TRGS 526 

7.3 Deductions
Fume cupboards must be maintained regularly and their functionality checked and documented. The test must be carried out at least once a year by a qualified person. The annual check of the ventilation function can be omitted if a self-monitoring function check of the individual fume cupboard ensures that a drop below the minimum volume flow is indicated optically and acoustically. The permanent monitoring device should be checked at intervals of no more than three years.