The disclosure generally relates to a process for the preparation of compounds of formula I, including synthetic intermediates which are useful in the process. Due to a single flexibly designed pressed screen having appropriate receptacles it is possible to cost-effectively manufacture various plug connectors having different contact element arrangements. The power consumption for the operation of a plurality of compressor units of a compression installation can be optimized by calculating a novel circuit configuration and automatically adjusting the novel circuit configuration by a control device. An automatic defect classification is then performed to divide the defects into different defect types according to a predetermined database.