Fluke - Proper use of clamp meters in commercial and residential environments

Application Note There’s nothing so annoying asa circuit breaker that keepstripping, usually at the mostinopportune times. Moreannoying yet is not being ableto figure out why as theproduction line stands silentwaiting for you to work yourmagic. The pressure’s on! Inthis application note, we’lldiscuss how to utilize all thecapabilities of your clampmeter so you can keep yourworld up and running. We all know that clamp meters are used to measurecircuit loading. But, with a bitof ingenuity, you can also useclamps to tell you which circuitbreaker controls which outlets,as well as to measure individ-ual loads (for both load andearth currents, if any). This canhelp you solve load problemsquickly and preserve yourreputation as an ace trouble-shooter. Clamp meters measure current by determining themagnetic field around acurrent-carrying conductor.There is simply no other practi-cal way to measure current onelectrical wiring systems.Breaking these circuits open tomake a series circuit measure-ment is impractical and mayeven be a career ending moveif you inadvertently take criticalloads offline. Usually, measure-ments are taken at the paneland include loading andbalance on three-phase feed-ers. With the prevalence ofharmonic loads, neutral meas-urements at panelboards arealso mandatory. Current meas-urements can also be used todiagnose motor health.  Beyond these basic measure- ments that clamps werespecifically designed for,modern digital clamp metershave voltage and resistancemeasurement capability as well.That means it’s possible tomake most, if not all, of thecommon, everyday measure-ments using a clamp meter. If anelectrician could take just onetest tool on the job, it wouldmake sense for him to choosehis clamp meter. Furthermore,that clamp meter should be atrue-rms model, like the Fluke335, 336, or 337. The alterna-tive is an average-sensing Proper use of clamp  meters in commercial and residential environments model, which costs less, butwill not measure the currentaccurately. Whenever there areelectronic loads (computers,TVs, lighting, motor drives, etc.)on a circuit, the average-sens-ing meter could be inaccurate.The greater the electronic load,the greater the inaccuracy.True-rms clamps will always beaccurate (assuming of coursethat you’ve kept it calibrated).So unless you feel comfortablesaying that you’re not likely torun into those types of loads,get a true-rms clamp. That wayyou can think about the job, notthe test tool.

2 Fluke Corporation Proper use of a clamp meter in commercial and residential settings  Clamp meters in residential applications  For residential electricians,clamps are a necessity tomeasure loads on individualbranch circuits at the distri-bution panel. While a spotcheck of current is oftensufficient, sometimes it doesn’tprovide the full picture as loadsare switching on and off, goingthrough cycles, etc. Voltageshould be stable in an elec-trical system, but current canbe very dynamic. To check thepeak or worst-case loading ona circuit, use a clamp with amin/max function which isdesigned to catch high currentsthat exist for longer than 100 ms, or about eight cycles.These currents lead to inter-mittent overload conditionswhich can cause nuisancetripping of circuit breakers. Take measurements on the load side of the circuit breakeror fuse. The breaker will openthe circuit in the event of anaccidental short circuit. This isespecially important with anykind of direct-contact voltagemeasurement. Even thoughclamp jaws are insulated andtherefore have a level ofprotection that doesn’t existwith direct-contact voltagemeasurement, it’s still a goodidea to be cautious.  A common problem in residential electrical work ismapping outlets to breakers.A clamp can be useful in identi-fying which circuit a particularoutlet is on. First take a base-line reading, at the distributionpanel, of the existing currenton the circuit. Then put theclamp in min/max mode. Go tothe outlet in question, plug in aload—a hair dryer is ideal—andturn it on for a second or two.Check the clamp to see if themax current reading haschanged. A hair dryer will typi-cally draw 5 A, so there shouldbe a noticeable difference. Ifthe reading is the same, you’vegot the wrong breaker. Clamp meters in commercial environments Clamp meters are used at thepanelboard to measure circuitloading on feeders as well ason branch circuits. Measure-ments on branch circuitsshould always be made at theload side of the breaker or fuse.  • Feeder cables should bechecked for balance as wellas loading: current on allthree phases should be moreor less the same, to minimizethe return current on theneutral.  • The neutral should also bechecked for overloading.With harmonic loads, it’spossible for the neutral to becarrying more current than afeeder—even if the feedersare balanced.  • Each branch circuit shouldalso be checked for possibleoverloading.  • Finally the earth circuitshould be checked. Ideallythere should be no currenton the earth, although levelsunder 300 mA can often be tolerated in certaininstallations. Testing for leakagecurrents To check if there is leakagecurrent on a branch circuit, putboth the live and neutral wiresin the jaws of the clamp. Anycurrent that is measured isleakage current, i.e., currentreturning on the earth circuit.The supply and return currentsgenerate opposing magneticfields. The currents should beequal (and opposite) and theopposing fields should canceleach other out. If they don’t,that means that some current,called leakage current, isreturning on another path, andthe only other available path isthe earth.  If you do detect a net current between the supply and return,consider the nature of the loadand the circuit. A mis-wiredcircuit can have up to half of thetotal load current strayingthrough the earth system. If themeasured current is very high,you probably have a wiring prob-lem. Leakage current may also becaused by leaky loads or poorinsulation. Motors with wornwindings or moisture in fixturesare common culprits. If yoususpect excessive leakage, a de-energized test using amegohmmeter will help evaluatethe integrity of the circuit’s insu-lation and help identity if andwhere a problem exists.

