Low resistance measurement is an invaluable diagnostic aid in a wide variety of applications and, most of the time, it’s fast and easy to carry out. There are, however, a few pitfalls that often result in calls to our technical support team. Here are some of the questions the team is regularly asked.
Q: Why is a four terminal connection frequently used when making low resistance measurements?
A: This arrangement is used so that the resistance of the test leads and connections doesn’t influence the results obtained. Such influence is usually unimportant in the measure of resistances from, say, a few tens of ohms upward, but with lower values the resistance of the leads can be similar to or even greater than the resistance of the device under test. Also any resistance of the connections from the test leads to the test piece can vary depending on the condition of the conductor. Four terminal connections use separate current and voltage leads between the test instrument and the device under test. This means that the voltage is measured directly at the device and the measurement is unaffected by any voltage drop in the current leads. Even though the current and voltage test leads are electrically separate, they’re usually combined in the same lead set, which may be terminated with Kelvin clamps. These are, in effect, modified crocodile clips that provide separate voltage and current connections to the device under test.
Q: When making low resistance measurements, how do I overcome problems with standing EMFs (electro motive force) on circuits involving connections between different metals?
A: These problems can be overcome fairly easily by making a measurement, then reversing the polarity of the test leads and making a second measurement. The required resistance value is the arithmetic average of the measurements. Some instruments, such as those in the DLRO10 range of digital low resistance ohmmeters, feature automatic current reversal, so that the correct result is displayed without operator intervention even if there is a standing EMF on the circuit under test.
Q: What type of instrument should I use for measuring circuit breaker contact resistance?
A: Ideally an instrument that’s designed for the job! A general-purpose ohmmeter is unlikely to give satisfactory results, even if it can nominally measure the low values of resistance involved. It’s important for the instrument to be capable of delivering a high test current – 50 A or greater is required by most standards – it should also support four-terminal testing, and ideally it should be suitable for use with DualGround test techniques. In cases where the measurement is being carried out to determine the remaining length of the arcing contact, the instrument will need to offer facilities for dynamic resistance measurement (DRM).
Q: Why does it take so long to measure the resistance of transformer windings?
A: The windings are highly inductive so, when a DC voltage is applied, the current in the winding increases exponentially, which means it increases rapidly at first but takes a long time (theoretically an infinite time!) to reach its final value. Only then can the resistance be measured. In practice, the best transformer ohmmeters, such as the MTO250, offer ways of reducing the time needed for tests. One method is to provide a high test current capability, which means that the rate of increase of current in the winding will never be limited by the test set. Another approach is to provide dual-channel operation, so that measurements can be performed on two windings simultaneously.