The ideal MRT return current system should be designed so that the negative return current runs back to the negative end of rectifier entirely via rails. However, when a current flows through the rails, it forms a voltage drop in the rail. Additionally, resistance of the rail against the ground is also unlikely to be fully insulated thus causing "stray currents" to escape from the rail and returning back to the rail via underground metal pipelines or structural RC.
The amount of stray current is directly dependent to the electrical potential of the rail. The amount is influenced by the rail's resistance and its ground resistance and also closely influenced by the grounding method of the rail. Therefore, TIAA MRT adopted the "ungrounded system" to directly increase ground resistance of the rail and reduce the amount of stray current leakage. In addition, SCCM is formed using structure RC under the rail to avoid current leakage from the rail bed. Then, SCRC is used to collect the stray currents to reduce electrical corrosion caused by current flowing through metal pipelines or structure RC. On account of recording the stray current as well as the return current which flows through the entire MRT rail network, PLCs are distributed in every TSS to measure and record the stray current leakage amount of that section. The data is then collected and sent to the TSS in Chingpu Depot main workshop and displayed, monitored and recorded through the SCMS (see figures for its structure and appearance). MRT rail network stray currents in all phases including initial phase, power supplied phase, operational test phase and official operation phase will be recorded forming a complete database which will be the reference for future station structures, pipelines and rail system maintenance.