機場捷運電聯車動力來自設於沿線之牽引動力變電站（Traction Supply Substation，TSS），經由TSS之整流器組將22kV交流電轉換為750V直流電，透過正電電纜傳輸電力至導電軌（第三軌）、負電回流部分則由鋼軌（行車軌）經由負電電纜連接至TSS負電直流盤，形成一完整電流路徑。但當電流流經鋼軌時，在鋼軌上形成電壓降，而鋼軌對地之電阻亦不可能完全絕緣，使得電流從軌道洩出，再經由周遭地表下的金屬管線或結構鋼筋又回到軌道，此種由軌道洩出之電流就是雜散電流。
The traction power of TIAA MRT EMUs is generated from the traction supply substations (TSS) along the MRT line. The power is alternated from 22kV AC power into 750V DC power through the TSS rectifier and provides power to the conductive rail (third rail) via positive electricity cable; the negative backflow is connected by the rail (running rail) to a TSS negative DC switchgear via negative cable to form a complete current path. However, when the current flows through the rail forming voltage drop on the rail causing the rail resistance to the ground unable to be completely insulated, current is dissipated from the rail which returns back to the track via the surrounding metal pipeline or structural steel. This kind of current is called the stray current.
In order to curb the stray current and reduce its damage to the civil structure of reinforced steel or other underground metal pipelines, the TIAA MRT was designed and constructed in accordance with the stray current control measures at its design stage.
The mechanism is briefed as follows:
1. The northbound/southbound rails are bridged across a 240 mm2 cable every 200 meters to reduce the resistance of the negative current path so that most of the return current can return to the TSS negative terminal through the rail to reduce its chances of dissipation.
2. The ground insulation resistance of rail construction and installation must be greater than 100Ω-km; as for the power supply system design, the system adopted the ungrounded (floating) design to increase the rail insulation resistance to the ground. The power of the main line and depots are insulated by the insulated rail joint (IRJ) to ensure the insulation resistance of the rail to the ground and to protect the safety of the personnel in the maintenance area of the depot.
3. In order to intercept the dissipated stray current to reduce its damage to other underground metal pipelines, the stray current collector mats (SCCM) are especially set below the rail to collect the primary stray current. Then, the cycle will connect with station steel structures to form idiostatic potential and link directly to the SCCM through the return cable to collect the secondary stray current.
To understand and monitor the TIAA MRT stray current changes and dissipation situation, stray current test boxes are installed in the TSS rooms along the entire MRT line. The PLC in the test boxes will record the stray current measurement data of the section and transmit it to the “stray current monitoring system (SCMS)” in the TSS of the main workshop of Chingpu depot to real-time display, monitor and record the stray current variation of the entire line as a long-term reference for system monitoring.