The atoms in insulating materials have very tightly-bound electrons, resisting free electron flow very well. However, insulators cannot resist indefinite amounts of voltage. With enough voltage applied, any insulating material will eventually succumb to the electrical ”pressure” and electron flow will occur.
However, unlike the situation with conductors where current is in a linear proportion to applied voltage (given a fixed resistance), current through an insulator is quite nonlinear: for voltages below a certain threshold level, virtually no electrons will flow, but if the voltage exceeds that threshold, there will be a rush of current.
Once current is forced through an insulating material, breakdown of that material’s molecular structure has occurred. After breakdown, the material may or may not behave as an insulator any more, the molecular structure having been altered by the breach.
There is usually a localized ”puncture” of the insulating medium where the electrons flowed during breakdown.
Dielectric strength comparison between materials
|Material *||Dielectric strength (kV/inch)|
|Air||20 to 75|
|Porcelain||40 to 200|
|Paraffin Wax||200 to 300|
|Bakelite||300 to 550|
|Rubber||450 to 700|
|Glass||2000 to 3000|
* = Materials listed are specially prepared for electrical use.
Thickness of an insulating material plays a role in determining its breakdown voltage, otherwise known as dielectric strength. Specific dielectric strength is sometimes listed in terms of volts per mil (1/1000 of an inch), or kilovolts per inch (the two units are equivalent), but in practice it has been found that the relationship between breakdown voltage and thickness is not exactly linear.
An insulator three times as thick has a dielectric strength slightly less than 3 times as much. However, for rough estimation use, volt-per-thickness ratings are fine.
Resource: Lessons in electric circuits – Vol. II