PURPOSE: To achieve higher reliability in the protection of the system by coinciding the tertiary winding wound almost equal to axial length of the master winding with the axial center of the master winding.

CONSTITUTION: One leg of an iron core 5 is wound with a primary winding 1, a master winding 2, a slave winding 3, and a tertiary winding 14 in such a manner that the winding 14 almost equals the master winding 2 in the axial length while coinciding each other in the center of the axial length. Now, when the winding 2 causes a short-circuit between terminals u1Wθ1, the main magnetic flux passes through the route 6 and bence, has little interlinkage with the winding 14 whereby the voltage induced in the winding 4 becomes zero. On the other hand, when the winding 3 causes a short-circuit between terminals u2Wθ2, the main magnetic flux passes through the winding 14 and hence, the rated voltage is induced in the winding 14. Accordingly, when three such master-slave type transformers with a tartiary winding for instrumentation are combined, only controlling the action of the relay suffices in case of short-circuiting in the winding 2 thereby upgrading the reliability in the protection of the system.