ABC de las mediciones eléctricas

8/22/2019 ABC de las mediciones elctricas

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Voltage spikesanunavoidable hazard

As distribution systems andloads become more complex,the possibilities o transientovervoltages increase. Motors,

capacitors and power con-version equipment, such asvariable speed drives, can beprime generators o spikes.Lightning strikes on outdoortransmission lines also causeextremely hazardous high-energy transients. I youretaking measurements on elec-trical systems, these transientsare invisible and largelyunavoidable hazards. Theyoccur regularly on low-voltagepower circuits, and can reachpeak values in the many thou-sands o volts. In these cases,youre dependent or protectionon the saety margin alreadybuilt into your meter. Thevoltage rating alone will nottell you how well that meterwas designed to survive hightransient impulses.

Early clues about the saetyhazard posed by spikes camerom applications involvingmeasurements on the supply

bus o electric commuterrailroads. The nominal busvoltage was only 600 V, butmultimeters rated at 1000 Vlasted only a ew minutes whentaking measurements while thetrain was operating. A closelook revealed that the trainstopping and starting generated10,000 V spikes. These tran-sients had no mercy on early

multimeter input circuits. Thelessons learned through thisinvestigation led to signicantimprovements in multimeterinput protection circuits.

Test tool safety standards

To protect you against tran-sients, saety must be built intothe test equipment. What per-ormance specication shouldyou look or, especially i youknow that you could be work-ing on high-energy circuits?The task o dening saetystandards or test equipment isaddressed by the InternationalElectrotechnical Commis-sion (IEC). This organizationdevelops international saety

standards or electrical testequipment.Meters have been used

or years by technicians andelectricians, yet the act is thatmeters designed to the IEC 1010standard oer a signicantlyhigher level o saety. Let's seehow this is accomplished.

Application Note

Dont overlook safetyyourlife may depend on it

Where saety is a concern, choosing a multi-meter is like choosing a motorcycle helmetiyou have a ten dollar head, choose a tendollar helmet. I you value your head, get asae helmet. The hazards o motorcycle ridingare obvious, but whats the issue with multi-meters? As long as you choose a multimeterwith a high enough voltage rating, arent yousae? Voltage is voltage, isnt it?

Not exactly. Engineers who analyze testersaety oten discover that ailed units weresubjected to a much higher voltage than theuser thought he was measuring. There are theoccasional accidents when the meter, ratedor low voltage (1000 V or less), was used tomeasure medium voltage, such as 4160 V. Just

as common, the knock-out blow had noth-ing to do with misuseit was a momentaryhigh-voltage spike or transientthat hit themultimeter input without warning.

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ABCs of

multimeter safetyMultimeter safety and you


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2 Fluke Education Partnership Program ABCs o electrical measurement saety

Transient protection

The real issue or multimetercircuit protection is not just themaximum steady state voltagerange, but a combination oboth steady state and transientovervoltage withstand capa-bility. Transient protection is

vital. When transients ride onhigh-energy circuits, they tendto be more dangerous becausethese circuits can deliver largecurrents. I a transient causesan arc-over, the high currentcan sustain the arc, producinga plasma breakdown or explo-sion, which occurs when thesurrounding air becomes ionizedand conductive. The result isan arc blast, a disastrous eventwhich causes more electricalinjuries every year than thebetter known hazard o electricshock. (See Transientsthehidden dangeron page 4.)

Measurement categories

The most important singleconcept to understand aboutthe standard is the Overvolt-age Installation Category. Thestandard denes Categories Ithrough IV, oten abbreviated asCAT I, CAT II, etc. (See Figure 1.)The division o a power distri-bution system into categories isbased on the act that a danger-ous high-energy transient suchas a lightning strike will beattenuated or dampened as ittravels through the impedance(ac resistance) o the system.A higher CAT number reers toan electrical environment withhigher power available andhigher energy transients. Thus,a multimeter designed to a CATIII standard is resistant to much

higher energy transients thanone designed to CAT II standards.Within a category, a higher

voltage rating denotes a highertransient withstand rating, e.g.,a CAT III-1000 V meter hassuperior protection compared toa CAT III-600 V rated meter. Thereal misunderstanding occurs isomeone selects a CAT II-1000Vrated meter thinking that itis superior to a CAT III-600 Vmeter. (See When is 600 Vmore than 1000 V?on page 7.)

Figure 1.Location, location, location.

