A Guide To Low ResistAnce TestinG - Instrumart

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A Guide to lowresistance testingUnderstanding and Measuring Low Resistanceto Ensure Electrical System Performancewww.MeGGer.coM

Front CoverLow Resistance Ohmmeter shown being usedto measure contact resistance of a low voltagemolded case breaker.

TABLE OF CONTENTSIntroduction. 2Evaluation/Interpretation of Results. 14Why Measure Low Resistance?. 3Repeatability .14What 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.4Spot Readings/Base Expectations for Readings.14Trending .14Circuit Breakers.15Stand-by Battery Back-up Systems.15Appendices. 16Specific Examples of Apparatus in Need of Low ResistanceTesting.4Motor Armature.5Potential Sources of Error/EnsuringQuality Results.16Test Leads/Probes.16Automotive Assembly.5Accuracy Statements.17Power Generation and Distribution(high current joints, connections and bus bars).5Interference.17Transformer Testing.5Taking the Measurement at a Stable Plateau.17Uninterruptible Power Supply - Battery Straps.5Cement Plants and other Raw MaterialProcessing Applications.6Circuit Breakers.6Aircraft Assembly.7Delivery of Stated Test Current Under Load .17Material Resistivity.17Effects of Temperature on Measured Resistance Values81Effects of Humidity.19Background Noise Conditions,Current and Voltage .19Strap and Wire Bonds between Rail Segments (RailroadIndustry).7Use and Misuse of Low Resistance Ohmmeters.19Graphite Electrodes.7Calibration.20Welding Spot or Seam.7Ingress Protection.20Cable Reels.7Various Test Modes.22Models Designed in the 1970s and 1980s.22How do You Measure Low Resistance. 82-Wire, 3-Wire and 4-Wire DC Measurements .8Two-Wire Measurements.8Three-Wire Measurements.9Four-Wire Measurements.9Brief History of Low Resistance Ohmmeters.20Recently Designed 10 Amp Models.22Nominal versus Absolute Test Current Levels.22Autoranging.23Transformer Testing.23Bar to Bar Testing.24DC vs. AC Testing.9Battery Strap Testing.26How Does a Low Resistance Ohmmeter Operate?.10Current Selection.10Wheatstone and Kelvin Bridges.27Wheatstone Bridge.27Probe and Lead Selection .11Kelvin Bridge.27Low Range Testing .11Safety.28Test on “Dead” Test Samples.12Types of Testers/How to Choose. 12Megger Products Overview. 29Milli-Ohmmeter.1210-A Micro-Ohmmeter.12100 A Micro-Ohmmeter.12Transformer Ohmmeter.13Lab Micro-Ohmmeter.13A GUIDE TO LOW RESISTANCE TESTING1

IntroductionThe quantitative study of electrical circuits originated in 1827,when Georg Simon Ohm published his famous book “Diegalvanische Kette, mathematisch bearbeitet” in which he gavehis complete theory of electricity. In this seminal work, heintroduced the relationship or “Law” that carries his name:Resistance (R) Voltage (E) / Current (I)At that time, the standards for Voltage, Current and Resistance had not been developed. Ohm’s Law expressed the factthat the magnitude of the current flowing in a circuit depended directly on the electrical forces or pressure and inversely ona property of the circuit known as the resistance. Obviously,however, he did not have units of the size of our present volt,ampere, and ohm to measure these quantities.At this time, laboratories developed resistance elements, constructed of iron, copper or other available alloy materials. Thelaboratories needed stable alloys that could be moved fromplace to place to certify the measurements under review. Thestandard for the ohm had to be temperature stable and withminimum effects due to the material connected to the ohmstandard.In 1861, a committee was established to develop a resistancestandard. This committee included a number of famousmen with whom we are now familiar, including James ClerkMaxwell, James Prescott Joule, Lord William Thomson Kelvinand Sir Charles Wheatstonei. In 1864, a coil of platinum-silveralloy wire sealed in a container filled with paraffin was usedas a standard. This was used for 20 years while studies weremade for a more reliable standard. These studies continuedas the old National Bureau of Standards (NBS), now knownas the National Institute of Standards and Technology (NIST),controlled the standard for the “OHM.” Today the industryuses Manganin alloy because it has a low temperature coefficient so that its resistance changes very little with temperature(see figure 1). The table below from Melvin B. Stout’s “BasicResistivityiiCompositionMicro-ohms Ohms for Cir.Percentfor cm Cubemil FootElectrical Measurements” highlights the key properties ofManganin.The thermal emf against copper indicates the thermocoupleactivity of the material whereby a voltage is generated simplyby connecting two different metals together. The goal is tominimize thermocouple activity as it introduces error into themeasurement.With the metric system, the measurements are in meters andthe resistivity is determined for a one-meter cube of the material. However, more practical units are based on a centimetercube. With the USA system, the resistivity is defined in ohmsper mil foot. The wire diameter is measured in circular mils(0.001)2 and the length in feet.Figure 1 shows the temperature-resistance curve for Manganin wire at 20º C. For Manganin shunts, the 20 C curveshifts to 50º C, as this material will be operating at a highertemperature due to the application. The Manganin alloy wasdesigned for use in coils used to perform stable measuringconditions at 20º C ambient room conditions.The alloy is modified for strips of material used in measuringshunts, which operate at a higher ambient, up to 50º C.The purpose of this booklet is to help the engineer, technicianor operator:nUnderstand the rationale behind low resistancetesting.nUnderstand how to make a low resistancemeasurement.nUnderstand how to select the proper instrument for thetesting application.nUnderstand how to interpret and use the results.TemperatureCoefficientper ºCThermal emfAgainst Copperµv/ ºC* 0.00001º1.7Cu 84%Mn 12%44 µΩ 264 ΩNi 4%*Manganin shows zero effect from 20º to 30º C.i Swoope’s Lessons in Practical Electricity; Eighteenth Edition; Erich Hausmann, E.E., ScD.; page 111ii Swoope’s Lessons in Practical Electricity; Eighteenth Edition; Erich Hausmann, E.E., ScD.; page 1182A GUIDE TO LOW RESISTANCE TESTING

