Under what circumstances reasonable opportunity under Article 311 (2) of the Indian Constitution is not available?

The protection of Article 311 (2) for giving reasonable opportunity is not available in the following circumstances : (a) where a person is dismissed are reduced in rank on the ground of misconduct which has led to conviction or criminal charges ; (b) where it is impracticable to give the civil servant an opportunity to defend himself but  the authority taking action against him shall record the reasons for such action ; (c)  where in the interest of the security of state, it is not expedient to give such an  opportunity to the civil servant [Article 311, proviso].