A stage-fused underground transformer loop system, which includes a series of transformers connected sequentially by a series of cables, and a series of fused elbow terminators connected to the inlets and outlets of the transformers. The fused elbow terminators are arranged in an order of decreasing the fuse capacity starting from the feed. The fused elbow terminator includes an elbow connector having a housing and a cable connector disposed within; and a fused pin connected to the cable connector. Further disclosed is a method of rapid diagnosis of a fault cable or a transformer failure using the stage-fused transformer loop system.