Table 1. Measurement categories. IEC 61010 applies to low-voltage(< 1000 V) test equipment.

Measurementcategory In brie Examples

CAT IV Three-phase atutility connection,any outdoorconductors

Reers to the origin o installation, i.e., where low-voltageconnection is made to utility power

Electricity meters, primary overcurrent protection equipmentOutside and service entrance, service drop rom pole to

building, run between meter and panelOverhead line to detached building, underground line to well

pump

CAT III Three-phasedistribution,including single-phase commerciallighting

Equipment in xed installations, such as switchgear andpolyphase motors

Bus and eeder in industrial plantsFeeders and short branch circuits, distribution panel devicesLighting systems in larger buildingsAppliance outlets with short connections to service entrance

CAT II Single-phasereceptacleconnected loads

Appliance, portable tools, and other household and similarloads

Outlet and long branch circuitsOutlets at more than 10 meters (30 eet) rom CAT III sourceOutlets at more that 20 meters (60 eet) rom CAT IV source

CAT I Electronic Protected electronic equipmentEquipment connected to (source) circuits in which measures

are taken to limit transient overvoltages to an appropriatelylow level

Any high-voltage, low-energy source derived rom a high-winding resistance transormer, such as the high-voltagesection o a copier

Understanding categories:Location, location, location


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ABCs o electrical measurement saety Fluke Education Partnership Program 3

What does theCE symbol indicate?

A product is marked CE(Conormit Europenne) toindicate its conormance tocertain essential requirementsconcerning health, saety,environment and consumerprotection established bythe European Commissionand mandated through theuse o directives. There aredirectives aecting manyproduct types, and productsrom outside the EuropeanUnion can not be imported andsold there i they do not complywith applicable directives.Compliance with the directivecan be achieved by provingconormance to a relevanttechnical standard, such as IEC61010 or low-voltage products.Manuacturers are permitted

to sel-certiythat they havemet the standards, issue theirown Declaration o Conormity,and mark the product CE.The CE mark is not, thereore,a guarantee o independenttesting.

Its not just the voltage level

In Figure 1, a technician work-ing on oce equipment in a CAT Ilocation could actually encounterdc voltages much higherthan thepower line ac voltages measuredby the motor electrician in the

CAT III location. Yet transients inCAT I electronic circuitry, what-ever the voltage, are clearly alesser threat, because the energyavailable to an arc is quite lim-ited. This does notmean there isno electrical hazard present inCAT I or CAT II equipment. Theprimary hazard is electric shock,not transients and arc blast.Shocks, which will be discussedlater, can be every bit as lethal asarc blast.

To cite another example, an

overhead line run rom a houseto a detached workshed might beonly 120 V or 240 V, but its stilltechnically CAT IV. Why? Anyoutdoor conductor is subject tovery high energy lightning-relatedtransients. Even conductors buriedunderground are CAT IV, becausealthough they will not be directlystruck by lightning, a lightningstrike nearby can inducea tran-sient because o the presence ohigh electromagnetic elds.

When it comes to OvervoltageInstallation Categories, the ruleso real estate apply: its location,location, location....(For more discussion o InstallationCategories, see page 6, Applyingcategories to your work.)

Independent testing is thekey to safety compliance

Look or a symbol and listingnumber o an independent

testing lab such as UL, CSA,TV or other recognized testingorganization. Beware o word-ing such as Designed to meetspecifcation ... Designersplans are never a substitute oran actual independent test.

How can you tell i youregetting a genuine CAT III orCAT II tester? Unortunatelyits not always that easy. It ispossible or a manuacturer tosel-certiy that its tester is CATII or CAT III without any inde-

pendent verifcation. The IECdevelops and proposes stan-dards, but it is not responsibleor enorcingthe standards.

Look or the symbol andlisting number o an indepen-dent testing lab such as UL,CSA, TV or other recognizedapproval agency. That symbolcan only be used i the productsuccessully completed testingto the agencys standard, whichis based on national/interna-

tional standards. UL 3111, orexample, is based on IEC 1010.In an imperect world, that isthe closest you can come toensuring that the multimeteryou choose was actually testedor saety.

Independent testing

Tool TipNon-contact voltage detectors are aquick, inexpensive way to check orthe presence o live voltage on accircuits, switches and outlets beoreworking on them.1. Veriy the voltage detector unction is working properly.2. Make sure the detector is rated or the level o voltage being

measured and is sensitive enough or your application.3. Make sure youre grounded (through your hand, to the foor),

to complete the capacitive voltage connection.

