Substance Dielectric Strength (KV/mm)
Helium (relative to nitrogen) 0.15
Air  3.0
Window glass 9.8 – 13.8
Silicone oil, Mineral oil 10 – 15
Polyethylene 18.9 – 21.7
Neoprene rubber 15.7 – 26.7
Distilled Water 65 – 70
High Vacuum (field emission limited) 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
Much More Detail at :- http://physics.info/dielectrics/
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
- Obtain a sample of material with a uniform thickness of about 1mm applied to a sheet of metal.
- 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.
- 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.
- Lift the HV electrode off the surface of the material and note the output voltage.
- 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.
- 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.