power line and this pulsecan be measured at any outlet connected to the samewiring circuit. Other offenders are motors of any sort:vacuum cleaners, refrigerators, sump pumps, etc., theseall create transients, noise and other EMI and feed themback into your house's wiring.




Powerful lightning strikes can consist of many strokes and many pulses per stroke

Image Courtesy of Panamax Corporation, Photo by Michael B. Wood


Transients may be thought of as “bundles of energy” ridingon the AC power sine wave; a potential seeking a pathwayto ground. Because they are of short duration with

discernible end points, they are usually described in terms of theirenergy content (Joules) rather than power (energy per unit time,Joules/sec or watts). Transient overvoltages, which may bepositive or negative in potential and be positioned at any phaseangle on the wave, originate from a variety of sources.

Types of Transients

Oscillatory or Ringwave Transients are characterized by fast risetime with oscillating exponential decay. They can be generated byinductive loads such as elevators, copiers, welders, airconditioning equipment, fuse clearing, motors and tools of alltypes. The Ringwave transient is generally the result of internalelectrical activity. Its amplitude an energy content are determinedby its source and environment. Impulse or UnidirectionalTransient are fast rise time, slower decay, high energy pulses.Sources include lightning, utility grid switching, industrialaccidents. Sometimes called a spike, the Impulse transient ifoften defined by both a voltage and a current wave-form. Thistakes into account the open circuit of pre-arc condition (thevoltage waveform) and the short circuit or a condition (the currentwaveform) of impulse behavior.

When Can Transients Strike?

Since transients originate with interruptions in current flow, andalso by magnetic coupling, they can occur any time of the day ornight. In some cases, it may be necessary to identify the sourceof transients within a facility. To do this, some sort of power linemonitor is generally used. A number of models are available torecord the amplitude, duration and frequency of transient activityon the line being checked. If the choice is made to use a powerline monitor, sufficient time must be allowed to build an accuratepicture of electrical disturbances. Generally, a period of 3 to 6weeks is needed to establish a useful base of data.

Measuring Transients:

As a result of their brief duration, transient pulses can be veryhigh in frequency. That means a typical volt meter won’t measurethem properly because of its limited upper-frequency responsecapability. What’s needed is an oscilloscope or power line monitorwith a very fast sampling rate and high upper-frequency

UnderstandingTransients

Images Courtesy of Leviton Corporation


response, meaning a mini-mum of 100-150 MHzbandwidth.

How transients affect equipment

Microprocessor-driven devices can be found in practicallyevery commercial, industrial and residential setting.Needsless to say, home theater equipment is loaded withmicroprocessors these days and is especially sensitive totransient volt-age surges because of certain characteristicscommon to integrated circuits and IC chips.

Trace Spacing — Most of the spacing betweencomponents of an integrated cir-cuit is substantially lessthan the thickness of ahuman hair. The methodsfor producing power andsignal circuit paths (calledtraces) in an integratedcircuit also producemicroscopic self-supportingstructures. These structurescan become overheatedand then sag when hit withsurges. Once this happens,tracks which should beisolated can touch, therebycausing internal shorts thatrender the IC useless.

Exceeding OperatingVoltage Limits— In strivingto extend operating time,computer manufacturers are designing machines withlower operating voltages to allow the use of lower-voltagebatteries. Many older computers use approximately 5 VoltDC logic levels. Current designs use 3.3 Volt DC, andfuture units will use even lower voltages. The result is thatany spike above 3.3 Volts that makes its way into the logicIC’s can cause disruption or permanent damage.

Confusing Computer Logic— The internal heart of acomputer is called the clock. Faster computers have fasterclock speeds so, for example, a 33MHz machine is fasterthan a 16 MHz unit. At these speeds, electrical noisebecomes a threat. When noise enters a computer, it canmimic clock frequencies, and can be mistaken for a validlogic command. When the computer acts on this falselogic command, the keyboard can lock up, or some otherundesired action occurs. Also noise can cause thecomputer to miss valid operating commands or clockpulses. If that happens, the computer creates erroneousoutput, or no output at all.

