  Thermomagnetic avalanches in superconducting films propagating extremely fast
while forming unpredictable patterns, represent a serious threat for the
performance of devices based on such materials. It is shown here that a
normal-metal frame surrounding a selected region inside the film area can
provide efficient protection from the avalanches during their propagation
stage. Protective behavior is confirmed by magneto-optical imaging experiments
on NbN films equipped with Cu and Al frames, and also by performing numerical
simulations. Experimentally, it is found that while conventional flux creep is
not affected by the frames, the dendritic avalanches are partially or fully
screened by them. The level of screening depends on the ratio of the sheet
conductance of the metal and the superconductor in the resistive state, and for
ratios much larger than unity the screening is very efficient.