Strip - ShuntResistanceWire Coil1.000.9920ϒC350ϒC50ϒCTemperatureFigure 1: Qualitative Resistance-Temperature Curve for ManganiniiiWhy Measure Low Resistance?Measuring low resistance helps identify resistance elementsthat have increased above acceptable values. The operationof electrical equipment depends on the controlled flow ofcurrent within the design parameters of the given piece ofequipment. Ohm’s Law dictates that for a specified energysource, operating on V ac or V dc, the amount of currentdrawn will be dependent upon the resistance of the circuit orcomponent.In the modern age of electronics, increased demands areplaced on all aspects of electrical circuitry. Years ago theability to measure 10 milli-ohms was acceptable, but, inthe present industrial electronic environments, the field testengineer is now required to make measurements whichshow repeatability within a few micro-ohms or less. Thesetypes of measurements require the unique characteristics ofa low resistance ohmmeter’s four-wire test method, which iscovered on page 9 in this booklet.Low resistance measurements are required to prevent longterm damage to existing equipment and to minimize energywasted as heat. They indicate any restrictions in current flowthat might prevent a machine from generating its full poweror allow insufficient current to flow to activate protectivedevices in the case of a fault.Periodic tests are made to evaluate an initial condition or toidentify unexpected changes in the measured values, and thetrending of this data helps indicate and may forecast possiblefailure conditions. Excessive changes in measured values pointto the need for corrective action to prevent a major failure.When making field measurements, the operator ought tohave reference values that apply to the device being tested(the manufacturer should include this information in theliterature or name-plate supplied with the device). If the testsare a repeat of prior tests, then these records may also beused to observe the range of the anticipated measurements.If, when conducting tests, the operator records the resultsand the conditions under which the tests were performed,the information becomes the beginning of a database thatcan be used to identify any changes from fatigue, corrosion,vibration, temperature or other condition that may occur atthe test site.What is a Low Resistance Measurement?A low resistance measurement is typically a measurementbelow 1.000 ohm. At this level it is important to use testequipment that will minimize errors introduced by the testlead resistance and/or contact resistance between the probeand the material being tested. Also, at this level, standingvoltages across the item being measured (e.g. thermal emfs atjunctions between different metals) may cause errors, whichneed to be identified.To allow a measurement to compensate the errors, a fourterminal measurement method is employed with a reversibletest current and a suitable Kelvin Bridge meter. Low resistanceohmmeters are designed specifically for these applications.In addition the upper span on a number of these meterswill range into kilohms, which covers the lower ranges of aWheatstone Bridge (please see the appendix for a discussionof the Wheatstone and Kelvin Bridge methods). The lowerrange on many low resistance ohmmeters will resolve 0.1micro-ohms. This level of measurement is required to performa number of low range resistance tests.What Does the Low Resistance Measurement Tellthe Operator?Resistance (R) is the property of a circuit or element thatdetermines, for a given current, the rate at which electricalenergy is converted to heat in accordance with the formulaW I2R. The practical unit is the ohm. The low resistancemeasurement will indicate to the observant operator whendegradation has or is taking place within an electrical device.Changes in the value of a low resistance element are one ofthe best and quickest indications of degradation taking placebetween two contact points. Alternatively, readings can becompared to “like” test specimens. These elements includerail bonds, ground bonds, circuit breaker contacts, switches,transformer windings, battery strap connections, motorwindings, squirrel cage bars, bus bar with cable joints andbond connections to ground beds.iii Basic Electrical Measurements; Melvin B. Stout; 1950; page 61A GUIDE TO LOW RESISTANCE TESTING3

