Current-limiting fuses are widely used to protect the thyristors in dc drive systems. In the event of the commutation failure when regenerating (inverting), the fuses need to interrupt in loop supplied by the ac and dc voltages acting in series which is the most difficult case for protection by fuse. In this paper, a detailed study of the complete interruption process has been investigated by modeling of arcing process of the fuse. The effect varying the motor time constant, supply impedance, number of fuses used to clear the fault and dc machine rating to the total response for fuses protecting against the regenerative circuit internal commutation fault has been studied. The model of 200A fuse is employed in this study and fuses in series with the semiconductor devices (F1) and fuses in ac line (F2) are both considered.