Regarding to dispersed generation field, an islanding is known as a situation that those generators are disconnected from the utility feeders and a bunch of local loads being powered by them is isolated. Accordingly, anti-islanding schemes are used to immediately detect that phenomenon in facilitating the detachment the DG or adaptive controlling of autonomous operation. Negative-sequence impedance measurement is considered as a novel method of detecting electrical islands in DG networks. However, in the case of high induction motor penetration, this method might malfunction. Using two-round impedance measurement, this paper deals with that problem while making typical assessments on how the impedance tolerant values are selected and how these values vary with network configuration. The simulation in PSCAD/EMTDC and experiments validate the methodology.