During field DC resistance testing, it is common to encounter inconsistent repeated measurement results on the same device, large differences among three-phase data, and fluctuating readings for small resistances. Most of these problems are related to wiring methods, measurement channel switching logic, and on-site interference, rather than faults in the instrument itself. The following summarizes common issues and troubleshooting approaches for three-channel DC resistance testers from the perspective of field operation and maintenance, point by point.
Point 1: Why three-phase measurement should use separate channels rather than one channel in rotation. A three-channel DC resistance tester can connect to three-phase windings simultaneously, apply excitation to each phase separately, and collect the voltage drop. Compared with single-channel phase-by-phase switching, three-channel parallel acquisition reduces measurement time and also lowers data dispersion caused by winding temperature changes and insufficient residual inductance discharge during phase-by-phase switching. If poor repeatability of three-phase data is found on site, priority should be given to checking whether the wiring of each channel is independent and whether the clamping points are oxidized or loose, rather than repeatedly restarting the instrument.
Point 2: How to handle fluctuating readings in the small resistance range. The DC resistance of transformer windings, busbar joints, and similar components is often in the milliohm range, so the proportion of lead resistance and contact resistance cannot be ignored. On site, a four-wire connection method can be adopted, clamping the current leads and voltage sampling leads separately; the clamps and the surface of the test object should be polished to remove paint film and oxide layers; and personnel should avoid touching the leads during measurement. If fluctuation still exists, it is necessary to investigate whether common-mode interference is caused by intermittent loads such as inverters and welding machines on site.
Point 3: Performance of anti-interference capability under actual working conditions. The DC resistance measurement signal is a slowly varying quantity and is sensitive to power-frequency and high-frequency interference. Instruments with filtering and power-frequency suppression design can maintain stable readings in environments with background electromagnetic fields, such as substations and distribution rooms. If the value drifts unidirectionally over time on site, it is mostly related to the temperature rise of the test object. The oil surface temperature or ambient temperature should be recorded and compared after temperature conversion, rather than directly judging it as a winding abnormality.
Point 4: Troubleshooting sequence for poor data repeatability. It is recommended to eliminate items one by one in the order of wiring, clamping, range, filtering, and temperature. First confirm that the four-wire connection is correct, then check the clamp contact, then verify whether the range matches the resistance of the test object, and finally consider the filtering settings and temperature correction. For three-channel models, it should also be checked whether all channels are under the same sampling conditions to avoid comparison deviations caused by different ranges on a single channel.
Point 5: Daily maintenance and usage precautions. The instrument should be stored in a dry environment, the test leads should avoid excessive bending, and the clamps should be cleaned regularly. If it has not been used for a long time, check the status of the battery and power adapter before powering on. After measurement is completed, wait for the test object to fully discharge before disconnecting the leads to ensure the safety of personnel and equipment.
