4. Based on the inspection plans referred to in paragraph 3, the competent authority shall regularly draw up programmes for routine inspections for all establishments including the frequency of site visits for different types of establishments.
The period between two consecutive site visits shall not exceed one year for upper-tier establishments and three years for lower-tier establishments, unless the competent authority has drawn up an inspection programme based on a systematic appraisal of major-accident hazards of the establishments concerned.