3. Power supply

While 750 V dc is the standard power supply on urban lines, suburban sections are best electrified with higher voltages to reduce the number of substations and allow regional, mainline or freight trains hauled by electric locomotives (Mulhouse – Thann, Strasbourg – Obernai): 1,500 V dc, 3,000 V dc, 15,000 V 16.7 Hz ac, 25,000 V 50 Hz ac.For single-phase, 15 kV ac or 25 kV ac sub-catenary operation, trainsets must be fitted with a power transformer. A 750 V direct-current sector is separated from a single-phase sector by a neutral section, de-energized. Switching from a 750 V dc supply to a 15 kV ac or 25 kV ac supply takes place on the fly. Once the driver has returned his joystick to the neutral position, the train rolls along. When the pantograph bow enters the neutral section, the zero-voltage relay reacts and opens the main circuit breaker. When the pantograph enters the single-phase,...