Megger DLRO200 Low Resistance Testing - Actoolsupply.com

www.actoolsupply.comTABLE OF CONTENTSIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . .2Why Measure Low Resistance? . . . . . . . . . . . . .3What is a Low Resistance Measurement? . . . . . . . .3What Does the Low Resistance MeasurementTell the Operator? . . . . . . . . . . . . . . . . . . . . . . . . . . .3What Problems Create the Need toPerform the Test? . . . . . . . . . . . . . . . . . . . . . . . . . . .4Industries with Significant Resistance Problems . . . .4Specific Examples of Apparatus in Need of LowResistance Testing . . . . . . . . . . . . . . . . . . . . . . . . . . .4Motor Armature . . . . . . . . . . . . . . . . . . . . . . . . . . .5Automotive Assembly . . . . . . . . . . . . . . . . . . . . . .5Power Generation and Distribution(high current joints, connections and bus bars) . . .5Transformer Testing . . . . . . . . . . . . . . . . . . . . . . . .5Uninterruptible Power Supply - Battery Straps . . .5Cement Plants and other Raw MaterialProcessing Applications . . . . . . . . . . . . . . . . . . . . .6Circuit Breakers . . . . . . . . . . . . . . . . . . . . . . . . . . .6Aircraft Assembly . . . . . . . . . . . . . . . . . . . . . . . . . .7Strap and Wire Bonds between Rail Segments(Railroad Industry) . . . . . . . . . . . . . . . . . . . . . . . . .7Graphite Electrodes . . . . . . . . . . . . . . . . . . . . . . . .7Welding Spot or Seam . . . . . . . . . . . . . . . . . . . . . .7Cable Reels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7How do You Measure Low Resistance . . . . . . .82-Wire, 3-Wire and 4-Wire DC Measurements . . . . .8Two-Wire Measurements . . . . . . . . . . . . . . . . . . . .8Three-Wire Measurements . . . . . . . . . . . . . . . . . . .9Four-Wire Measurements . . . . . . . . . . . . . . . . . . . .9DC vs. AC Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . .9How Does a Low Resistance Ohmmeter Operate? .10Current Selection . . . . . . . . . . . . . . . . . . . . . . . . . . .10Probe and Lead Selection . . . . . . . . . . . . . . . . . . . .11Low Range Testing . . . . . . . . . . . . . . . . . . . . . . . . .11Test on “Dead” Test Samples . . . . . . . . . . . . . . . . . .12Types of Testers/How to Choose . . . . . . . . . . .12Evaluation/Interpretation of Results . . . . . . .14Repeatability . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14Spot Readings/Base Expectations for Readings . . . .14Trending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14Circuit Breakers . . . . . . . . . . . . . . . . . . . . . . . . . .15Stand-by Battery Back-up Systems . . . . . . . . . . . .15Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . .16Potential Sources of Error/EnsuringQuality Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16Test Leads/Probes . . . . . . . . . . . . . . . . . . . . . . . . .16Accuracy Statements . . . . . . . . . . . . . . . . . . . . . .17Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17Delivery of Stated Test Current Under Load . . . .17Taking the Measurement at a Stable Plateau . . .17Material Resistivity . . . . . . . . . . . . . . . . . . . . . . . . .17Effects of Temperature on Measured ResistanceValues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18Effects of Humidity . . . . . . . . . . . . . . . . . . . . . . . . .19Background Noise Conditions,Current and Voltage . . . . . . . . . . . . . . . . . . . . . . . .19Use and Misuse of Low Resistance Ohmmeters . . .19Brief History of Low Resistance Ohmmeters . . . . . .20Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20Ingress Protection . . . . . . . . . . . . . . . . . . . . . . . . . .20Various Test Modes . . . . . . . . . . . . . . . . . . . . . . . . .22Models Designed in the 1970s and 1980s . . . . . .22Recently Designed 10 Amp Models . . . . . . . . . . .22Nominal versus Absolute Test Current Levels . . . . .22Autoranging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23Transformer Testing . . . . . . . . . . . . . . . . . . . . . . . . .23Bar to Bar Testing . . . . . . . . . . . . . . . . . . . . . . . . . .24Battery Strap Testing . . . . . . . . . . . . . . . . . . . . . . . .26Wheatstone and Kelvin Bridges . . . . . . . . . . . . . . .27Wheatstone Bridge . . . . . . . . . . . . . . . . . . . . . . .27Kelvin Bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . .27Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28Megger Products Overview . . . . . . . . . . . . . . .29Front CoverLow Resistance Ohmmeter shown being usedto measure contact resistance of a low voltagemolded case breaker.Milli-Ohmmeter . . . . . . . . . . . . . . . . . . . . . . . . . . . .1210-A Micro-Ohmmeter . . . . . . . . . . . . . . . . . . . . . .12100+ A Micro-Ohmmeter . . . . . . . . . . . . . . . . . . . . .12Transformer Ohmmeter . . . . . . . . . . . . . . . . . . . . . .13Lab Micro-Ohmmeter . . . . . . . . . . . . . . . . . . . . . . .13www.actoolsupply.comA GUIDE TO LOW RESISTANCE TESTING 1