A current injecting two-pole device (12) is inserted either between the network's negative terminal (2) and the ground terminal (8), or between the ground terminal (8) and the network's positive terminal (3). The two-pole device (12) has a capacitor (20) and a diode (15) injecting current into the network and restoring the network's voltage conditions relative to the ground terminal to near ground fault free conditions. The insulation resistance of the network is defined as the quotient of the DC voltage measurable between the two-pole device's two terminals (13,14), and the direct current flowing through the device. In networks with high resistance ground fault, the fault is located by maintaining the restored voltage and tracking the direct current injected into the network from the device.