ABSTRACT
The return circuit of DC traction systems requires specific treatment because the running rails are insulated from earth and installations connected to earth. Reason for that is that electrolytic corrosion erodes metallic installation laid in the soil when DC current leaves versus soil. The DC current escaping to earth or to neighbouring metallic installations which are in contact with earth are called stray currents, because they don’t follow the intended path in the return circuit. As a consequence damage could occur caused by the so-called stray current corrosion, both for the railways installations and third parties. The level of corrosion depends on the amount of stray current and on the duration of its flow.
Stray currents escaping along the line cannot be measured. Therefore an alternative method is used for the assessment of stray current corrosion, using the rail potential of DC traction systems. Rail potential is a very sensible indicator for stray currents flowing from the running rails the soil or to neighbouring metallic installations.
As a consequence of the rail insulation, the voltage drop along the line causes rail potential, which can be bridged by persons. Voltage limiting devices installed between running rails and Structure Earth, preferably in passenger stations, survey the rail potential and cause a protective earthing connection in case of extraordinary operation circumstances for limited time. This proceeding allows fulfilling the permissible touch voltage for safety of persons without the need of interruption of operation.
This article explains the principle how to treat the return circuit of DC traction systems in order to fulfil both, the electrical safety and requirement against stray current corrosion. It depicts as solutions the voltage limiting devices Sitras® SCD on the one hand and the stray current Monitoring System Sitras SMS
Keywords: Mass transit, Metro, Return circuit, Touch voltage, Voltage limiting devices, Stray current monitoring.
