Influence of shield capacity on the breakdown voltade of vacuum tubes

With a normal vacuum switch, shield electrodes are built into the system to control the electric field distribution. The shield electrodes thus represent an additional capacity, which is in parallel to the contact electrodes. The prediction of the high-voltage strength of such electrode arrangements in vacuum is complex. Effects of a wide range of parameters like electrode material_ surface finish: surface area, conditioning state of the surface and the geometry of the electrode arrangement exist. It is well known that the electrode's area influences the dielectric strength in vacuum. With increasing area the dielectric strength decreases, in the opposite direction the capacity of the electrode arrangement rises. This work shows that shields in a vacuum tube which represent a parallel capacity, do not alter the breakdown behaviour during lightning impulse voltage of the contact system. Since the gap distance of the shields lies clearly over that of the contact system, a reduction of the breakdown voltage is not to be expected from the increasing total surface by the ¿area effect¿. The empirical relation between the breakdown voltage and the area of the electrodes surface can also be described as a capacity influence. But the increasing capacity is not causal for the phenomenon. For this purpose. coplanar electrodes are stressed and conditioned with lightning impulse voltage (1.2/50μs). The test vessel is enabled to pick up more than one pair of electrodes. That makes possible to vary the total capacitance by a second or even third parallel connected pair of electrodes. The results of the measurements are discussed and compared to the area effect.