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Cable voltage rating

I was looking for guidelines of how to select appropriate cables for applications (I am interested in DC), I found IEC 183 standard which talks about high voltage cable selection for AC. Also, the status of the document is shown as withdrawn, so I assume it is not applicable.

[I found another document referencing IEC 183](https://www.helukabel.com/media/publication/de/cor_docs/qt_23/QT_COR-DOCS_X_018_Nominal_voltage_and_Operating_voltage.pdf)

What I see at the bottom of the page: ![Image_alt_text](https://electrical.codidact.com/uploads/kj10fd2wj0fsmbhsqn1fld45sbcv)

U<sub>0</sub> = cable nominal voltage between the conductor and the metal covering or earth.

U = cable nominal voltage between the phase conductors.

First question I have is why conductor/earth voltage rating for DC is specified lower at 0.9kV? I would imagine that due to capacitance of the insulating material the total impedance between conductor and whatever is on the other side of insulation is lower, so the rated voltage would be lower. Why isn't that the case, or perhaps the reference is wrong?

My second question is if there are any standards for selecting proper insulation for high voltage DC cables. Is it application dependent as well, such as free air, submerged, vacuum, or medical/military grade etc.? If you could point me to some IEC/IPC standards that mention this I would highly appreciate it.

My last question is the margin for voltage. For capacitors, on an output of 8V power supply we do not place capacitors rated 10V, we leave a bigger margin. What about cables? I read that due to partial discharge cable insulation degrades over time. What is a reasonable margin for insulation rating?