By hvhipot 
DC resistance tester measurement method and several points of attention:
1. Measurement method
Measuring DC resistance is an important item in transformer testing. Through measurement, it is possible to check whether the conductive circuit of the equipment has defects such as poor contact, poor welding, coil failure, and wiring errors. In the actual measurement of medium and small transformers, most of the DC bridge method is used. When the resistance value of the tested coil is above 1 ohm, it is generally measured with a single-arm bridge, and when the resistance value of the tested coil is below 1 ohm, it is measured with a double-arm bridge. When using double-arm bridge wiring, the potential pile head of the bridge should be close to the resistance to be measured, and the current pile head should be connected to the potential pile head. Before measuring, you should first estimate the resistance value of the tested coil, place the bridge magnification button in the appropriate position, short-circuit the non-tested coil and ground it, then turn on the power switch to charge, and press the galvanometer switch after it is fully charged. Quickly adjust the measuring arm to move the pointer of the galvanometer to the zero line in the middle of the scale of the galvanometer, and make fine adjustments. When the pointer stops at the zero position, the resistance value is recorded. At this time, the resistance value of the measured coil = the multiplying factor. ×Measure the arm resistance value. After the measurement is completed, first open the galvanometer button, and then release the power switch.
2. Matters needing attention
In the measurement process, in addition to strictly complying with the electrical safety regulations and equipment test regulations, special attention should also be paid to:
1) Measure when the temperature of the coil is stable, and it is required that the temperature difference between the upper and lower parts of the transformer tank should not exceed 3°C;
2) Due to the inductance of the transformer coil, the charging current during measurement is not stable, so the counting must be performed after the current is stable, and measures to shorten the charging time should be taken if necessary;
3) Minimize the contact resistance of the wires in the test circuit. The transformer taps in operation are often affected by oil film and other dirt to make their contact poor. Generally, it is necessary to switch several times before measuring to avoid misjudgment.