Dielectric strength of glass

Some approximate values for the dielectric strength of glasses are a field strength of 20-40 kV/mm for a glass thickness of 1 mm at 50 Hz at 20 °C, and 10-20 kV/mm for greater thicknesses. At higher temperatures and frequencies, decreasing values can be expected. 

Temperature and Time. The dielectric strength of glass decreases with increasing temperature because of the greater ionic conductivity at the higher temperatures (see Fig. 2.32). Regions of instabihty due to the channeling of the current and the formation of hot spots result in dielectric breakdown by the puncture of the dielectric material. 

Frequency. The dielectric strength of glass is lowered at higher frequencies by dipole relaxation processes (see Fig. 2.34). Data listed in Table 2.8 indicate that the dielectric strength of ordinary glass at 100 MHz is 2 to 5 percent of the corresponding value measured at 60 Hz. 

In general addition of 10-60% glass fibre causes an improvement in the dissipation factor in PC and PA 6,6, and in surface arc resistance in PPO and epoxy resins but causes deterioration of dielectric constant in PA 6,6 and in tracking resistance of PP epoxies and phenylene oxide. Glass fibre caused an improvement in the dielectric strength of epoxies. 

Dielectric Constants of Glasses, 12-56 Dielectric Strength of Insulating Materials,... 

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