The DC resistance measurement is to check the quality of the winding welding and the short circuit fault between the windings, whether the tapping power switch position is appropriate, whether the specific position is consistent with the label, whether the wire is broken or loose, and the winding of the series winding is not broken. The measurement of the DC resistance tester is not only the maintenance of the transformer, the basic test report after the predictive analysis test and the change of the tap power switch position, but also the key inspection item after common failures.

In the process of using the DC resistance tester, when the resistance value measured by the device exceeds the limit, first consider whether there is a measurement error. For example, whether the external lead is connected, whether the test lead is too long or too thin, whether the contact is good, whether the battery voltage in the bridge is insufficient, and so on.

Secondly, we must understand that the resistance value of DC resistance is greatly affected by temperature. Therefore, it must be converted to the same temperature for comparison, and generally based on the oil temperature of the upper layer.

Furthermore, the three-phase distribution transformer currently in use has a Y-shaped connection for the high-voltage winding. When the resistance value exceeds the limit value, the following formula can also be used to find the defective phase.

RA=（RAB+rac-rbc）/2，RB=（RAB+rac-rac）/2，RC（RBC+rac-rab）/2

Finally, it is more common to analyze whether the tap switch has poor contact, which leads to a high resistance value. If the switch is clean and electroplated, insufficient spring pressure, uneven stress, and carbon deposits on the overvoltage contacts will cause the resistance value to be turned on. At this time, the tap switch cover should be opened and turned several times, usually to eliminate it.

If the limit is still exceeded after the above four points of inspection, it may be due to false welding, welding, wire breakage or layer short circuit or winding burnout of the winding and lead. The user can proceed according to the actual situation.