Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

E-field break down

  1. Nov 15, 2011 #1
    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.
  2. jcsd
  3. Nov 15, 2011 #2


    User Avatar

    Staff: Mentor

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


    (Click on the graph to enlarge it)
  4. Nov 16, 2011 #3
    Assuming standard temp/pressure, is V=ε E*d valid ?
  5. Nov 16, 2011 #4
    It depend on humidity also.
  6. Nov 17, 2011 #5
    Yes! Given a constant humidity, is V=ε E*d valid ?
  7. Nov 17, 2011 #6
    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.
  8. Nov 17, 2011 #7
    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook