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Islanding detection is one of the most important issues for distributed generation (DG) systems connected to an electric power grid. The conventional passive islanding detection methods inherently have a non-detection zone (NDZ), and active islanding detection methods might cause problems with safety, reliability, stability, and power quality on power system. This paper proposes a passive islanding detection method with the zero NDZ property by considering the switching frequency of an inverter of the DG system. Moreover, the proposed method does not cause any negative effects in the above problems because it avoids applying the intended changes such as variations of reactive power and/or harmonics, which are required in the active islanding detection methods. Several case studies are carried out to verify that the proposed method can detect the islanding operation within 20 milliseconds, which is less than 150 milliseconds required in a recloser as the shortest mechanical reclosing (delay) time. This value also satisfies the requirement of 2 seconds given in the IEEE Std. 1547.