Use either a voltage detector wand or a digital multimeter(DMM) with non-contact measurement built in.

This meter has a bui lt in non-contact voltage tester.


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Protection against two major electrical hazards

Lets take a look at a worst-casescenario in which a technicianis perorming measurementson a live three-phase motorcontrol circuit, using a meterwithout the necessary saetyprecautions.

Heres what could happen:1. A lightning strike causes a

transient on the power line,which in turn strikes an arcbetween the input terminalsinside the meter. The circuitsand components to preventthis event have just ailed orwere missing. Perhaps it wasnot a CAT III rated meter.

The result is a direct shortbetween the two measure-ment terminals through themeter and the test leads.

2. A high-ault currentpossiblyseveral thousands o ampsfows in the short circuitjust created. This happensin thousandths o a second.When the arc orms inside

the meter, a very high pres-sure shock wave can cause aloud bangvery much like agunshot or the backre roma car. At the same instant,the tech sees bright blue arcfashes at the test lead tipsthe ault currents superheatthe probe tips, which start toburn away, drawing an arcrom the contact point to theprobe.

3. The natural reaction is topull back, in order to breakcontact with the hot circuit.But as the techs hands arepulled back, an arc is drawn

rom the motor terminal toeach probe. I these twoarcs join to orm a single arc,there is now anotherdirectphase-to-phase short, thistime directly between themotor terminals.

4. This arc can have a tempera-ture approaching 6000 C(10000 F), which is higher

than the temperature oan oxyacetylene cuttingtorch! As the arc grows, edby available short circuitcurrent, it superheats thesurrounding air. Both ashock blast and a plasmareball are created. I thetechnician is lucky, theshock blast blows him awayand removes him rom theproximity o the arc; thoughinjured, his lie is saved. Inthe worst case, the victimis subjected to atal burninjuries rom the erce heato the arc or plasma blast.

In addition to using a multi-meter rated or the appropriatemeasurement category, anyoneworking on live power circuitsshould be protected with fameresistant clothing, should wearsaety glasses or, better yet, asaety ace shield, and shoulduse insulated gloves.

A lightning strike causes a transienton the power line, creating an arcbetween the meters input terminaland resulting in loud noises.

Then, a high current fows in theclosed circuit which is ormed.An arc starts at the probe tips.

I those arcs join, theresulting high-energyarc can create a liethreatening situationor the user.

When you pull the probes awayas a reaction to the loud noise,arcs are drawn to the motorterminals youre probing.

Transientsthe hidden danger

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Figure 2. A worst-case scenariopotential arc blast sequence.


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ABCs o electrical measurement saety Fluke Education Partnership Program 5

Transients arent the only source opossible short circuits and arc blasthazard. One o the most commonmisuseso handheld multimeterscan cause a similar chain o events.

Lets say a user is makingcurrent measurements on signalcircuits. The procedure is to selectthe amps unction, insert the leadsin the mA or amps input termi-nals, open the circuit and take aseries measurement. In a seriescircuit, current is always the same.The input impedance o the ampscircuit must be low enough so thatit doesnt aect the series circuitscurrent. For instance, the inputimpedance on the 10 A terminalo a Fluke meter is .01 . Comparethis with the input impedance onthe voltage terminals o 10 M(10,000,000 ).

I the test leads are let in theamps terminals and then acciden-tally connected across a voltagesource, the low input imped-ance becomes a short circuit! Itdoesnt matter i the selector dial isturned to volts; the leads are stillphysically connected to a low-impedance circuit.* Thats why theamps terminals must be protectedby uses. Those uses are theonly thing standing between aninconvenienceblown usesand apotential disaster.

Use only a multimeter with ampsinputs protected by high-energyuses. Never replace a blown usewith the wrong use. Use onlythe high-energy uses specifedby the manuacturer. These usesare rated at a voltage and with ashort circuit interrupting capacitydesigned or your saety.

Overload protection

Fuses protect against overcur-rent. The high input impedance othe volts/ohms terminals ensuresthat an overcurrent condition isunlikely, so uses arent necessary.Overvoltageprotection, on the otherhand, isrequired. It is providedby a protection circuit that clampshigh voltages to an acceptablelevel. In addition, a thermal protec-tion circuit detects an overvoltage

condition, protects the meter untilthe condition is removed, and thenautomatically returns to normaloperation. The most common ben-et is to protect the multimeter romoverloads when it is in ohms mode.In this way, overload protectionwith automatic recovery is providedor all measurement unctions aslong as the leads are in the voltageinput terminals.

