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 Also, the status of the document is shown as withdrawn, so I assume it is not applicable.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
What I see at the bottom of the page:
U0 = 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?
1 answer
First question I have is why conductor/earth voltage rating for DC is specified lower at 0.9kV?
Imagine two cables running side to side. To get from one cable to the other, an electron would have to cross the insulator twice, but if those two cable ran against a conductive surface (a metal plate or beam, for example), the same electron would only have to cross the insulator once, and since most regulations call for all conductive surfaces to be grounded, there is your conductor-to-earth max voltage.
Half the insulation, half the maximum voltage.
My second question is if there are any standards for selecting proper insulation for high voltage DC cables.
That is taken care of by the cable manufacturer. The designer usually selects their cable from the catalog of reputable manufacturers and distributors, and according to their project's specific requirements. How the manufacturer reaches said requirements is up to them.
[...] we leave a bigger margin. What about cables?
If necessary, this is often called for by the customer through a design specification. The practice is called "derating" and is expressed in percentage. For example, if your design case calls for a 20% derating, you would ensure that a cable rated for 1kV never carries more than 800V (1kV - 20%).
Regarding degradation over time, the reputable manfacturers I mentioned before would supply, along with voltage and current rating, duration of safe operation for specific environment (marine, contact with solvent, vacuum, etc.
In conclusion,when designing/selecting cable, focus on the constraints you have. How the manufacturer manages them is their issue (once again, assuming you use good suppliers).
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