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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 h...

posted 9mo ago by Sclrx‭ · edited 9mo ago by Sclrx‭

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#2: Post edited by

Sclrx‭ · 2023-08-08T14:16:31Z (9 months ago)

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> 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.
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).

> 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).

#1: Initial revision by

Sclrx‭ · 2023-08-08T14:11:20Z (9 months ago)

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 > 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.

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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