A traction substation converts energy from the distribution grid into the voltage used to supply the overhead contact line - in Poland, 3 kV DC traction predominates, alongside AC systems on lines supplied from the industrial power grid. From the equipment-selection perspective, the facility can be divided into three sections with different requirements: the AC incoming feeder bay, the rectifier unit, and the DC outgoing feeder bays with the return circuit.
The DC side is more demanding than the AC side. Direct current has no natural current zero, so an arc does not extinguish on its own - the breaking capability of the apparatus depends on the L/R time constant of the circuit, not only on the prospective short-circuit current. The same fuse or switch-disconnector behaves differently in circuits with low and high inductance. This is compounded by switching and lightning overvoltages entering from the overhead contact line and by the need for insulation coordination at rated voltages higher than those found in typical industrial installations.
Equipment used in this area includes: fuse-links and fuse bases for AC and DC circuits, DC switching apparatus, surge protection devices on both sides of the converter, high-voltage measurement transducers with galvanic isolation, power supplies and relays for auxiliary-service circuits, as well as terminal blocks and cabinet systems.
Selection criteria for this area:
In an AC circuit the arc is extinguished when current crosses zero. In a DC circuit there is no natural current zero, so the fuse-link must force arc extinction itself, and the energy to be dissipated increases with the circuit L/R time constant. Manufacturers therefore specify separate DC voltages and limiting currents for DC fuse-links together with the assumed time constant - AC data cannot simply be applied to DC.
For DC switchgear in fixed installations - EN 50123; insulation coordination - EN 50124-1; supply voltages of traction systems - EN 50163. The IEC/EN 61643 series is used for low-voltage surge protective devices. Compliance is confirmed by the datasheet or the manufacturer's declaration.
No. EN 50155 applies to electronic equipment installed on rolling stock. Fixed installations are governed by standards applicable to stationary equipment and by relevant product standards. This distinction matters at the specification stage, because requiring EN 50155 for equipment installed in a substation unnecessarily narrows the supplier market.
By superimposing the time-current characteristics of the outgoing-feeder protection and the upstream protection over the full range of prospective fault currents, including the let-through energy (I²t) for high-current faults. Selectivity confirmed only at rated current does not guarantee selectivity during a short circuit.