Limit of Commutation of DC High Current from Main to Parallel Circuit

Commutation of fault current from a main contact to its bypass circuit is usually employed to transfer the current at fault interruption to the bypass circuit which involves arcing contacts, solid state switches, or nonlinear current limiting resistors. Factors which affect the limit of commutation have been studied experimentally. The main contact we tested is rated at 3000 A de and moves at I m/s to a full stroke of 4 mm. The main contact is kicked by a plunger accelerated with a repulsion coil Main currents of up to 8.5 kA have been eommutated to the bypass circuit at various impedance values. Precise measurements of voltage and current transients and time resolved recording of the intensity of the are across the main contact have been made during the commutation by synchroscopes and photomultipliars with light guides. As a result of the study, the limit of commutation has been revealed as a function of arcing time, arc voltage and impedance of the bypass circuit. Rate of erosion on the surface of the contact was observed microscopically and correlations with arc energy and current density of an electrode spot were found. Dielectric recovery characteristics of the main contact was also studied briefly.