Low Resistance ohmmeters have an important role in testing and commissioning electrical power systems. Rather than simply checking continuity using a basic multimeter, the low-resistance ohmmeter's measurement gives a much more accurate and meaningful result. This is especially true for conductors designed to carry high current. A power system main is often rated for hundreds or thousands of amps. Digital Low Resistance Ohmmeters (DLRO) are able to push a substantial test current through the test object and yield a result with resolution of 0.1 micro-ohm. This type of precise resolution enables detection of loose connections, defective materials. High Resistance connections are the primary source of energy and heat lost from a power system. Energ lost from resistive heating is proportional to the current of the load multiplied time the resistance squared. The resistance counts a lot! A small increase in resistance leads to a large increase in heat dissipated. This is why torque specs and micro-ohmmeters are so important.
On a slightly different topic; If you ever tried to measure a transformer's winding resistance by using a micro-ohmmeter, then you are probably aware that it won't give you an accurate measurement - if it will make the measurement at all. Instead of a typical ductor or DLRO, you must use a special ohmmeter called a winding resistance meter- which can be quite expensive. However, the unique Sonel MMR Ohmmeters are equipped to measure winding resistance and inductive object resistance measurement. MMR also has a DEMAG function for transformer demagnetization after the resistance test. These features fortunately come at no additional cost to the user. The Sonel MMR-650 (10A) and the Sonel MMR 6700 (200A) are priced similarly to other micro-ohmmeters with no such capability. That would explain why Sonel MMR is becoming one of the most popular micro-ohmmeters
Measuring resistance of a breaker contact, bolted connection, or any conductor is done using ohm's law. The test instrument pushes a known test-current through the object via the current leads. Then a second set of leads measures the volt drop. This is called the Kelvin Method and thus the dual-measuring test leads are called Kelvin probes. Most any micro-ohmmeter or Low resistance ohmmeter will perform this measurement using some pre-determined test current. In fact Micro-ohmmeters are usually classified based on the maximum test current output. common Amperages are 10A, 100A and 200A max. A higher test current allows for smaller differences in ampacity to become evident. However, even the best micro-ohmmeters are struggles to measure resistance of highly inductive objects such as transformer windings and motor winding resistance. This is because inductive objects make it difficult to get stable test current through the winding. For the measurement to stabilize, the core of the inductor must polarize and magnetize completely before the test current stabilizes and the measurement can be made accurately. For this reason, winding resistance test sets are equipped with more powerful outputs compared to a micro-ohmmeter. For example, a micro-ohmmeter may have an max output 200A @ 6V (1200W) whereas a winding resistance meter may have an output of 60A @60V (3600W max) so it can provide enough power to charge the core of the transformer to full saturation in a short period of time. Problem is a winding resistance meter cost is about $17K. Meanwhile, Sonel has combined the micro-ohmmeter and the winding resistance meter into one test set for under $7K. It even has the Auto DEMAG built in. Its made by Sonel in Poland and distributed in USA through Nemec Industries. Please contact Louis Nemec to set up your free demo of this versatile ohmmeter. You can test the resistance of breakers, bolted connections, transformers, and motors with Sonel MMR650 (10A) or Sonel MMR 6700 (200A).
Please contact Louis Nemec (770) 331-1393 Louis@nemecindustries.com