The insulation resistance of technical glasses is set by the volume resistivity and the surface resistivity. At room temperature, SCHOTT sealing glasses have an electric volume resistivity ranging from 1013 to 1020 Ω cm, which makes them good insulators. But under normal conditions (T = 20°C, 50% relative humidity), this high electric volume resistivity is rendered meaningless by the surface resistivity, which is considerably lower. The surface resistance is essentially determined by the absorption of water on the free glass surface and thus depends on the chemical resistance of the sealing glasses used.
For this reason, SCHOTT uses sealing glasses with high chemical resistance. Under normal conditions, the insulation resistance of current glass-to-metal seals ranges from 1010 to 1012 Ω, depending on the type of glass used. It is largely independent of the electric creep distance. For even higher insulation resistance (1012 to 1013 Ω) and for use in very humid climates, special surface treatment processes can be applied by SCHOTT.
At temperatures above 100°C, the effect of the water film disappears almost completely and the insulation resistance is determined by the volume resistance alone. Since the sealing glasses are ion conductors, their electrical conductivity increases with the temperature; i.e. the temperature characteristic of the electric resistivity is negative. The temperature dependence of the electric resistivity for certain sealing glasses is shown in fig. 16.
Flashover Voltage & Dielectric Characteristics
Glass has a high dielectric breakdown strength of approximately 20 kV/mm. But for glass-to-metal seals, the characteristic flashover resistance is that of the creep distance between the live metal elements, which is much lower. Fig. 17 shows the admissible alternating test voltage (50 Hz) as a function of the creep distance.
Standard glass-to-metal seals have intrinsic capacities of 0.5 to 3 pF with dissipation factors of tan δ = 25 to 250 x 10-4. The intrinsic capacity (CE) and the dissipation factor (tan δ) of glass-to-metal seals are determined in a type test with 1 MHz at room temperature. To a great extent, these values depend on the dielectric properties of the sealing glass used and on the geometrical characteristics of the glass-to-metal seal. Upon request, the intrinsic capacity tolerance of glass-to-metal seals can be limited to approximately ± 10%.
Current-carrying capacity I of various sealing alloys and of copper for a temperature
increase of metal of 30 K, as a function of the diameter D of the conductor