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Stray current is a kind of current generated because of external conditions. In electrical high voltage test, DC leakage or DC withstand test, for instance, current may flow through the capacitor since the high-voltage part has capacitance over the ground.

Due to the grounding and leakage of electric equipment in electrical railways, mines, factories, and ports, circulation of stray current may form in soil as well.

Refer to the current outside the preset power network, and its major sources usually include: 1. current of electric traction flowing through metals (namely those rather than tracks) or current returned to DC substation from the ground; 2. leakage of power and lighting AC circuits; 3. natural current of the ground; 4. induction current of thunder and electromagnetic radiation.

In subway system, traction power system usually adopts the DC way, thus generating stray current. Due to the poor or incomplete insulation between running track and ground, the current flowing through running track can not completely flow back to the cathode of traction substation. Part of the current may leak into the ground, and then flow back to substation. The part of current is called stray current. Stray current in subway is a kind of harmful current, which can affect the normal operation of electrical equipment and facilities in subway to different degrees, do harm to structural steel of tunnel and track bed, and metal pipelines nearby, and even endanger the safety of human body, thus brining about unfavorable impact on the safe operation of subway. Therefore, the prevention of stray current in subway becomes a hot topic of many experts and scholars.

The primary task of stray current prevention is to isolate and control all possible leakage means of stray current so as to prevent stray current from flowing into subway’s main structure, equipment and related facilities; through the collection and drainage system of stray current, the system offers a passage for stray current to flow back to the negative busbar of traction substation so as to avoid the continuous leakage of electricity and further reduce corrosion; there should also be complete stray current monitoring system monitoring and measure the value of stray current and offering basis for operation and maintenance.