ContentsINTRODUCTION 3Brief history of low resistance ohmmeters 4WHY MEASURE LOW RESISTANCE? 4What is a low resistance measurement? 5What does a low resistance measurement tell the user? 5What problems create the need for a test? 5Saving money by low resistance testing 5Industries with significant resistance problems 6What equipment needs low resistance testing 6Motor armature 6Automotive assembly 7Power generation and distribution 7Transformers 7Uninterruptible power supply - battery straps 7Cement plants and other raw material processing applications 8Circuit breakers 8Aircraft assembly 8Strap and wire bonds between rail segments (railroad industry) 9Graphite electrodes 9Welding spot or seam 9Cable reels 10Measuring cable resistance of multicore cable of at least 3 cores 10Using low resistance measurements to set torque 11HOW IS LOW RESISTANCE MEASURED? 12Two, three and four wire d.c. measurements 12Two wire measurements 13Three wire measurements 13Four wire measurements 13D.C. vs. A.C. 13The difference between continuity and low resistance 14Test modes 14Models designed in the 1970s and 1980s 1410 amp models 14100 amp and above models 14HOW DOES A LOW RESISTANCE OHMMETER OPERATE? 15Safety 15Test on de-energized samples 15Use and misuse of low resistance ohmmeters 16Current selection 16Probe and lead selection 16Low range tests 17TYPES OF TESTERS - WHICH ONE? 17Milli-ohmmeter 1710 Amp micro-ohmmeter 17100 Amp and above micro-ohmmeter 18Nominal vs. absolute test current levels 18Auto range 19Ingress protection 19EVALUATION / INTERPRETATION OF RESULTS 20Repeatability 20Spot readings / base expectations for readings 20Trending 21Circuit breakers 21Stand-by battery back-up systems 21Measuring components of a system 23High currents in low resistance measurement 23Potential sources of error / ensure quality results 23Test leads / probes 23Accuracy statements 24Interference 24Delivery of stated test current under load 25Taking a measurement at a stable plateau 25Material resistivity 25Effects of temperature 26Effects of humidity 26Background noise, current and voltage 26Thermal emf / Seebeck voltage compensation 27Contact resistance contamination 27Noise ratio and induced currents 27Hot spots 28Calibration in the field 28APPENDICES 29Testing of transformers 29Motor bar to bar tests 29Battery strap tests 31Ramp testing 31Wheatstone and kelvin bridges 32Wheatstone bridge 32Kelvin bridge 32DLRO microohm and milliohm applications lists 33MEGGER PRODUCTS OVERVIEW 35DLRO100 series 35DLRO10 / DLRO10X 35DLRO10HD / DLRO10HDX 36DLRO600 36DLRO200 36MOM2 37MJÖLNER200 / MJÖLNER600 37MOM690A 38MOM200A / MOM600A 38BT51 38Series 247000 39Duplex connect test lead system 39PRODUCT COMPARISON CHART 41www.megger.com 1