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Figure 3. Misuse o DMM in Ammeter Mode.

While most people are aware o the

danger rom electric shock, ew real-ize how little current and how low avoltage are required or a atal shock.Current fows as low as 30 mA canbe atal (1 mA = 1/1000 A). Lets lookat the eects o current fow througha typical 68 kilogram (150 pound)male: At about 10 mA, muscular paraly-

sis o the arms occurs, so that hecannot release his grip.

At about 30 mA, respiratory paraly-sis occurs. His breathing stops andthe results are oten atal.

At about 75 to 250 mA, or expo-sure exceeding ve seconds,ventricular brillation occurs,causing incoordination o the heartmuscles; the heart can no longerunction. Higher currents causebrillation at less than ve seconds.The results are oten atal.

Now lets calculate the threshold ora hazardous voltage. The approxi-mate body resistance under the skinrom hand-to-hand across the bodyis 1000 . A voltage oonly 30 V

across 1000 will cause a currentfow o 30 mA. Fortunately, the skinsresistance is much higher. It is theresistance o the skin, especially theouter layer o dead cells, that protectsthe body. Under wet conditions, or ithere is a cut, skin resistance dropsradically. At about 600 V, the resis-tance o the skin ceases to exist. It ispunctured by the high voltage.

For multimeter manuacturers andusers, the objective is to preventaccidental contact with live circuits atall costs.Look or: Meters and test leads with double

insulation. Meters with recessed input jacks

and test leads with shrouded inputconnectors.

Test leads with nger guards and anon-slip surace.

Meter and test leads made o high-quality, durable, non-conductivematerials.

Use the right high-energyfuses

Electric shock

* Some multimeters have an input alert which gives a

warning beep i the meter is in this conguration.

Arc blast and electric shock


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Work safely

Always wear appropriate personal protectiveequipment (PPE), including fame-resistantclothing, leather over rubber gloves, saetyglasses, and an arc-rated ace shield or hood,

both with hard hat and hearing protection.

Multiple categories

Theres one scenario that some-times conuses people trying toapply categories to real worldapplications. In a single piece o

equipment, there is oten morethan one category. For example,in oce equipment, rom the120 V/240 V side o the powersupply back to the receptacle isCAT II. The electronic circuitry,on the other hand, is CAT I. Inbuilding control systems, suchas lighting control panels, orindustrial control equipmentsuch as programmable con-trollers, it is common to ndelectronic circuits (CAT I) and

power circuits (CAT III) existingin close proximity.What do you do in these

situations? As in all real-worldsituations, use common sense.In this case, that means usingthe meter with the highercategory rating. In act, itsnot realistic to expect peopleto go through the category-dening process all the time.What is realistic, and highlyrecommended, is to select amultimeter rated to the highest

category in which it could pos-sibly be used. In other words,err on the side o saety.

Shortcuts tounderstanding categoriesHere are some quick ways toapply the concept o categoriesto your every day work:

The general rule-o-thumbis that the closer you are tothe power source, the higherthe category number, and thegreater the potential dangerrom transients.

It also ollows that the greaterthe short-circuit currentavail-able at a particular point, thehigher the CAT number.

Another way o saying thesame thing is the greaterthe source impedance, the

lowerthe CAT number. Sourceimpedance is simply the totalimpedance, including theimpedance o the wiring,between the point where youare measuring and the powersource. This impedance iswhat dampens transients.

Finally, i you have anyexperience with the applica-tion o transient voltage surgesuppression (TVSS) devices,you understand that a TVSSdevice installed at a panel

must have higher energyhandling capacity than oneinstalled right at the com-puter. In CAT terminology, thepanelboard TVSS is a CAT IIIapplication, and the computeris a receptacle connectedload and thereore, a CAT IIinstallation.

As you can see, the concepto categories is not new andexotic. It is simply an exten-sion o the same common-sense

concepts that people who workwith electricity proessionallyapply every day.

Saety is everyones responsibility but ulti-mately its in yourhands.

No tool by itsel can guarantee your saety.Its the combination o the right tools andsae work practices that gives you maximum

protection. Here are a ew tips to help you inyour work.

