Directional element to determine faults on ungrounded power systems

directional element for detecting ground faults in systems ungrounded, comprising: means (30) when activated to calculate a zero sequence impedance for a particular protected line, using a voltage of zero sequence and a stream zero sequence on that line; a drive circuit that allows operation of the calculation means under conditions of zero current preselected; and means (56, 57) for comparing the values ​​of zero sequence impedance originating from the calculation means with the appropriate systems ungrounded selected threshold values ​​in which the means for comparing the zero sequence are characterized in that one forward fault indication is provided when the zero sequence impedance is above a first threshold of sensitivity (26) and there is provided a reverse fault indication when the zero sequence impedance is below a second threshold of sensitivity (28 ).