2 the machine control system 5 switches to the next lower number of activated vacuum pumps 4 1 4 3 also shown in fig1 is a device 9 for providing and or conditioning seal water jointly for all the vacuum pumps 4 1 4 3 of the vacuum device 3 the device 9 forms part of a water circuit through the vacuum pumps 4 1 4 3 and can include among other things a device for cooling the seal water and connections for venting supplying fresh water and removing waste water taking account of the power consumption of the vacuum unit 3 and the overall characteristic curve 7 a function and fault monitoring of the entire installation is furthermore possible the machine control system 5 knows how many vacuum pumps 4 1 4 3 need to be operated for a certain operating status of the installation at a specified frequency of the supply voltage or what target energy consumption arises in the particular operating status if the corresponding value i e the number of activated vacuum pumps 4 1 4 3 the frequency of the supply voltage for these pumps and thus also the energy consumption for the maintenance of the target negative pressure differ from the target values by more than a particular amount which is defined by a specified admissible tolerance range then there is a fault in the vacuum device 3 or in the installation 1 for example in the form of a relatively large leak in this case a warning or indication signal or a warning or indication message.