PURPOSE: To make a production and a stock management of a current transformer resonable, by making the use of the same current transformer possible regardless of the largeness of load equipments, and to contribute the saving of resources, by eliminating a level converting auxiliary current transformer, and further, to prevent a generation of an overvoltage when the secondary electric path of the current transformer is opened and prevent a surge from penetrating the current transformer from a main circuit, in a protective instrumentation system.

CONSTITUTION: In a protective instrumentation system, a current transformer 3 is provided wherein its secondary current I2 is subjected to a small energization of the transformation into the current adapted to the use of an electronic circuit and wherein a load resistor R whereby its secondary winding terminals are connected and an electrostatic shield 32 laid between its primary and secondary windings are included. Further, in the system, an electronic type protective instrumentation device 5 is provided which has a voltage amplifying part 8 for amplifying an output voltage V2 of the current transformer 3 using it as an input and has a gain setting part 10 for so setting the gain of the device 5 that an output voltage 9 of the voltage amplifying part 8 can be changed into respective full-scale voltages correspondingly to a plurality of rated values of the different current flowing through the primary side winding of the current transformer 3.