The best 10 A micro-ohmmeters offer measurements from 0.1 microhm to 2000 ohms with a best resolution of 0.1 microhm at the low end of the range and accuracy of ±0.2%, ±0.2 microhm. On some instruments, different measurement modes can be selected which address different types of test conditions. Measurement modes could include manual, automatic or a continuous test, or a high power test on windings. The following is a selected list of key d.c. resistance measurement applications for 10 A micro-ohmmeters. ■Switch and contact breaker resistance ■Bus bar and cable joints ■Aircraft frame bonds and static control circuits ■Welded joint integrity ■Intercell strap connections on battery systems ■Resistive components (quality control) ■Small transformer and motor winding resistance ■Rail and pipe bonds ■Metal alloy welds and fuse resistance ■Graphite electrodes and other composites ■Wire and cable resistance ■Transmitter aerial and lightning conductor bonding100 Amp and above micro-ohmmeterAccording to IEC62271-100, a test of the contact resistance of high voltage a.c. circuit breakers calls for a test current with any convenient value between 50 A and the rated normal current. ANSI C37.09 specifies that the test current should be a minimum of 100 A. Most electrical utilities prefer to test at higher currents, as they believe this is more representative of working conditions. Field portable micro-ohmmeters are available that can deliver anywhere from 100 A up to 600 A (subject to the load resistance and supply voltage). The best instruments have measurement resolution to 0.1 microhm and offer variable test current to address a wider range of applications. If a test is done at 10 A and then at a higher current, the user can get a better understanding of the maintenance requirements for the circuit breaker. As previously stated, 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.Industry standard test currents were originally developed according to available technology in metering. With early technology, enormous currents were needed to develop a measurable voltage across a test sample with negligible resistance. By Ohm’s Law, a typical meter of one millivolt full scale would require 100 A to measure as little as a microhm. The microhm being the preferred unit of measurement for low resistance tests, this made 100 A testers the standard design for early instrumentation.Unfortunately, this design made for testers that were large, difficult to move, and of limited practicality in the field. The development of cross-coil movements, with the balancing of voltage and current in two separate coils driving the pointer, produced a dramatic increase in sensitivity, and brought workable test currents down to the familiar 10 A level. Of course, microprocessors have further extended the sensitivity of modern instruments. But this process is limited by the need for adequate noise suppression. Low resistance ohmmeters measure at levels several powers of ten lower than common multimeters. Noise becomes large by comparison, and makes noise suppression critical to the adequate function of the instrument. The tester, therefore, must maintain an adequate signal-to-noise ratio.Testers with large current outputs are still widely used, however, for tests on specific types of equipment. The limiting factor on the high end is principally the generation of heat. Tests at too high a current can cause a heating effect on the measurements, be injurious to the test item, and even cause welding of contacts. Certain types of equipment such as high voltage a.c. circuit breakers (see IEC62271-100) have sufficiently large conductors and areas of contact to carry currents of several hundred Amps without experiencing these harmful effects. The demand for test current is critical when coils are tested, transformers or other magnetic components due to the inductive characteristics of these types of components. Industry standards may call for some specified high current. Such selection is typically a compromise between various factors as discussed above, with a view toward practicality, rather than scientifically justified demands. Sophisticated testers will automatically balance current against the load, for maximum precision and minimum heat effect, so that it is not necessary to impose specific, pre-selected values on the test procedure. Some suppliers specify 200+ Amps for SF6 breaker contacts to overcome oxidation on the contact surfaces.Note: The Kelvin Bridge instrument, which has been used to make measurements in the sub-microhm region, uses approximately 5 A of test current.Auto rangeAuto range capability on an instrument allows the user full use of the test probes. An auto range instrument will automatically select the range to give the best use of the display, provide the most sensitive reading for the measurement and optimize the resolution of the reading. When taking a series of readings, the user will be able to maximize the use of their time.Ingress protectionSomewhere in the fine print (specifications) of most test instrument product bulletins is an IP rating, a number that gives the user vital information. In fact, the IP rating lets the user know whether a piece of test equipment is suitable for an application and / or test environment.'IP' stands for 'ingress protection'. That is the degree to which the instrument can withstand invasion by foreign matter. The IP rating system was established by the IEC (International Electrotechnical Commission), in their Standard 529, and is used as a guide to help the user protect the life of the instrument. It also can help the user make a more informed purchase decision by ensuring that the test equipment is designed to work in the environments that a user faces.The IP rating comprises of two numbers, each signify a separate characteristic. The designation shows how well the item is sealed against invasion by foreign matter, both moisture and dust (the higher the number, the better the degree of protection). What would a typical rating of IP54 tell a buyer about the application capabilities of a model? If you want to sound thoroughly knowledgeable, that’s IP five-four, not fifty-four. Each number relates to a separate rating, not to each other.The first number refers to particulate ingress, reflecting the degree to which solid objects can penetrate the enclosure. A level of '5' means 'dust protected', as well as protected from invasion with a wire down to 1.0 mm. There is only one higher category: 'dust tight'.The second number refers to moisture. A rating of '4' means a resistance to 'splashing water, any direction'. The higher ratings of 5 through to 8 indicate 'jetting water' and 'temporary' or 'continuous' immersion.As an example, suppose an instrument under consideration is rated to IP43. What would that tell the user about its usability? Could it be thoroughly utilized in a quarry or cement plant? Hardly! The particulate rating 4 means 'objects equal or greater than 1 mm'. That’s a boulder in comparison to particles typically produced by industrial processes. Flying dust could put the instrument out of commission.In addition to circuit breakers, electrical utilities and test companies use higher current micro-ohmmeters on other high voltage apparatus, including: ■Cables ■Cable joints ■Overhead line joints ■Ground connections ■Lightning protections ■Welds ■Bus bars ■Switchgear in generalWhen a 100 A (or above) micro-ohmmeter is used, users should be aware of certain technical issues related to tests at high currents. Some users have shown that they do a 10 A test and then see improved resistance readings with 100 A (or more) test currents. This difference in the measurements raises the question of whether there is a need for additional maintenance. A strict reading of Ohm’s Law does not indicate the need for the higher current to do the measurement. In the equation R = V/I, the magnitude of the current is not defined. Is this a situation where the high current is blasting contaminants away from the contacts, and at the same time welding the contacts together? The user should be aware that they could be masking a potential problem in a power distribution system and avoiding necessary maintenance.Users should also be aware that high current meters are intended to be used at high currents. Their accuracy may reduce considerably at low currents, particularly when measuring small resistances. Nominal vs. absolute test current levelsBattery operated digital low resistance ohmmeters have different test currents, which are a function of the selected range. The lowest range has the highest current level and as the range increases the current decreases. As the range increases by a factor of 10, the test current will decrease by a factor of 10. This action allows for a balance of weight and function; if the current were to increase as the range increases, this field instrument would lose much of its portability, and its usefulness for field tests would decrease significantly. In power plants, substation and distribution sites, the test equipment is exposed to interference from high currents generated in the area. The user will have to determine the test current level to provide the most accurate and repeatable measurements. www.megger.com 1918 A guide to low resistance testing www.megger.com
