measurement and produce an error in the reading. The ideal current connection injects current above the potential measurement position. When these points are close to each other the Kelvin clip or C-clamp connectors are used, injecting current 180º from the potential connection (see Fig 25).The test leads are matched to battery operated meters to ensure that the nominal level of test current will be delivered to the test specimen.Finally, probes are designed to make electrical connection with the test sample. They are not intended to be used to clean surfaces, open tins, etc.Fig 25: Correct and incorrect probe placementsProbes are available in five basic styles. Each probe is designed to address specific field and / or application situations. Fig 26 shows some of the different styles.Fixed Point: Most economical and lightweight probes.Kelvin Clips: Feature spade lugs on the outboard end and alligator clips with insulated silver or gold plated jaws.Linear Spring Points: These probes are designed with spring points, which recess into the handle to allow for unevenness of the surface. They are designed for clean surfaces as they have no 'cutting' action to allow them to bite through surface contamination. Helical Spring Points: The tips rotate and compress into the body of the probe, allowing the probes to break through any grease or surface film, ensuring an accurate measurement. Additionally, these probes will leave a mark on the test surface to identify the points where the test was done. Care should be taken when using these probes if the surface being contacted is sensitive to pressure points.C-Clamps: A current passes through the C-clamp and screw thread while the potential passes through a four point anvil insulated from the clamp metal.Fig 26: Basic styles of probesAccuracy statementsQuality low resistance ohmmeters will show their accuracy statement as '±X.X% of reading, ±X LSD'. Beware of instrument accuracies stated as a percent of range rather than a percent of reading. While these accuracy statements can look alike, the measurements made on an instrument with (% of range) accuracy would provide readings that are less accurate.The resolution of an instrument reading is typically one half the least significant digit (LSD) noted in the accuracy statement. The magnitude of the LSD influences the repeatability of the measurement. A large LSD number is due to the low sensitivity of the instrument, adding an additional error to the measurement.Check the temperature coefficient of the selected instrument. The temperature coefficient (% of reading per degree) is multiplied by the site temperature difference from the instrument’s calibrated temperature and will influence the accuracy of the field measurements. An instrument that includes an accuracy notation of +0.2% / ºC should not be used in the field, as its best utilization would be in a laboratory with a constant ambient environment.The user must be aware of all these characteristics when selecting the test instrument.InterferenceA strong electrical field, flux linkage from a high current circuit or voltage induced from a high voltage conductor can cause interference at the test site. In addition ground currents can induce noise on a conductor. Interference can reduce sensitivity and produce unstable readings. An instrument with low noise rejection, or hum attenuation may be stable when tested on the bench, but be erratic in selective field conditions.Modern electronics can detect the level of noise and some instruments use this to show when excessive noise is present to make a valid measurement.A simple technique to minimize noise problems is to measure at high current since the measured signal gets larger than the noise it self.Delivery of stated test current under load Battery operated, digital low resistance ohmmeters have different test currents dependent on the selected range. The lowest resistance range has the highest current level and as the range increases the current will decrease (as the range increases by a factor of ten the test current will decrease by a factor of ten). This feature allows for an effective balance between weight and functionality.The output current delivered by the instrument is not critical, as the instrument will be measuring the actual test current at the time of the test. However, the instrument must be able to deliver sufficient current to produce a clear signal in the presence of typical noise. A typical instrument can have a 10% to 20% tolerance on the maximum current rating. But, to make a good potential measurement, the current must be stable. The critical factor for the measurement is the voltage measurement via the potential leads (Ohms Law).The one test area where the test current is critical is on a transformer, due to the magnetic characteristics of the winding. Sufficient current is required to saturate the winding, and then a lower constant current is used to do the measurement.Taking a measurement at a stable plateauA de-energized test specimen provides a stable platform on which to make the measurement. Live circuits can produce an unstable test platform. An example of the latter is the test of battery straps on a UPS system. The charging and / or discharging currents may induce noise across the battery straps being measured, and at the same time cause the resistance values to increase (due to heating of the strap and its connections). When collecting data, the user must define the test conditions. As noted previously, temperature can have a significant influence on any measurements made. The user should note the temperature and document any electrical equipment that is in operation in the test area.Material resistivityConductors of the same dimensions have different resistances if they are made of different materials, due to the varying number of free electrons in varying substances. We account for these differences with the term resistivity, which is the resistance of a sample of the material having dimensions with specified unit values. While scientists tend to look at cubes of material as the measurement standard (one centimeter cube or one inch cube), conductors tend to be circular, making a circular standard important for practical use. The resistivity of a material is defined in ohm-circular mils per foot; that is, the resistance (in ohms) of a piece of material one foot long and one circular mil cross section. It is defined at a temperature of 20 ºC (68 ºF). Table 5 shows the resistivities for a number of conducting materialsv:In most field applications the user determines the suitability of a field measurement against a pre-selected specification. In most cases, these specifications have been generated from the following formula (at 20 ºC (68 ºF)):R = ρL/Aρ = Resistivity of the material in ohm-CM per foot.L = Distance between two points on the material, in feet.A = Cross section area measured in circular mils.Table 5: Resistivities of conductors SubstanceMicrohmsOhm-cm per Footcm cubein cubeAluminum2.831.1117.0Carbon (Graphite)7002754210Constantan (Cu 60%, Ni 40%)4919.3295Copper (annealed)1.720.6810.4Iron (99.98% pure)103.9460.2Lead228.66132Manganin (Cu 84%, Ni 4%, Mn 12%)4417.3264Mercury95.7837.7576Platinum9.93.959.5Silver1.650.659.9Tungsten5.52.1733.1Zinc6.12.436.7v Electrical Metermen’s Handbook; Third Edition; 1965; page 479www.megger.com 2524 A guide to low resistance testing www.megger.com