The measurement will alert the operator to changes havingtaken place from the initial and/or subsequent measurements.These changes can occur from a number of influencesincluding temperature, chemical corrosion, vibration, lossof torque between mating surfaces, fatigue and improperhandling.These measurements are required on a regular timed cycle inorder to chart any changes taking place. Seasonal changesmay be evident when summer and winter data are reviewed.What Problems Create the Need to Perform the Test?Assuming a device has been correctly installed in the firstplace, temperature, cycling, fatigue, vibration and corrosionall work to cause the gradual degradation of the resistancevalue of an electrical device. These influences build up over aperiod of time until a level is reached at which the device nolonger operates correctly. The critical degrading factor will bedetermined by the application.Environmental and chemical attacks are relentless. Even airwill oxidize organic materials while the ingress of moisture,oil and salt will degrade connections even more rapidly.Chemical corrosion can attack the cross sectional area of anelement, reducing the area while increasing the resistanceof the component. Electrical stresses, particularly sustainedovervoltages or impulses, can cause welds to loosen.Mechanical stress from vibration during operation can alsodegrade connections, causing resistance to rise. Theseconditions result in excessive heating at the location whenthe component is carrying the rated current, based on theformula W I2R. For example:6000 A across a 1 µΩ bus 36 Watts.6000 A across a 100 µΩ bus 3,600 Watts, which will resultin excessive heating.If left unattended, these types of problems can lead to failurein the electrical system containing the affected components.Excessive heating will ultimately result in failure due toburnout, which may open an energized circuit.Backup battery power supplies provide a good practicalexample of how degradation can occur under normaloperating conditions. Changes in current flow causeexpansion and contraction of the terminal connections,causing them to loosen or corrode. Additionally, connectionsare exposed to acid vapors, causing further degradation.These conditions result in a decrease in the surface-to-surfacecontact area with an associated increase in surface-to-surfacecontact resistance, ultimately causing excessive heating at thejunction.4A GUIDE TO LOW RESISTANCE TESTINGIndustries with Significant Resistance ProblemsIndustries that consume vast amounts of electrical powermust include low resistance ohmmeter measurements intheir maintenance operations. Not only does abnormallyhigh resistance cause unwanted heating, possibly leadingto danger, but it also causes energy losses which increaseoperating costs; in effect you are paying for energy which youcan’t use.In addition, there are industries that have critical specificationson bond connections to ensure solid connections to “groundbeds.” Poor connections reduce the effectiveness of theground bed and can cause significant power quality-relatedproblems and/or catastrophic failure in the event of majorelectrical surge. A number of sub-assembly operations supplycomponents to aircraft manufacturers that specify lowresistance connections to the airframe. Strap connectionsbetween cells on a power back-up battery system also requirevery low resistance. A general list of industries include:nPower generation and distribution companiesnChemical ns companiesnAutomotive manufacturersnAircraft manufacturersnAnyone with UPS battery back-up systemsSpecific Examples of Apparatus in Needof Low Resistance TestingAs we have shown, low resistance ohmmeters haveapplication in a wide range of industries and can help identifya number of problems that could lead to apparatus failure.In general manufacturing industries, motor windings, circuitbreakers, bus bar connections, coils, ground bonds, switches,weld joints, lightning conductors, small transformers andresistive components all require low resistance testing.Following are some of the more typical applications.Motor ArmatureArmature windings can be tested to identify shortingbetween adjacent coils or conductors. Squirrel cage bars inthe rotor can separate from the end plates, resulting in lossof performance. If a motor appears to be losing power, a lowresistance test should be performed. Alternatively, tests canbe made when bearings are being replaced at a periodic orannual shutdown.

C1should have historical data to make the determination on thesuitability of the connection. If left uncorrected, loss of powerand/or excessive heating could lead to a meltdown at theconnection.P1Transformer TestingC2Transformer winding tests are performed in the factory andthen periodically in the field. The factory test is performedat ambient temperature. A second factory test is a heat runto verify that, at rated power, the resistance of the windingsremains within its designed temperature rise characteristics.P2Figure 2: Bar to Bar Testing on DC Motor RotorP1BP1AP1CP1DP1NC1C2P2AP2BP2CP2DP2NFigure 3: Bus Bar ConnectionsBar to bar testing on dc motor rotors is performed toidentify open or shorted coils (see Figure 2). These tests areperformed with spring loaded hand probes. This is a dynamicmethod to determine the conditions of the windings andthe soldered connections to the riser on the commutatorsegments. When test data is reviewed periodically, the effectsof overheating due to excessive temperature rise can beidentified.Automotive AssemblyCable leads in a “robot” spot welder can work-hardenthrough continual flexing. Eventually fatigue can occurcausing strands to break. This condition results in a high leadresistance with loss of power to the weld, producing a poorspot-weld (nugget) or even complete failure of the machine.Power Generation and Distribution (high current joints,connections and bus bars)Bus bars in a power system consisting of lap joints and otherconnections, are used to deliver current to the elements inthe system. These bolted connections can be degraded byvibration and corrosion. (See Figure 3.) The bolts are stressedto a specific tightness (torque), and the quickest and mosteconomical way to determine the quality of the connectionis to measure the resistance

2 A GUIDE TO LOW RESISTANCE TESTING introduction The quantitative study of electrical circuits originated in 1827, when Georg Simon Ohm published his famous book “Die galvanische Kette, mathematisch bearbeitet” in which he gave his complete theory

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