how to calculate breakdown voltage

[Ashford]

hey,
can anyone tell me the formula to calculate breakdown voltage. I came across this one:

Vb= B*p*d/(C+In(p*d)) where

Vb=breakdown voltage
p=pressure
d=distance of the air gap
B=a constant
C=a constant

But I'm not too sure about this and basically this was all the info the site gave me, it didn't tell me what the constant meant or anything. Also, this formula seems to be, to calculate the breakdown voltage of air but I was hoping for a more general formula that could calculate the breakdown voltage of any insulator. If such a formula does not exist then one that would simply calculate the breakdown voltage of air would do.

Thanks in advance.

 

[vogels]

response

I just looked through my notes for the equation but couldn't find it. However, you need to know the relative permittivity and relative permeability of the material. Then to find the absolute permittivity you multiply the materials relative permittivity and relative permeability by, ε0 =(8.85x10^-12(F/m)) and µ0 = 12.56x10^-7(H/m) respectively. For air εr = 1. Permittivity is an electrical attribute, and permeability is magnetic attribute. These are the 'constants', which are specific to the material you are working with. Hope this helps
good luck

 

[vogels]

didn't notice this was from Jan..

 

[Ashford]

doesn't matter...it's still helpful

 

[Misterbenn]

The equasion you have there is for air only as you stated, but more importantly the relationship between pressure and gap distance is only relevent for gas insulators. So it depends what kind of insulator your looking for?

I can tell you though that there is no general formular for all types of insulator, sorry

 

[Ashford]

yeah, I figured out that the reason pressure and distance are needed is because they alter the resistance of the air, so for every insulator the factors that alter its resistance must be included (some just have less than others), and ultimately you are simply finding the resistance of the insulator (that happens to be 'L' long and infinitely thin)