A method for differential protection of a three-phase electric connection of a three-phase electric network, the method comprising: a) detecting an earth fault in the three-phase electric network; b) in response to detecting the earth fault in the three-phase electric network, determining a value for an operate quantity, the determining comprising: b1) forming a cumulative phasor sum of at least two values, determined at different instants of time, of an electric phasor quantity, wherein the electric phasor quantity is based on a residual current at a first point of the three-phase electric connection, a residual current at a second point of the three-phase electric connection, and a residual voltage of the three-phase electric connection, or quantities indicative thereof; and setting the cumulative phasor sum as the value for the operate quantity, or b2) forming a first cumulative phasor sum of at least two values, determined at different instants of time, of a first electric phasor quantity, wherein the first electric phasor quantity is based on the residual current at the first point of the three-phase electric connection and the residual current at the second point of the three-phase electric connection, or quantities indicative thereof; forming a second cumulative phasor sum of at least two values, determined at different instants of time, of a second electric phasor quantity, wherein the second electric phasor quantity is based on the residual voltage of the three-phase electric connection, or a quantity indicative thereof; and setting a quotient of the first cumulative phasor sum and the second cumulative phasor sum as the value for the operate quantity; c) comparing the determined value for the operate quantity with one or more thresholds or an operation characteristic; and d) determining, on the basis of the comparing, whether the detected earth fault is in the three-phase electric connection between the first point and the second point of the three-phase electric connection. a) detecting an earth fault in the three-phase electric network; b) in response to detecting the earth fault in the three-phase electric network, determining a value for an operate quantity, the determining comprising: b1) forming a cumulative phasor sum of at least two values, determined at different instants of time, of an electric phasor quantity, wherein the electric phasor quantity is based on a residual current at a first point of the three-phase electric connection, a residual current at a second point of the three-phase electric connection, and a residual voltage of the three-phase electric connection, or quantities indicative thereof; and setting the cumulative phasor sum as the value for the operate quantity, or b2) forming a first cumulative phasor sum of at least two values, determined at different instants of time, of a first electric phasor quantity, wherein the first electric phasor quantity is based on the residual current at the first point of the three-phase electric connection and the residual current at the second point of the three-phase electric connection, or quantities indicative thereof; forming a second cumulative phasor sum of at least two values, determined at different instants of time, of a second electric phasor quantity, wherein the second electric phasor quantity is based on the residual voltage of the three-phase electric connection, or a quantity indicative thereof; and setting a quotient of the first cumulative phasor sum and the second cumulative phasor sum as the value for the operate quantity; b1) forming a cumulative phasor sum of at least two values, determined at different instants of time, of an electric phasor quantity, wherein the electric phasor quantity is based on a residual current at a first point of the three-phase electric connection, a residual current at a second point of the three-phase electric connection, and a residual voltage of the three-phase electric connection, or quantities indicative thereof; and setting the cumulative phasor sum as the value for the operate quantity, or b2) forming a first cumulative phasor sum of at least two values, determined at different instants of time, of a first electric phasor quantity, wherein the first electric phasor quantity is based on the residual current at the first point of the three-phase electric connection and the residual current at the second point of the three-phase electric connection, or quantities indicative thereof; forming a second cumulative phasor sum of at least two values, determined at different instants of time, of a second electric phasor quantity, wherein the second electric phasor quantity is based on the residual voltage of the three-phase electric connection, or a quantity indicative thereof; and setting a quotient of the first cumulative phasor sum and the second cumulative phasor sum as the value for the operate quantity; c) comparing the determined value for the operate quantity with one or more thresholds or an operation characteristic; and d) determining, on the basis of the comparing, whether the detected earth fault is in the three-phase electric connection between the first point and the second point of the three-phase electric connection.