The value of a system is in its ability to work on demand. Operations are predicated on many systems being available at an instant’s notice. When elements break, production is lost and time is wasted making emergency repairs. Taking and analyzing periodic low resistance measurements saves companies money by helping identify problems before they result in catastrophic failure.The practical example shown in Fig 22 shows how trending low resistance measurements made on a periodic basis provides critical information to the user. When low resistance measurements are made on stranded cables on spot welding robot #23, the user is gathering data to estimate when fatigue to the current conductor will degrade the quality of the weld nugget. The test data starts with the wire manufacturer’s specifications. The example shows that a resistance increase of up to 10% is acceptable. In this case, measurements are made after a specific number of weld operations. When charting this data, observe the rate of change as the readings approach the end of life for the stranded cable. The critical factor could have been long-term exposure to a chemical solvent. In other operations the critical factor is time, with tests done on a seasonal basis or on specified number of days.Fig 22: Trending analysis of low resistance readingsFig 23: C1 clip being connected to end of circuit being testedFig 24: Duplex hand spike being used to perform same test as shown in Fig 23Measuring components of a systemWhen using the current and potential as split test leads there is the ability to locate faulty components and connections by probing at each connection or joint and looking at the increase in resistance.An example is measuring the resistance of a cable to lug joint or lug to bolted connection while still connected to a system.In Figs 23 and 24, a kelvin clip is shown connected to a bus bar for the C2 and P2 connections, although these connections could easily be done using separate clips.Fig 23 shows a large C1 clip being connected to the end of the circuit being tested, which in this case is the end of a cable. A single probe tip is being used for the P1 connection to easily probe to the point the measurement is required.In Fig 24, a duplex hand spike is being used to perform the same tests. Roughly the same resistance values will be measured, although in practice they will have slight differences due to the current density difference produced by the different C1 connection point.The test results in Fig 24 show a jump in resistance of nearly 1.8 mW at the connection between the cable and the crimp lug. This would not be detected using a continuity test of 200 mA or a multimeter. This additional resistance will, over time, develop into a larger value eventually causing a breakdown or even a fire. As it stands, the additional resistance will at least create power losses.High currents in low resistance measurementLow resistance measurements are good for identifying resistive elements that change over time due to environmental conditions. Conditions that can degrade devices or materials include, temperature, noise ratio or induced currents, thermal EMF / Seebeck voltage, fatigue, corrosion, vibration, oxidization, hot spots (see "Potential sources of error / ensure quality results" below).Low resistance measurements are typically below 1 A, so it is important that test equipment errors be minimized. To minimise these errors as much as possible use the four wire (Kelvin) test method, which gives accurate results when low resistance is measured. ■High currents are recommended by International high voltage Circuit Breaker test standards and by Megger (taking care of heating issues) ■Higher test currents give a better chance of good reliable test results ■Bad low current results do not always indicate that a contact is in a bad state (contamination) or that a good result indicates a good contact condition (hot spots)The International Standards for high voltage Circuit Breaker tests can be found in IEC 622 7 1 and IEEE C32.09.Test Current (d.c.) ■Minimum 50 A (IEC): 100 A (ANSI)Potential sources of error / ensure quality resultsThe user can compromise low resistance measurements if the wrong test equipment is used or the temperature at the test site is not determined and noted on the test data sheet. Before a test, surface preparation can be critical. Heavy scale or oxide coatings should be removed to expose a clean surface and ensure good current connections.Test leads / probesAn instrument’s specification should have a recommended listing of suitable test leads. The user should always check that the correct leads are being used as leads can look alike but have different resistances, which can limit the maximum current that the instrument can produce.Do not use thermocouple extension wire in place of copper leads as the material mismatch will produce erratic data that will change as the site temperature varies with the seasons.The probe selection is also critical. High current tests require secure connections to the work surface because high resistance at the contacting point can limit the expected level of test current, causing a poor signal-to-noise ratio, with erratic results. Use of unsuitable probes for the particular application can lead to unreliable results.In all cases tests are done with current injection and potential measurements made at separate locations on the component. Potential test clips must never be connected to the current connection as the voltage drop at the current interface will be added to the potential www.megger.com 2322 A guide to low resistance testing www.megger.com
