Use and misuse of low resistance ohmmetersThe effective operation of a low resistance ohmmeter relies on the user using the correct test leads. Battery operated instruments are designed for a specific lead resistance, based on the operational life of the test sequence. The specified leads allow for a reasonable current drain from the power supply for the test cycle. If leads with a higher resistance are used, the current used for the test can be lower than the meter requires, potentially causing a signal-to-noise problem that can reduce the accuracy and / or repeatability of the measurement.If leads with lower than the specified resistance values are used, the test cycle for the instrument will be shorter than anticipated. This situation may be suitable if the meter is to be used in a test program with high background electrical noise. The use of special leads with shielding can also be a solution for these high noise situations. A common error in the field is to use a low resistance ohmmeter to sample the resistance of a ground bed. This application is incorrect, as the ground bed test method requires an instrument that toggles the test signal at a known frequency and current level. A low resistance ohmmeter used in this application will provide an erroneous reading as the ground current will have an undue influence on the measurement. A genuine ground tester works in essentially the same way as a low resistance ohmmeter, that is, by injecting a current into the test sample and measuring the voltage drop across it. However, the earth typically carries numerous currents originating from other sources, such as the utility. These will interfere with the d.c. measurement being taken by a low resistance ohmmeter. The genuine ground tester, however, operates with a definitive alternating square wave of a frequency distinct from utility harmonics. In this manner, it is able to do a discrete measurement, free of noise influence.Current selectionDepending on the selected instrument, the current selection can be either manual or automatic. The user should select the highest current suitable for the test to provide the best signal to noise ratio for the measurement. On instruments that offer current levels in excess of 10 A, care is required to minimize any heating of the sample that would itself cause the resistance of the sample to change. Instruments designed to test circuit breakers have much higher current characteristics. For high current paths, like overhead line joints, bus bars and circuit breakers, it is important to make the measurement with the highest current possible, to be able to detect degraded current paths. Phenomena called 'hot spots' heat up the current path at high currents and the heat increases resistance even more, which makes the situation worse. This problem needs to be detected before it happens within nominal currents and creates a problem. To be compliant with circuit breaker standards, a minimum 50 A (IEC) and 100 A (ANSI) is required when performing low resistance measurements.In circuit breakers, contaminations have been seen which influence the results and cause a higher than expected reading. Using a high current can break through the contamination and thus provide an accurate and correct value.Instruments designed specifically to test transformers have a special high voltage power level at the start of a test, to saturate the winding. These instruments then switch to a lower constant current mode to measure the winding on the transformer. It is also important that the instrument discharges the transformer when the measurement is completed. If not, lethal voltages can be present at disconnection. Dedicated test instruments with these features integrated are available.Warning: Never use a non-dedicated LRO to measure the winding resistance on a power transformer, since lethal voltages can be present if a winding is not discharged correctly before the leads are disconnected.Probe and lead selection The potential and current leads are either connected separately or to a probe. When probes are used the potential connection is identified with a P. The connections are placed in contact with the sample so that the P-identified contacts or leads are positioned towards each other. The current contacts are then positioned outside or away from the potential connections. This causes the current to flow with a more uniform current density across the sample being measured.For the more rigorous tests, separate test leads are used and the current connections are positioned away from the potential connections by a distance that is 1.5 times the circumference of the sample being measured. ASTM Standard B193-65 provides guidelines for making a measurement that will establish uniform current density. This standard suggests separating the current probes from the potential probes by 1.5 times the cross sectional perimeter of the test specimen. Fig 20 on the following page shows a test being made to the standard on a cylindrical test item.The use of probes, Kelvin Clips, or C-clamps will meet most field requirements as the user should be making repetitive measurements under the same conditions. The sharp points on the probes should leave a mark on the specimen for future tests. In some situations a marker pen can show the test area and the probe positions will be identified by the probe indents.Leads are available in a number of lengths to meet different field application requirements. The probe selection is made from separate current and potential leads with clips to connect to the test sample. Helical spring point probes have both potential and current probes in the same handle. The 'P' identification on the probe identifies the position on the sample at which the measurement is taken. This probe arrangement provides a practical method when making repetitive measurements (ideal for tests on strap connections in UPS battery supply systems).Kelvin Clips and C-clamps have the current and potential connections 180º from each other, providing separate current and potential connections. The size of the terminal connection determines which one to select. See Fig 21 for the different probe / lead configurations.Note: The order of connection of potential and current clips is not important. However, never connect the potential clip to the current clip as this will cause an error in the measurement due to the voltage drop at the current connection interface at the sample. Fig 20: ASTM standard B193-65 Fig 21: Probe / lead configurationsLow range tests When measuring on the extreme edge of precision and sensitivity, factors that would be too small to be of consequence in conventional tests, become significant.In low resistance tests, thermal emfs (electromotive forces), also known as Seebeck voltage, can produce voltage gradients across the test sample. Although only on the millivolt level, and of little or no influence on common multimeter tests, these can cause fluctuations of several digits. Such instability defeats the purpose of a high precision measurement. In addition, a.c. interference can be induced by nearby electric or magnetic fields, or can be present from the float charge on standby battery systems, or through leaky switches, electrical imbalance and so on.This problem is readily overcome by taking readings in forward and reverse polarity and then averaging them. Some models accomplish this with a manually operated reversal switch, while others do the two measurements automatically, then show the average reading. If unidirectional measurement is required (to save time (as in battery strap tests)), the tester may have an override function. Another sophisticated technique automatically measures the magnitude and slope of thermal emfs and subtracts from the shown reading.However, the simplest technique is to test with high current if it is a high current path. Since the measured voltage becomes significantly higher than the thermal emf voltage the accuracy will be kept. This simple method also saves time since there is no need for reversed polarity.Types of testers - which one?Milli-ohmmeterAs the name implies, a milli-ohmmeter is less sensitive than a micro-ohmmeter, with measurement capability in the milliohm rather than microhm range (minimum resolution of 0.01 millohm. This type of instrument is normally used for general circuit and component verification. Milli-ohmmeters also tend to be less expensive than micro-ohmmeters, making them a good choice if measurement sensitivity and resolution are not critical. The maximum test current is typically less than 2 A and as low as 0.2 A.10 Amp micro-ohmmeterThe field portable micro-ohmmeter with a 10 A maximum test current is the 'work horse' instrument for most users because it covers the majority of field applications. The 10 A output not only provides a comfortable and suitable test current through the test sample to make the measurement, but also allows for reduced weight and improved battery operation.www.megger.com 1716 A guide to low resistance testing www.megger.com
