The direct current system is used both in railway traction,
and in urban and suburban electric traction (surface and
underground). The electric energy for supplying the line
contact (usually with rated voltages of 600 V, 750 V, 1500 V,
etc.) is taken from the three-phase high voltage power network
and converted from ac to dc in the electric traction substations.
These substations are indoor facilities, the equipment being
arranged in a structure of cell type (in enclosed spaces) [1], [2].