A method for detecting an isolation fault in an electrical installation comprises: measuring an AC electrical voltage between phase conductors of an electrical load to be monitored and ground, and an electric fault current flowing between said electrical load and ground; identifying, in the measured electrical voltage, at least one first component oscillating at the predefined first frequency and one second component oscillating at the predefined second frequency; calculating an impedance of the electrical fault from the measurements and an impedance of the electrical installation from the identified first and second components; selecting a predetermined case from a predefined list; and identifying an operating condition of the electrical installation on the basis of the selected predetermined case.