The critical state stability in a 'hard' type-II superconductor with respect to the flux jumps has been studied both for adiabatic and isothermal boundary conditions. The sample geometry as well as initial magnetic field and current distributions are found to be important for the stability criterion. The increase of the ratio of the thermal and magnetic diffusivities results in an increase of the stability. This effect is most noticeable in the case of intensive external cooling. For the extremely 'hard' superconductors the simplified scheme of the investigation is proposed.