How Transients damage equipment

The most common failures produced by transient withinelectronic devices are disruptive, dissipative, anddestructive.

Disruptive effects — are usually encountered when atransient enters the equipment by inductive coupling. Theenergy source for this inductive coupling can act on thedata lines. The electronic components then try to processthe transient as a valid logic command. The result issystem lock-up, malfunctions, erroneous output, lost orcorrupted files, and a variety of other undesirable effects.

Dissipative effects — areassociated with repeatedstresses to IC components.The materials used to fab-ricate IC’s can withstand acertain number of repeatedenergy level surges, but notfor long. Long-termdegrada-tion begins, andsooner or later, the devicefails to operate properly forno apparent reason.Actually, the failure is dueto the cumulative build-upof transient-createdstresses which haveresulted in arc-overs,shorts, open circuits, or

semiconductor junction failures within the IC.

Destructive effects — include all conditions wheretransients with high levels of energy cause equipment tofail instantaneously. Very often, there is actual physicaldamage apparent, like burnt PC boards, melting ofelectronic components, or other obvious faults. Destructiveeffects can occur when noise pulses are too fast forpower-supply regulator circuits to respond by limitingtransient energy to acceptable levels. Also, transients onthe power line may subject electronic components withoverwhelming energy levels. For example, componentslike rectifier diodes can fail immediately when their PeakInverse Voltage rating (PIV) is exceeded. PIV diode rat-ings in a well-designed computer can be in the 1 kV - 1.5kV range. Transients on AC lines can easily exceed 1,500Volts, and often by a wide margin.



UnderstandingHarmonics

Harmonics are voltages or currents with frequenciesthat are integer multiples of the fundamental powerfrequency. In the case of the AC powerline environ-

ment, the fundamental frequency is 60 Hz. The secondharmonic would be 120 Hz; the third harmonic 180 Hz,and so on. Harmonics occur on the AC pow erlinewhenever the sine wave shape is distorted.The figure above shows the pure AC sine wave, the linevoltage with harmonics, and the line current withharmonics as they would appear on an oscilloscope.Harmonics are caused by non-linear loads within an ACpower distribution system. Linear loads, such as aresistive heating element, do not cause harmonicdistortion and the AC current that flows will be a rela-tivelypure sine wave. However, if the load is non-linear, drawingshort bursts of current each cycle, the current wave shapewill be non-sinusoidal and har monic currents will flow. Thetotal resultant current will be a combination of thefundamental frequency plus each of the harmonics.

Switching Power Supplies Generate Harmonics

Power supplies that use semiconductors to switch the linecurrent on and off abruptly during each AC cycle generateharmonic currents. Switching Power Supplies, sometimescalled Electronic or Solid State Power Supplies, are usedin a wide variety of modern electronic equipment found incommercial and residential facilities. Solid state ballastsare also used in fluorescent lighting systems. Of particularconcern is the fact that all personal computers, printersand microprocessor-based equipment use switching typepower supplies.

Problems Caused By Harmonics

Line voltage harmonics can not only radiate interferenceinto telephone and communication systems they can travelaround a branch circuit to all the devices connected. Theycan cause over loading and malfunctions in circuitsdesigned to primarily handle 60 Hz. loads. Standbygenerators may overheat and/or experience internalcontrol circuit malfunction due to harmonics produced by

the very microprocessor loads they’re connected to.Certain consumer electronis motor drives are particularlyvulnerable to higher frequency harmonics and mustoperate in an environment where the Total HarmonicDistortion is less than 5%. Switching supplies use diode-capacitor circuits to convert AC line voltage to lowervoltage DC. These large capacitors charge up with narrowpulses of current that are timed with the peak line voltage.This process generates odd harmonics which are mostlythe 3rd and 5th, and to a lesser degree the 7th, 9th, etc.Note that the 3rd and 9th har-monics will algebraically addin the neutral conductor of a 3-phase distribution system,causing conductor overload-ing and transformer heating.

