  Quantum tunnelling phenomenon allows a particle in Schr\"odinger mechanics
tunnels through a barrier that it classically could not overcome. Even the
infinite potentials do not always form impenetrable barriers. We discuss an
answer to the following question: What is a critical magnitude of potential,
which creates impenetrable barrier and for which the corresponding
Schr\"odinger evolution system separates? In addition we describe some
quantitative estimates for the separating effect in terms of cut-off
potentials.
