Arcing behaviour on TMF-contacts at high-current interrupting operations

Vacuum circuit breakers are widely used in the medium voltage area. The majority of the installed vacuum tubes is equipped with electrodes using the transverse magnetic field (TMF) design forcing the electric arc on a circular motion to avoid severe local overheating. A vacuum test switch was used to investigate the arc movement behaviour between spiral-shaped and cup-shaped electrodes at high-current interrupting operations. The switch was equipped with viewing ports allowing an observation from two rectangular views. Mounted sample contacts were made of CuCr 75/25 at different diameters. A digital 8 bit high-speed camera was used to record the arcing process with frame rates of 33,000 frames per second. Behaviour patterns were investigated and compared with arc voltage and instantaneous current. Parameters as arc velocity and current density on the contacts could be determined by means of the recordings. The experiments were conducted with short circuit currents from 20 kA to 60 kA rms with a frequency of 50 Hz.