Reducing Line Voltage Harmonics

Ironically, the ever-increasing quantity of microprocessor-based equipment used in any given installation maygenerate enough harmonics to create a significant powerquality problem. Power conditioners are valuable devicesfor reducing harmonics, and will be discussed in moredetail in the rest of this document. Another considerationfor facility electrical engineers is lowering the powersource impedance in the electrical distribution system.Larger transformers, larger and shorter conductors, andbetter connectors will reduce harmonics. Designing for agood power quality Infrastructure requires that whereverpossible, harmonic-generating loads do not share branchcircuit wiring with important motor loads. The IEEEStandard 519, “IEEE Recommended Practices andRequirements for Harmonic Control in Electrical PowerSystems”, is a valuable resource for information onHarmonics.


17Chapter Three: Understanding Sequencing

CHAPTER THREE:Power ManagementThrough Sequencing

Home theater equipment can draw enormousamounts of current from your electrical service butthere’s a catch; the peak draw is just for a few

milliseconds after turn on. Just because this is a very shortperiod of time, however, doesn’t mean that you can ignorethis “in-rush” current. In fact, if all the devices in a hometheater are turned on at exactly the same time there is avery real possibility that there will not be enough current topower up all the devices.

The diagram above shows the in-rush current draw for asmall electric motor. Although it is hard to see on thisoscilloscope photo, the in-rush current draw is many timeshigher than the steady state current draw. Multiple this in-rush peak times all your home theater gear and you’ve gota potential power starvation problem.

Fortunately some power management devices offer avaluable feature called "Power Sequencing". Powersequencing allows you to program your home theaterequipment to power up in a staggered time sequence.This can be a lifesaver in many installations. In particular,if you have a home theater with a motorized screen, aprojection monitor and large power amps, you will mostlikely need to sequence the turning on of your componentsor you will find some power starved on turn on.

The photo below shows the rear panel of a MonsterHTS500 Power Center. The switches identified by thearrow allow you to program the outlets for delayed turn ontimes. This allows you to spread the in-rush current drawover several seconds and thus decrease the peak currentdraw of your system by many times.

Electric Motor In-Rush Current

18Chapter Three: Understanding Sequencing

In this home theater system the A/V processor and the VCR receive continuous power. Thisallows the A/V receiver to “stay awake” and receive IR commands (from a remote) to turn on thesystem. The VCR is keep powered so the clock keeps time. After the power strip is turned on the12VDC control line turns the power amplifier on and the DVD player and video projector asequenced on several seconds later. This spreads the system power draw over several seconds.

Example of Power Sequencing in a Home Theater System

19Chapter Three: Eliminating G round Loops

What are Ground Loops ?

Ground loops generally occur when there is a difference inpotential between the various grounding points in a hometheater system. When potentials like this occur, 60hzground currents and high frequency noise can flow aroundthe system and cause hum in both audio and videosignals. Audio hum is the most obvious evidence of 60 hzground loop currents as one can hear it directly from thespeakers. Video hum can be more mysterious. It usuallymanifests itself as a series of faint lines that rise upthrough the video image. Sometimes they are very faintbut, occasionally, can be quite severe. The figure belowillustrates what these 60Hz ground currents can look like(video hum bars) as seen on a projection video system.

The difference in ground potentials in an A/V system canbe caused by a number of factors. One of the mostnotorious involves the grounding of the cable TV coaxwhere it enters the building. As shown on the diagram on

the next page, the Cable Grounding Block should besecurely grounded to the electrical service ground as perthe National Electrical Code (NEC). If this grounding ispoor or nonexistent, external 60 Hz currents can circulatethroughout the shield of the A/V system interconnectcables.

If the system is properly grounded and video hum stillexists (which is very common, incidently), it is probablycaused by grounding problems in from the local house ACwiring.

How do you get rid of ground loops ?

We suggest several ways to eliminate audio and videohum in a home theater system. First, however, verify thatthe Cable Grounding Block is well grounded to the serviceground (and not just to a nearby water pipe, for example).If this appears grounded properly then see if you can plug

Video Hum is usually manifested as a series of faint lines tha t

rise up through the video image(period= 12-14 seconds)

CHAPTER FOUR:Eliminating Hum and

Ground Loops


the entire A/V system into the same outlet(s). This is oftenall that is needed because all the ground connections arereferenced to the same point.