Motors and motor control circuits One of the most challengingplaces to make current meas-urements is in a control circuitcabinet, especially if it usesIEC-style components. Euro-pean-originated IEC-stylecomponents are much morecompact than the equivalentNEMA parts, and the wiringcan be packed pretty tight. Thetapered jaw and “backlight”function of the Fluke 330Series clamp meters are wellsuited to this measurementtask. Three-phase induction motors are commonly used incommercial buildings to drivefan and pump loads. Motorscan either be controlled byelectromechanical starters or byelectronic variable speeddrives. Variable speed drivesare more and more common,since they save considerableenergy.  The Fluke 337 is the ideal clamp to make these motor anddrive measurements: • Loading: The current drawof the motor, measured asan average of the threephases, should not exceedthe full load amperes ratingof the motor (times the serv-ice factor). On the otherhand, a motor that is loadedbelow 60 percent of full loadamperes — and many ofthem are — is less and lessefficient, and the powerfactor decreases as well.  • Current balance: Currentimbalance can be an indica-tion of problems with themotor windings (for exam-ple, different resistances onfield windings due to inter-nal shorts). Generallyspeaking, imbalance shouldbe under 10 percent. (Tocalculate imbalance, firstcalculate the average of thethree phase readings; thenfind the highest deviationfrom the average and divideby the average.) Theextreme of current imbal-ance is single phasing,when there is no current onone of the three phases.This is usually caused by anopen fuse. • Inrush current: Motorsstarted across the line (bymechanical starters) willhave an inrush current(variable speed drives donot have inrush current).The inrush current isapproximately 500 percenton older motors to as highas 1,200 percent on energy-efficient motors. This inrushcurrent, if it is too high, is acommon cause of voltagesags as well as nuisancetripping. The “inrush” func-tion of the Fluke 337 clampmeter is a unique capability,designed to trigger on theinrush current and captureits true value. • Peak loading (shock loads):Some motors are subject toshock loads, which cancause enough of a currentsurge to trip the overloadcircuit in the motor controller. Think of a saw running intoa knot. The min/max func-tion can be used to recordthe worst-case currentdrawn by shock loads. Whether in residential orcommercial settings, the clampmeter is the electrician’sindispensable test tool. Work safely The high voltage and currentspresent in electrical powersystems can cause seriousinjury or death by electrocutionand burns. Consequently, onlytrained, experienced electri-cians who have knowledge ofelectrical systems in generaland the equipment under testshould perform testing andmodification of electricalsystems. Fluke cannot anticipate all possible precautions that youmust take when performing themeasurements described here.At a minimum, however, youshould: • Use appropriate safetyequipment such as safetyglasses, insulated gloves,insulating mats, etc. • Be sure that all power hasbeen turned off, locked out,and tagged in any situationwhere you will be in directcontact with circuit compo-nents. Be certain that thepower can’t be turned on byanyone but you. • Read and understand all ofthe applicable manualsbefore applying the informa-tion in this application note.Take special note of allsafety precautions andwarnings in the instructionmanuals. • Do not use instruments onapplications for which theyare not intended, andalways be aware that if theequipment is used in amanner not specified by themanufacturer, the protectionprovided by the equipmentmay be impaired. Proper use of a clamp meter in commercial and residential settings     Fluke Corporation  3

Fluke. Keeping your world up and running. Fluke CorporationPO Box 9090, Everett, WA USA 98206 Fluke Europe B.V.PO Box 1186, 5602 BD Eindhoven, The Netherlands For more information call:In the U.S.A. (800) 443-5853 or Fax (425) 446-5116In Europe/M-East/Africa (31 40) 2 675 200 or Fax (31 40) 2 675 222In Canada (800)-36-FLUKE or Fax (905) 890-6866From other countries +1 (425) 446-5500 or Fax +1 (425) 446-5116Web access: http://www.fluke.com/ ©2003 Fluke Corporation. All rights reserved. Printed in The Netherlands. 3/2003  10561-eng. Rev. 01 4 Fluke Corporation     Proper use of a clamp meter in commercial and residential settings