Earthing and Negative Return Systems in the Melbourne DC Railway

The Melbourne metropolitan passenger railway system uses a 1,500 V DC overhead contact line traction power system to power the rolling stock. The running rails are used as conductors for the return current path. As there is a finite resistance between the running rails and the earth, a portion of the current will flow through earth and metallic buried structures back to the substation source. The rails are intentionally insulated from earth to mitigate return current leaving the rails as stray currents and causing stray current corrosion of underground metallic services. The adverse effect of increasing rail to earth resistance is elevation of rail potentials which may exceed allowable levels, thus creating a hazard to passengers, persons working on the railway and the general public. Therefore the design objectives for the negative system are to meet the conflicting requirements of stray current mitigation and reduction of rail to earth voltages. This paper details the negative return system employed in the Melbourne railway system and the parameters which need to be considered in its design, construction and maintenance.