This paper presents a new islanding detection method for synchronous Distributed Generation (DG). The proposed method is developed based on the combination of rate of change of exciter voltage (RCEV) and open-close circuit breaker (OCCB) at DG connection point. Whenever the RCEV parameter is not sufficient enough to detect islanding condition efficiently and correctly, the OCCB strategy will be triggered. The proposed OCCB strategy sends a switching command to one-phase of the three-phase circuit breaker (CB) which connects the DG to the grid. In this way, the CB will be immediately opened and then closed. This switching condition will causes a variation in the value of RCEV. This variation is small for the conditions when DG operates in parallel with the main grid, whereas it will be large during the occurrence of islanding. The implementation of the proposed strategy is cost-effective because it only uses the CB at the DG output and does not require any extra equipment. The proposed approach is tested on two different systems including a hypothetical power system and the IEEE 33-bus test system in MATLAB/Simulink environment. The simulation results show that the proposed method eliminates the non-detection zone (NDZ), and also reveals high accuracy compared with previously introduced methods. Moreover, it has been proved that the performance of the proposed approach is not affected by DG capacity, DG location, the number of DG units, as well as network configuration.