A current of only around 1,500 A which is smaller than the load current flows in DC 1,500-V feeding circuits, even when the grounding resistance at the fault point is about 1 Ω. It is therefore not possible to detect faults with existing protection devices. In DC feeding system, an EMU is supplied from two substations simultaneously. As an EMU moves the currents supplied from each substation change. With the addition of the phenomenon known as cross current (the current that flows into the adjoining feeding circuit through substations), it is not possible to judge the status of EMU load current from a substation.

The current detection system using telecommunication lines is developed. It gathers EMU load current data from adjacent substations into one place through telecommunication lines and uses the current evaluation table that weights current loads based on the normal load conditions on each load current division of different lines. As the load current becomes zero after a certain length of time, the system repeats integration and resetting of the current evaluation value. Meanwhile the value is not reset when a fault is present since the fault current flows continuously. This causes the integrated current evaluation value to reach the preset level at which a fault is judged to have occurred (Fig. 1). The RTRI implemented a fault test in the field, and confirmed that a fault current of about 300 A can be detected in 62 seconds (Fig. 2).