When an emergency makes it impossible to fill a position in the classified service under any other provision of this chapter the appointing authority may appoint any qualified person to such position in order to prevent stoppage of public business or loss or serious inconvenience to the public. Any such person shall be employed only during the emergency and for a period not exceeding 60 days in any twelve-month period. A vacancy of which the department head concerned has had a reasonable notice, or an employment condition of which he has, or might with due diligence have had, previous knowledge, shall not be considered an emergency under this section.