A method for monitoring a production process of a voltage source, which consists of modules (6a to 6f, 8a to 8f, 10) connected in series and having an electrically conductive casing, said modules being set up step-by-step during the production process starting from a first connection (HV+) to form a first partial string of the module and starting from a second connection (HV-) to form a second partial string of the module and a lastly installed module connecting both partial strings of the modules (4a, 4b) to each other so as to form a closed module string, the method comprising the steps: - connecting a first coupling branch of a coupling circuit of an insulation monitoring device to the first connection (HV+), connecting a second coupling branch of the coupling circuit of the insulation monitoring device to the second connection (HV-), and connecting a ground connection of the insulation monitoring device to the casing of the voltage source, - feeding a measuring signal (U P) between the first connection (HV+) and the ground connection and between the second connection (HV-) and the ground connection, - capturing and determining a first partial voltage of the module (U HV+/E) at the first connection (HV+) every time a module has been added to the first partial string of the module, - capturing and determining a second partial voltage of the module (U HV-/E) at the second connection (HV-) every time a module has been added to the second partial string of the module, - detecting a leakage resistance (R f) every time a module has been added and detecting a leakage capacitance (C e) for the partial string of the module (4a, 4b) to which the module was added, - signaling an insulation fault should the leakage resistance (Rf) drop below a given leakage resistance threshold value by adding the module, - signaling a line disruption should the leakage resistance (R f) not have decreased by adding the module, - signaling a line disruption should the leakage capacitance (C e) not have increased by adding the module, - testing whether a voltage is measured between the first connection (HV+) and the second connection (HV-) as a fault voltage every time a module has been added, - testing whether a nominal voltage has been set between the first connection (HV+) and the second connection (HV-) every time the lastly installed module has been added. - connecting a first coupling branch of a coupling circuit of an insulation monitoring device to the first connection (HV+), connecting a second coupling branch of the coupling circuit of the insulation monitoring device to the second connection (HV-), and connecting a ground connection of the insulation monitoring device to the casing of the voltage source, - feeding a measuring signal (U P) between the first connection (HV+) and the ground connection and between the second connection (HV-) and the ground connection, - capturing and determining a first partial voltage of the module (U HV+/E) at the first connection (HV+) every time a module has been added to the first partial string of the module, - capturing and determining a second partial voltage of the module (U HV-/E) at the second connection (HV-) every time a module has been added to the second partial string of the module, - detecting a leakage resistance (R f) every time a module has been added and detecting a leakage capacitance (C e) for the partial string of the module (4a, 4b) to which the module was added, - signaling an insulation fault should the leakage resistance (Rf) drop below a given leakage resistance threshold value by adding the module, - signaling a line disruption should the leakage resistance (R f) not have decreased by adding the module, - signaling a line disruption should the leakage capacitance (C e) not have increased by adding the module, - testing whether a voltage is measured between the first connection (HV+) and the second connection (HV-) as a fault voltage every time a module has been added, - testing whether a nominal voltage has been set between the first connection (HV+) and the second connection (HV-) every time the lastly installed module has been added.