Isolated neutral is used in networks where continuity of power supply is essential, which is often in medium-voltage distribution systems of industrial power utilities. Despite the continuous operation during an earth fault, quick fault location is crucial due to overvoltages and the risk of developing a multiphase fault. Existing solutions for earth fault detection, such as sensitive directional homopolar overcurrent protection, are sometimes not economically justified. Additionally, in small networks with low earth-fault current amplitudes, such solutions may not be sufficiently sensitive. Signal injection is one of the known fault location methods applied in unearthed low-voltage networks, and commercially available solutions based on signal injection exist. However, not much research has been done on the possibility of signal injection method application at medium-voltage level, and there are no commercially available solutions for earth-fault detection based on signal injection for medium-voltage networks. The possibility of application of this method at medium-voltage level is dependent on how to inject the signal into unearthed phases with voltages ranging from 5 kV to 35 kV. In this research, low-frequency signal injection through the primary windings of a group of three single-phase insulated voltage transformers, that are normally installed in isolated networks, is proposed. The possibility of injecting a low-frequency signal through primary windings of inductive voltage transformers in order to locate the earth-fault is verified and the influence of the signal injection on voltage transformers is analysed. The low frequency is used to maximize the capacitive reactance of healthy phases and increase the probability for the injected signal to close its path through fault location. The injected signal is traceable in the frequency spectrum of residual currents throughout the network. Simulations of simultaneous earth fault and signal injection are conducted on a model of small, industrial network and larger, distribution network. The possibility of high-impedance fault detection is analysed as well as the influence of the network size and the injected signal parameters on method efficiency. Based on the simulation results, a possible modification of the voltage transformer design is proposed, to achieve better low-frequency signal injection performance, while preserving the primary function and characteristics of voltage transformers. The method is experimentally verified in laboratory environment.