Voltage sag is identified as a common undesirable power quality phenomenon in the distribution systems, which causes malfunction of sensitive loads and may even damage them. Implementation of dynamic voltage restorer (DVR) makes it possible to suppress the effect of this phenomenon to protect sensitive loads. For this purpose, the time response of voltage sag detection is crucial. This paper proposes a simple detection method to achieve fast response without the need of phased locked loop (PLL), look-up table or complex computations. The proposed method has been implemented through simulation and experiment; and it has been tested under various conditions considering possible cases such as different amount of voltage sag, different amount of voltage sag angle at which voltage sag occurs and line frequency variation. According to the obtained results, the best and worst detection time of proposed method were measured 0.26 and 0.68 ms, respectively, which shows fast response of proposed method. Both simulation and experimental results demonstrate the performance and effectiveness of proposed method.