3.5.3 Electrical feedthroughs
A major factor in engineering an electric feedthrough is the current capacity and voltage for which it will be used and the requirements it will have to satisfy with respect to vacuum-tightness and temperature resistance. Feedthroughs with organic insulating materials can only be used for lower voltages. Cast-resin feedthroughs are frequently used for moderate current loads and for moderate temperatures. With respect to their insulating resistance, feedthroughs with glass-to-metal fusings are suitable for high-voltage and weak-current feedthroughs for electronic devices.
Feedthroughs with ceramic insulation offer greater mechanical stability and temperature resistance than glass. In addition, ceramic (e.g. aluminum oxide) can also be produced in an insulating form that is suitable for high voltage. This is why ceramic feedthroughs are superior to glass feedthroughs for high voltages and high performance. Only rigid metal-to-ceramic connections should be considered for the most rigorous electrical, thermal and vacuum technology requirements.
It must be generally considered that higher temperatures reduce the electrical insulation effect and also decrease the ampacity of the conductor. Unless otherwise stated, the electrical data refer to room temperature. Moreover, the maximum operating voltages apply for one vacuum of less than 1 · 10-4 hPa. At higher pressures, small clearances between conductors with high voltage differences can lead to gas discharges and flashovers. In the vulnerable pressure range between 1 · 10-3 hPa, appropriate clearances must be provided between high-voltage conductors. Alternatively, potting with cast resin or shields made of glass or ceramic pipes can be useful in this regard.
Electrical feedthroughs are available as wire feedthroughs, as multiple feedthroughs with a plug or a feedthrough with coaxial connector. There are variations specifically for the transfer of high voltages or currents, or voltages of many signal voltages in a narrow installation space.
Figure 3.20: Electrical feedthrough with ceramically insulated wire conductor made of copper