A fault signal indicates a single-pole or a double-pole fault in a three-phase electrical power grid which occurred during a present electric oscillation in the electrical power grid. The method assures that single-pole or double-pole faults occurring during oscillation can be detected with high reliability in that a symmetry signal is produced during the oscillation, which indicates whether the oscillation is symmetrical or unsymmetrical, and the phases of the electrical power grid are checked for an existing fault, wherein the symmetry signal is used for carrying out the check. The fault signal is produced if a fault was detected during the check. A protective device has an accordingly equipped control unit.