Voltage sag has been the focus of considerable research in recent years. It can cause expensive downtime. Research on voltage sag detection has also grown up and it is an essential part of the voltage sag compensator. There are many methods have been introduced to measure and detect voltage sags. Among these are RMS Value Evaluation, Peak Value Evaluation, Missing Voltage technique and Hysteresis Voltage Control technique. In this paper, a study is carried out to observe the different techniques of voltage sag detection. Since, precise and fast voltage detection is an essential behavior for a voltage sag compensator, therefore, observation on detection time of each detection methods will also be carried out.