# E-field break down

likephysics
What is the realistic break down value for E field in air.
Text books say its 3*10^6 V/m
But there are many websites, which go down to 1*10^6 V/m.

If there is a dielectric like FR4, then the break down increases
V=ε E*d ?, where ε is the dielectric constant for FR4.

## Answers and Replies

Mentor
What is the realistic break down value for E field in air.
Text books say its 3*10^6 V/m
But there are many websites, which go down to 1*10^6 V/m.

If there is a dielectric like FR4, then the break down increases
V=ε E*d ?, where ε is the dielectric constant for FR4.

It varies with pressure -- the relevant reference is the Paschen Curve:

http://en.wikipedia.org/wiki/Paschen_curve

(Click on the graph to enlarge it)

likephysics
Assuming standard temp/pressure, is V=ε E*d valid ?

yungman
It depend on humidity also.

likephysics
It depend on humidity also.

Yes! Given a constant humidity, is V=ε E*d valid ?

Carl Pugh
The book
Reference Data for Radio Engineers, fifth edition has a great graph for breakdown voltage.
If you are serious about working with high voltage, you should get this book.
Volts/mil breakdown voltage varies with spacing.
Breakdown voltage varies with pressure and temperature.
Breakdown voltage is different for sharp points and spheres.
Breakdown voltage varies only slightly with humidity unless there is condensation.

likephysics
The book
Reference Data for Radio Engineers, fifth edition has a great graph for breakdown voltage.
If you are serious about working with high voltage, you should get this book.
Volts/mil breakdown voltage varies with spacing.
Breakdown voltage varies with pressure and temperature.
Breakdown voltage is different for sharp points and spheres.
Breakdown voltage varies only slightly with humidity unless there is condensation.

Thanks for the book suggestion. I will look it up.
I am not working with high voltage, but need the breakdown information.
Certain products need to meet standards set by UL (underwriter labs). Some of these standards are a mystery. Lot of ppl at work think breakdown voltage calculation(even under standard conditions) is some what blackmagic.