If that isn’t possible (and in many larger home theaterinstallations it simply isn’t), you can employ special groundbreaking devices. These devices are specially builttransformers that are designed to pass the AC signals butblock 60hz ground currents.

There are two types of DC blocking transformers. The firstis an RF ground breaker that is used to isolate the cablecompany’s grounds from your home theater system.These DC blocking transformers are inexpensive and inour experience the cause of ground loop problems in 60-

70% of the home theater systems in this country.

The second type of DC blocking transformer works ataudio and video frequencies. These transformers areplaced directly in-line on the audio and video signalcables. The diagram shows the Jensen Iso-Max VB-1BBisolation transformer which is used on video signalconnections.

In some rare circumstances we have seen Actual ACisolation transformers used to separate a devices’s powercompletely from attached grounds. These isolationtransformer are available from a variety of manufacturers.

60Hz Ground Currents in a Home Theater System


1) PLUG ALL THE EQUIPMENT INTO THE SAME POWER FEED

Because audio and video hum is caused by 60 Hz ground currents, oftenthe only thing you need to do is plug your entire A/V system into thesame power feed from your breaker box. This removes the potentialdifference between grounds. This is easily done with small systems butmay be difficult to do with larger home theater systems because they areoften distributed throughout the house.

Ground Loop Elimination Techniques

3) USE A VIDEO GROUND BREAKER

These devices are designed to break the path for 60 Hz groundcurrents circulating through the video cables. They are wide bandtransformers with the primary and secondary physically separated sothat the ground path is broken. They are typically are placed in serieswith the video cable to the projector. The illustration is for Jensen VR-1BB Ground Breaker. It retails for $99.95

4) USE AN AC POWER ISOLATION TRANSFORMER

AC isolation transformers are common devices in electronic repair shops.Technicians use them to isolate the power on their repair benches fromphysical grounds in the repair shop (pipes, radiators, etc.) This is primarily forsafety reasons.You can use the same isolation transformers to electrically isolate your A/Vequipment from connected AC power and thus break any ground loops. Theyare typically placed in series with the power cord to the projector We recommend the Tripp-Lite IS 250, IS500 or IS500 models

2) RF GROUND BREAKERS

These devices are often all that is needed to break the path for 60 Hzground currents flowing through the RF cables. They are just a small RFtransformer with the primary and secondary physically separated so thatthe ground path is broken. They are typically placed inline with the coaxcable that feeds into the cable box. The illustration is for Jensen VR-1FFGround Breaker. It retails for $49.95


Grounding Theor yFrom an Engineer’ s

Prospective

It is tempting to think that the solution to all grounding problems isconnect the power cords from all your A/V equipment to any outlet thathas a ground connection. The problem is that the physical ground

wires that run around a building may not be referenced to the samepoint. The diagram below shows an ideal grounding scenerio with all theequipment connected to a single grounding point. The result is thatsince everything is referenced together, the equipment can “float” at thesame relative potential and no ground currents will flow through theinterconnects.

Another grounding issue involves high frequency noise signals.Electrical engineers know that straight pieces of wire act very differentlydepending on the frequency of the signal applied. The grounding wirecontained in your houses Romex wiring will behave very differently atpower frequencies (60Hz) versus noise frequencies (10 mHz). On thetop right we illustrate an electrical circuit. When one analyzes the circuitat power line frequencies it behaves in one fashion. When analyzed athigher noise frequencies, it behaves differently. Fortunately the laws ofphysics still apply here. If you have single point grounding the systemcan still float and reduce the effects of noise-based ground loops.

When A/V Equipment isReferenced to a Single

Ground, G round Loops ar eEliminated

23GLOSSARY

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

24GLOSSARY

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

25GLOSSARY

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

26GLOSSARY

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.

27GLOSSARY

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.

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