Work on de-energized circuits wheneverpossible. Use proper lock-out/tag-outprocedures. I these procedures are notin place or not enorced, assume that thecircuit is live.

On live circuits, use protective gear: Use insulated tools. Wear saety glasses or a ace shield. Wear insulated gloves; remove watches

or other jewelry. Stand on an insulated mat. Wear fame resistant clothing, not ordi-

nary work clothes. When making measurements on live

circuits: Hook on the ground clip rst, then make

contact with the hot lead. Remove thehot lead rst, the ground lead last.

Hang or rest the meter i possible. Try toavoid holding it in your hands, to mini-mize personal exposure to the eects otransients.

Use the three-point test method, espe-cially when checking to see i a circuitis dead. First, test a known livecircuit.

Second, test the target circuit. Third,test the live circuit again. This veriesthat your meter worked properly beoreand ater the measurement.

Use the old electricians trick o keepingone hand in your pocket. This lessensthe chance o a closed circuit acrossyour chest and through your heart.

Applying categories to your work


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Understanding voltage

withstand ratingsIEC 61010 test procedures takeinto account three main crite-ria: steady-state voltage, peakimpulse transient voltage andsource impedance. These threecriteria together will tell you amultimeters true voltage with-stand value.

When is 600 V more than1000 V?

Table 2 can help us understandan instruments true voltagewithstand rating:1. Within a category, a higher

working voltage (steady-state voltage) is associatedwith a higher transient,as would be expected. Forexample, a CAT III-600 Vmeter is tested with 6000 Vtransients while a CAT III-1000 V meter is tested with8000 V transients. So ar, sogood.

2. What is not as obvious isthe dierence between the6000 V transient or CATIII-600 V and the 6000 Vtransient or CAT II-1000 V.They are notthe same. Thisis where the source imped-ance comes in. Ohms Law(amps = volts/ohms) tells usthat the 2 test source orCAT III has six times the cur-rento the 12 test sourceor CAT II.

The CAT III-600 V meter clearlyoers superior transient pro-tection compared to the CATII-1000 V meter, even thoughits so-called voltage ratingcould be perceived as beinglower.It is the combination othe steady-state voltage (calledthe working voltage), and thecategory that determines thetotal voltage withstand ratingo the test instrument, includingthe all-important transient volt-age withstand rating.

Working Voltage Peak Impulse

Measurement (dc or ac-rms Transient Test SourceCategory to ground) (20 repetitions) ( = V/A)

CAT I 600 V 2500 V 30 Ohm source

CAT I 1000 V 4000 V 30 Ohm source

CAT II 600 V 4000 V 12 Ohm source

CAT II 1000 V 6000 V 12 Ohm source

CAT III 600 V 6000 V 2 Ohm source

CAT III 1000 V 8000 V 2 Ohm source

CAT IV 600 V 8000 V 2 Ohm source

Table 2: Transient test values or measurement categories.(50 V/150 V/300 V values not included.)

How to evaluate a testers safety rating

A note on CAT IV: Test values

and design standards or CATIV voltage testing are addressedin IEC 61010 second edition.

Creepage and clearance

In addition to being tested toan actual overvoltage transientvalue, multimeters are requiredby IEC 61010 to have minimumcreepage and clearancedistances between internalcomponents and circuit nodes.Creepage measures distance

across a surace. Clearancemeasures distances throughthe air. The higher the categoryand working voltage level, thegreater the internal spacingrequirements. One o the maindierences between the oldIEC 348 and IEC 61010 is theincreased spacing requirementsin the latter.

The bottom line

I you are aced with the task o

replacing your multimeter, doone simple task beore you startshopping: Analyze the worst-case scenario o your job anddetermine what category youruse or application ts into.

Firstchoose a meter ratedor the highest categoryyoucould be working in. Then, lookor a multimeter with a voltagerating or that category match-ing your needs. While youreat it, dont orget the test leads.

Look or category and voltage ratings o test leads and multimeters.

Fluke CorporationPO Box 9090, Everett, WA USA 982062006-2009 Fluke Corporation.Printed in U.S.A. 6/2009 2096653D A-EN-N

Web access: http://www.fuke.com


IEC 61010 applies to test leadstoo: They should be certiedto a category and voltage ashigh or higher than the meter.When it comes to your personalprotection, dont let test leadsbe the weak link.

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