Protection from islanding is one of the most important R&D items for photovoltaic (PV) and other dispersed power systems connected to a utility power grid and supplying back surplus power. To protect against islanding, dispersed power systems should be able to detect loss of power in the power grid and be disconnected from the grid immediately. However, a reliable method for detecting the islanding, especially when multiple dispersed systems are connected to one power distribution line, has not yet been developed. We have developed a new method to detect the islanding using correlation techniques. In this article the detecting ability of the correlation method is analyzed when multiple power systems are connected to a power distribution line and compared with that of a usual voltage perturbation method. It is shown that the correlation technique is effective for the case of multiple connection of the PV systems to line, while a usual voltage perturbation method is not effective.