Rx, is defined as earth resistance under test, and R1, R2...Rn are defined as earth resistance of other measuring objects.

These earth resistances, R1, R2, Rn can be considered that they are connected in parallel. And they can be regarded as a combined resistance Rs. The Rs can be regarded small enough against Rx since a combined resistance consists of several resistances. Following is an equivalent circuit diagram of this circuit.

Voltage V is applied to the object (Resistance Rx) measured from the voltage injection transformer CT1, and the current I corresponding to the earth resistance is flowed.

The current I is detected with detection transformer CT2, and object (Resistance Rx) measured can be put out by the calculation. (refer to the right diagram)