n European distribution networks the resonant-ground system dominates. The advantages of the resonance neutral earthing are based mainly on the very low single-pole fault current, the residual earth fault current IRES. The growing use of both, cables and power electronics, increases the residual earth fault current IRES. In the case of a single-pole earth fault, harmonic components in residual earth fault current IRES can reach values, which can violate the limits defined by standards. Up to now, the estimation of the residual earth fault current IRES is usually done with measurements in the field by solid grounding of one phase at the bus bar in the substation. However, these measurements do not correspond to the real fault conditions. Ground faults occur anywhere in the network with relatively high fault resistances. In this paper a new method is presented to enable an accurate estimation of the residual earth fault current IRES during the earth fault by using the harmonics measurement of the voltages during the healthy state of the network, the network configuration and the expected impedance at the fault location. The results of the new method were verified in real field tests and will be also presented in this paper.