Recently, there has been significant increase in interest on Smart Grid security. Researchers have proposed various techniques to detect cyber-attacks using sensor data. However, there has been little work to distinguish a cyber-attack from a power system physical fault. A serious operational failure in physical power grid may occur from the mitigation strategies if fault is wrongly classified as a cyber-attack or vice-versa. In this paper, we utilize a data-driven approach to accurately differentiate the physical faults from cyber-attacks. First, we create a realistic dataset by generating different types of faults and cyber-attacks on the IEEE 30 bus benchmark test system. With extensive experiments, we observe that most of the established supervised methods perform poorly for the classification of faults and cyber-attacks specially for the practical datasets. Hence, we provide a data-driven approach where labelled data are projected in a new low-dimensional subspace using Principal Component Analysis (PCA). Next, Sequential Minimal Optimization (SMO) based Support Vectors are trained using the new projection of the original dataset. With both simulated and practical datasets, we have observed that the proposed classification method outperforms other existing popular supervised classification approaches considering the cyber-attack and fault datasets.