2X voltage rating between two insulated conductors

[big john]

So that makes sense to me, however why is this not an issue with LV conductors? Why cant a MV conductor just have X times the insulation thickness as a unshielded LV conductor?

That's a good question and I don't know why. Having seen unshielded conductors used up through 15kV I can say they fare very poorly: They often have severe corona damage.

But the insulation thickness is very similar to the same v/mil level you see at 600V. So apparently there is some mechanism that becomes more destructive as the voltage increases.

 

[electrofelon]

That's a good question and I don't know why. Having seen unshielded conductors used up through 15kV I can say they fare very poorly: They often have severe corona damage.

But the insulation thickness is very similar to the same v/mil level you see at 600V. So apparently there is some mechanism that becomes more destructive as the voltage increases.

Was researching this further and found this:

Medium Voltage Cable
•
The semi-conductive layer between conductor and insulation which
compensates for air voids that exist between conductor and insulation.
•
Air is a poor insulator, having a nominal dielectric strength of only 55
volts per mil, while most cable insulations have dielectric strengths over
700 volts/mil. Without strand shielding an electrical potential exists that
will over-stress these air voids.
•
As air breaks down or ionizes, it goes into corona (partial discharges).
This forms ozone which chemically deteriorates cable insulations. The
semi-conductive strand shielding eliminates this potential by simply
“shorting out” the air

 

[big john]

But even unshielded cable uses semiconductor, and I assume all MV manufacturers try to prevent voids in the cable.

But that's an interesting idea and it seems to make sense: Is mild corona *always* occuring on the outside of unshielded cable, and this is the destructive agent not the symptom?

 

[GoldDigger]

But even unshielded cable uses semiconductor, and I assume all MV manufacturers try to prevent voids in the cable.

But that's an interesting idea and it seems to make sense: Is mild corona *always* occuring on the outside of unshielded cable, and this is the destructive agent not the symptom?

The minumum voltage at which corona will take place at the interface between a metal (or other) conductor and air will be inversely proportional to the radius of curvature at the metal surface. The semiconductor layer provides a uniform voltage over a large radius smooth surface to minimize the chances of corona discharge from the outside of the insulation.

Manufacturers do their best to minimize voids in the insulation itself, but at the microscopic level that can be very difficult.
Another factor in damage to the insulation itself is water trees, which I will not go into any deeper.