Where the interval really comes from
The answer “once a year” is common and often right – but it is not the law. Section 3 of the German Industrial Safety Regulation obliges the operator to set the type, scope and intervals of inspections themselves, in the risk assessment. The accident insurers’ rules then name a minimum interval of one year. With heavy duty, an aggressive environment or a safety-critical task the correct interval can be shorter – and then the annual date is simply not enough.