The problem of stability of the charged interface between two ideal fluids is considered with presence of the velocity jump across the interface. It was take into account that fluids having various dencity is not mixing up and each of which fills in half-infinite space. The upper liquid move along interface with variable in time velocity. The time dependence of velocity results to instability of interface and to decrease of critical surface density of a charge necessary for an initiation of the instability Tonks-Frenkel and the st. Elmo fires.