Electrical Properities of Sealing Glass
Insulation ResistanceThe insulation resistance of glass is set by its 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 determined by the absorption of water on the free glass surface, which is dependent 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 treatments can be applied by SCHOTT.
At temperatures greater than 100°C, the effect of the water film disappears almost completely and the insulation resistance is measured only by the volume resistance. Since the sealing glasses are ion conductors, their electrical conductivity increases with the temperature. In other words 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.
Electrical resistance ρ of SCHOTT sealing glasses as a function of the glass temperature
Flashover Voltage & Dielectric CharacteristicsGlass 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 of Conductors in Glass-to-Metal SealsFig. 18 shows the current-carrying capacity of wires of different sealing alloys and of copper conductors as a function of the wire diameter. The current load used in this example causes the temperature of the conductor to rise by 30K under normal ambient conditions.
The sealed-in conductors can be exposed to higher current loads if shock loads (extremely short high-current impulses) are avoided and efficient heat dissipation is provided for. Special seal types capable of withstanding extremely high current loads (up to several thousand amps) use tubes of a sealing alloy with brazed-in solid copper pins.
When the conductors are exposed to very high current loads with steep edge pulses (power-up condition), the electrodynamic forces on the seals (Pintsch effect) should not be neglected. Glass-to-metal seals for these special applications can be shock type-tested on request.
Admissible alternating test voltage Ueff (50 Hz) as a function of the creep distance KW between two live electrodes
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