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This paper presents a study of instabilities and of spontaneous extinctions which occur in low current dc arcs between metal electrodes. A theoretical model for the behavior of emitting sites on the cathode surface is proposed. This model explains experimental results obtained in arc duration measurements and arc noise correlation. The model yields figures for site life time and elementary current of emitting sites. These values are compared with those given by other authors. The proposed model can be applied to explain arc duration and related erosion occuring in telecommunication relays and circuit breakers.