IEC 61991 :2000 pdf download-Railway applications – Rolling stock – Protective provisions against electrical hazards.
3.2.1 4 nominal voltage voltage by which an installation or part of an installation is designated NOTE 1 The voltages are expressed by the value between poles, ripple-free for d.c., and by the r.m.s. value between phases for a.c. NOTE 2 The actual voltage may differ from the nominal voltage by a quantity within permitted tolerances. For further information about traction systems supply voltages, see IEC 60850. 3.2.1 5 obstacle part preventing unintentional direct contact , but not preventing direct contact by deliberate action 3.2.1 6 power circuit circuit carrying the current of the machines and equipment, such as convertors and traction motors, which transmit the traction output 3.2.1 7 protective bonding equipotential connection for protective purposes 3.2.1 8 protective conductor conductor used for some protective measures for protection against electric shock for electrically connecting any of the following parts: – exposed conductive parts ; – main earthing terminal; – earth electrode; – earthed point of the source or artificial neutral.
4 Classification of voltage bands 4.1 General principles This standard is based on the maximum supply voltages at which the equipment or electrical circuits are subjected. The voltages are classified into bands according to the nominal value as shown in tables 1 to 3. Different installation rules apply to each of these bands. The power supply of the various circuits installed in railway rolling stock are of different types such as – batteries; – transformers; – voltage dividers; – rotating machines; – static converters; – capacitors; – special sources.
4.2 Connections between circuits Circuits operating at different nominal voltages connected by power conversion equipment which does not provide a conducting path between them, or circuits which are linked only by a connection direct to the vehicle body external to the power conversion circuitry, shall be individually classified at the nominal voltage of each circuit. If the conducting paths referred to in this subclause include capacitive or inductive connections, whose impedance is low enough to induce hazardous voltages into any circuit under either normal or fault conditions, then all the circuits so connected shall be classified at the nominal voltage of the highest voltage circuit. NOTE This condition can apply to circuits connected, for example, by means of a chopper converter with impedance coupling.