After measurement, it can be checked 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 small and medium-sized transformers, the DC bridge method is basically adopted for most of them. When the resistance value of the tested coil is more than 1 ohm, the single-arm bridge is generally used, and the double-arm is used for the resistance value below 1 ohm. Bridge measurement. 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 galvanometer pointer to the zero line in the middle of the galvanometer scale. After fine-tuning, record the resistance value when the pointer stops at the zero position. At the same time, the measured coil resistance value = Multiplying factor × measuring arm resistance value. After the measurement is completed, the galvanometer button should be opened first, and then the power switch should be released.

During the measurement series, in addition to strictly complying with the electrical safety regulations and equipment test regulations, the following items must be paid special attention to:

1. Measure when the temperature of the coil is stable, and the temperature difference between the upper and lower parts of the transformer tank should not exceed 3°C;

2. Because the transformer coil has inductance, the charging current during measurement is not stable, so you must count 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 contaminants to make their contact poor. Generally, it is necessary to switch several times before measuring to avoid misjudgment.