Dielectric Strength of common insulators

Substance Dielectric Strength (KV/mm)

Helium (relative to nitrogen)[2] 0.15
Air [3] 3.0
Alumina 13.4
Window glass 9.8 – 13.8
Silicone oil, Mineral oil 10 – 15
Benzene[ 163
Polyethylene 18.9 – 21.7
Neoprene rubber 15.7 – 26.7
Distilled Water 65 – 70
High Vacuum (field emission limited)[6] 20 – 40 (depends on electrode shape)
Fused silica 25–40 at 20 °C
Waxed paper 40 – 60
PTFE (Teflon, Extruded ) 19.7
PTFE (Teflon, Insulating Film) 60 – 173
Mica 118
Diamond 2000

Much More Detail at :- http://physics.info/dielectrics/


From:- http://www.buckleys.co.uk/technical/calculating-the-dielectric-strength/

The dielectric strength is the maximum working voltage a material can withstand without breaking down.

It is normally expressed in Volts/mm. The material manufacturer should be able to supply this information but if not an approximate value can be found using a Holiday Detector.

Calculating the Dielectric Strength

  1. Obtain a sample of material with a uniform thickness of about 1mm applied to a sheet of metal.
  2. Connect the Holiday Detector to the sample with the earth lead connected to the metal and the high voltage probe (via a pointed probe) to the surface of the material.
  3. Starting with the output voltage set to minimum, slowly increase the volts until the material breaks down and the alarm on the Holiday Detector sounds.
  4. Lift the HV electrode off the surface of the material and note the output voltage.
  5. Repeat this test a number of times on a new area of the sample at least 20mm from where any previous breakdowns have occurred, noting the breakdown voltage each time.
  6. Take an average of the voltages and then 75% of that is approximately the dielectric strength of the material.

So now you have a value for the dielectric strength we can look at how this relates to the test voltage calculated previously.