Islanding detection is one of the most dominant challenges for distributed generation systems connected to the main power grid. Rudimentary islanding detection methods (frequency relays, voltage relays), have considerable non-detection zones (NDZ). Nod-detection zone which defines as the area that the method fails to detect the islanding, is the primary criterion to evaluate the detection method performance. In this paper, factors which affect this zone are introduced, and their effects on the passive islanding detection methods, like voltage and frequency relays, are examined. Parameters such as load dependence on frequency and voltage, inverter control system, filters, load's quality factor, SCR and X/R were considered. Finally, a most sever condition for a passive method to detect the islanding is presented.