Dielectric constant vs voltage breakdown 1

[alwaink]

Hi,

I know that higher dielectric constant (relative permittivity) yields higher capacitance. So I always assumed that the higher dielectric constant, the better insulation material it is...therefore, higher voltage breakdown.

But I just came across some references showing that voltage breakdown has inverse relationship with dielectric constant. So dielectric breakdown actually drops as the constant goes up. Could somebody explain? Thanks.

Albert

 

[BobbyNewmark]

When a material is between the two plates of a charged capacitor, all the electrons move towards the positive plate, while all the nuclei move towards the negative plate. The farther they move, the more work is done on them by the electric field, which means more work to charge the capacitor, which means higher capacitance, which implies a higher dielectric constant.

However, how far they move is proportional to how strongly the electrons are bound to the nuclei. The weaker that force, the more they move, but the more easily they can be ripped completely free. Ripping an electron free is dielectric breakdown.

 

[alwaink]

If the purpose of the material is for cable insulation, then higher dielectric breakdown is preferred. So it actually means that lower dielectric constant is better because there's less movement of electrons due to stronger bond between the electrons and the nuclei? Which also means less capacitance.

Based on this, correct me if I"m wrong, I can say that a new EPDM insulated cable should have less capacitance than an aged EPDM insulated cable. Is this a legit assumption?

Thanks.

 

[BobbyNewmark]

Interesting idea. I presume you want to make a non-destructive dielectric strength tester?

I expect that most aging processes will increase the capacitance. The exception would be some sort of "foamy" process that fluffs up the insulation, decreasing its density. You'd want to do lots of testing to see what aging does in the real world.

I see a possible complication: the effect of changing geometry on capacitance as the cable settles under its own weight. For horizontal cables, I'd expect them to squish together over time, gradually increasing the capacitance. For vertical cables, I'd expect them to splay out over time, decreasing the capacitance. Changes in a liquid environment would also have a big effect, as for buried cables. Again, you should do lots of experiments to characterize the background changes.