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In this paper the ampacity of cathode spots and structures of cathode spots on an AMF contact at short gap distance are investigated by observation of the arc behavior with a high-speed, high resolution CCD video camera. While the current carried by a single cathode spot is independent of the short circuit current, the number of the cathode spots and the voltage drop between the contacts increase linearly with the total current. Further the dynamical structure of the arc is studied and the variation of the density of the cathode spots over the current is measured. At the limit of the breaking capability of the contacts the maximum average current density is about 1.8 kA/cm 2 (RMS), which conforms well to the experience with commercial vacuum interrupters. Above this point the CuCr contact surface melts homogeneously over several square centimetres. Although the molten surface layer thickness is assumed to be a few tens to hundreds of microns only, millimetre-sized protrusions grow rapidly, providing a rough and quickly changing cathode surface. We discuss the growth and dynamics of these surface structures in terms of hydrodynamic instabilities and provide a model for the formation process.XXIII-rd Int. Symp. on Discharges and Electrical Insulation in Vacuum -Bucharest -2008 978-1-4244-2841-0/08/$25.00 ©2008 IEEE

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