The application of Distributed Generations (DGs) has changed the characteristics of break fault and increased the threats in a distribution network. However, there is a lack of protection methods for break fault in distribution networks with DGs, and even the characteristics of DGs under break fault are unclear. Based on the observability of an active distribution network, this paper proposes a new idea to identify the break fault in a distribution network based on the current variation rate of DGs. First, the output characteristics of DGs under a single-phase break fault in a distribution network are analyzed, and the equivalent models of DGs are established. Then, the equivalent circuit of a distribution network with DG under a single-phase break fault is established. The expressions of the current contributed by the DG before and after the single-phase break fault are derived. Subsequently, the current characteristics of DG before and after the break fault are analyzed, and a protection criterion for a single-phase break fault in a radial distribution network is established according to the current variation of the DG. The simulation shows that the proposed method can quickly detect and locate the break fault. This work is supported by National Natural Science Foundation of China (No. 51877018).