just a SWAG... perhaps dielectric strength is dependent on volume, and the increased 'thickness' is assumed to be thickness-for-a-constant-surface-area. If that's the case then an increasing thickness is also an increased volume which also increases available charge carriers, reducing breakdown voltage.
A test of my SWAG would be to incrementally increase dielectric thickness and determine if breakdown voltage eventually finds a minimum and then begins to increase with thickness. interesting question. On Sun, Jun 23, 2024, 13:32 Richard Nute <[email protected]> wrote: > > Why does air (or any insulating material) have decreasing dielectric > strength, kV/mm, with increasing distance through the dielectric > substance? Assume homogenous field. (I have assumed the dielectric > strength was constant for the material.) In other words, what is the > physical basis for the non-constant dielectric strength clearance tables > in various safety standards? (I have yet to find the answer from the web. > ) How can I predict the dielectric constant for a given distance through > air (or any insulation)? > > Charles J. Fraser, in Mechanical Engineer's Reference Book (Twelfth > Edition) > <https://www.sciencedirect.com/book/9780750611954/mechanical-engineers-reference-book>, > 1994: > > If the potential difference across opposite faces of a dielectric material > is increased above a particular value, the material breaks down. The > failure of the material takes the form of a small puncture, which renders > the material useless as an insulator. The *potential gradient* > <https://www.sciencedirect.com/topics/engineering/potential-gradient> > necessary > to cause break down is normally expressed in kilovolts/millimetre and is > termed the ‘dielectric strength’. The dielectric strength of a given > material decreases with increases in the thickness. Table 2.2 gives > approximate values for some of the more common dielectric materials. > > Table 2.2. Dielectric strength of some common insulators > > *Material* * Thickness (mm)* * Dielectric strength (kV/mm)* > Air 0.2 5.75 > 0.6 4.92 > 1.0 4.36 > 10.0 2.98 > Mica 0.01 200 > 0.10 115 > 1.00 61 > > Thanks, and best regards, > > Rich > > > ------------------------------ > > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to > [email protected] > All emc-pstc postings are archived and searchable on the web at: > https://www.mail-archive.com/[email protected]/ > > Website: https://ewh.ieee.org/soc/pses/ > Instructions: https://ewh.ieee.org/soc/pses/list.html (including how to > unsubscribe) <https://ewh.ieee.org/soc/pses/list.html> > List rules: https://ewh.ieee.org/soc/pses/listrules.html > > For help, send mail to the list administrators: > Mike Sherman at: [email protected] > Rick Linford at: [email protected] > > For policy questions, send mail to: > Jim Bacher at: [email protected] > ------------------------------ > > To unsubscribe from the EMC-PSTC list, click the following link: > https://listserv.ieee.org/cgi-bin/wa?SUBED1=EMC-PSTC&A=1 > - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to [email protected] All emc-pstc postings are archived and searchable on the web at: https://www.mail-archive.com/[email protected]/ Website: https://ewh.ieee.org/soc/pses/ Instructions: https://ewh.ieee.org/soc/pses/list.html (including how to unsubscribe) List rules: https://ewh.ieee.org/soc/pses/listrules.html For help, send mail to the list administrators: Mike Sherman at: [email protected] Rick Linford at: [email protected] For policy questions, send mail to: Jim Bacher: <[email protected]> _________________________________________________ To unsubscribe from the EMC-PSTC list, click the following link: https://listserv.ieee.org/cgi-bin/wa?SUBED1=EMC-PSTC&A=1
