The mechanism of the so-called fast quench, i.e., an abnormal quench process with a high propagation velocity of normal zone in the multi-strand cables is discussed. Firstly, a set of basic equations were proposed so as to describe precisely the electromagnetic coupling among strands, propagation of normal zone and thermal interaction among strands. A good agreement of the theoretical value of transport current, at which the fast quench began to appear, with our observed data for a 2-strand cable supported strongly the availability of the proposed equations for a description of the fast quench. Then numerical calculations were also carried out for the commutation of transport current and temperature distribution inside a 2-strand cable when one strand was locally quenched. With the aid of the obtained results, a detailed mechanism for the occurrence of the fast quench was proposed.
