IEC 61991-2019 pdf free.Railway applications- Rolling stock-Protective provisions against electrical hazards.
4.2connections between circuits
Circuits operating at different nominal voltages connected by power conversion equipmentwhich provides a sufficient insulation between them are individually classified at the nominalvoltage of each circuit.
lf the conducting paths referred to in this subclause include capacitive or inductiveconnections，whose impedance is low enough to induce hazardous voltages into any circuitunder either normal or fault conditions, then all the circuits so connected are classified at thenominal voltage of the highest voltage circuit.
NOTEThis condition can apply to circuits connected，for example，by means of a chopper converter withimpedance coupling.
Where circuits are linked conductively to a higher voltage source other than directly by circuitbonds connected to the vehicle body (for example by an auto-transformer or potential divider),all circuits in the group are treated as if energized at the nominal voltage of the source, unlessthe conditions of 4.3 have been met.
4.3Exceptions
lf voltage conversion from one band to another involves overvoltage detection resulting indisconnection of the primary or the secondary circuit,or having other means capable ofpreventing excessive voltage in the secondary circuit,then the secondary circuit is permittedto be classified according to the highest voltage at which the detection equipment will operate.Where overvoltage detection is used the integrity of the equipment should be evaluated asappropriate.
Circuits not connected to the vehicle body, for example floating supplies, are permitted to beclassified in any voltage band.The band selected should be appropriate，taking due accountof the various potentials possible in such circuits under normal or fault conditions so as toensure that the requirements of this document are met.
The limit between bands lll and lV is allowed to be lowered to take account of special nationalconditions.
5 Protective provisions against direct contact
5.1General
Live parts capable of causing an electric shock shall be protected against direct contact.Alltypes of equipment shall be capable of being operated without loss of protection against directcontact. Protection against direct contact shall be provided， where possible, by at least one ofthe means described in 5.2 to 5.3.
Where it is not possible to provide protective provisions in accordance with 5.2 or 5.3， then itis permitted to use protection by the use of band l voltage in accordance with the conditionsof 5.4.
Additionally where required to ensure adequate protection against direct contact，warninglabels as described in 5.5 shall be provided.
5.2Protection by insulation
ln addition to the requirements of lEC 62497-1, the insulating materials used to cover liveparts shall be appropriate to the rated equipment operating voltage and the conditions of use.Further provisions should be considered in order to minimize the consequences of damage.IEC 61991 pdf download.