According to the character of DC resistance measurement of power transformer windings

an automatic switch circuit of measured winding is presented to overcome the shortcoming of conventional transformer DC resistance instrument.In the measurement process

this technique needs only to change the transformer tap-switch

and the internal power relay automatically accomplishes the measurement of three-phase windings

which greatly decreases the intensity of the labor.An initiative discharge method is proposed to reduce inverse voltage peak in shortest time and protect the equipment and person.Moreover

Grubbs rule is applied to remove large error and a least squares formula of transformer DC resistance is also given.Test results show that the effects due to field interference and accidental error on measurement results can be minimized by using the Grubbs rule and least square formula of DC resistance.