Method for operating a motor vehicle on-board network having an electrical machine which can be operated as a generator and having an active bridge rectifier which is connected to the electrical machine via phase connections (VU - VY) and can be operated in a rectifier operating mode and a short-circuit operating mode, wherein - in the rectifier operating mode, phase voltages (VU - VY) present at the phase terminals (U - Y) are converted by driving active switching elements (AH - EL) of the bridge rectifier into an output voltage which is output at DC voltage terminals (B+, B-) of the bridge rectifier, - in the short-circuit operating mode, the phase terminals (U - Y) are shortcircuited by driving the active switching elements (AH - EL) so that no output voltage is output from the bridge rectifier at the DC voltage terminals (B+, B-) of the bridge rectifier, and - the short-circuit operating mode is initiated as soon as a signal (VB+) indicative of a voltage which drops between the DC voltage terminals (B+, B-) of the bridge rectifier exceeds an upper threshold value, wherein - in a first method phase, the short-circuit operating mode is cancelled as soon as the signal (VB+) subsequently falls below a lower threshold value, and a driving signal with a first switching time is used to drive the active switching elements (AH - EL), characterized in that - after the bridge rectifier has been operated at least once in the short-circuit operating mode in the first method phase, it is determined on the basis of at least one feature of the signal (VB+) whether the signal (VB+) exceeds the upper threshold value due to a cable break in a cable at at least one of the DC voltage terminals (B+, B-), wherein a duration of the at least one short-circuit operating mode in which the bridge rectifier was operated in the first method phase is used as the at least one feature of the signal (VB+), and if that is the case, - the bridge rectifier then is operated in the short-circuit operating mode for a prolonged period of time in a second method phase by presetting a corresponding short-circuit operating mode time or lowering the lower threshold value, and/or in the second method phase, a driving signal with a second switching time which is shorter than the first switching time is used to drive the active switching elements (AH - EL). - in the rectifier operating mode, phase voltages (VU - VY) present at the phase terminals (U - Y) are converted by driving active switching elements (AH - EL) of the bridge rectifier into an output voltage which is output at DC voltage terminals (B+, B-) of the bridge rectifier, - in the short-circuit operating mode, the phase terminals (U - Y) are shortcircuited by driving the active switching elements (AH - EL) so that no output voltage is output from the bridge rectifier at the DC voltage terminals (B+, B-) of the bridge rectifier, and - the short-circuit operating mode is initiated as soon as a signal (VB+) indicative of a voltage which drops between the DC voltage terminals (B+, B-) of the bridge rectifier exceeds an upper threshold value, wherein - in a first method phase, the short-circuit operating mode is cancelled as soon as the signal (VB+) subsequently falls below a lower threshold value, and a driving signal with a first switching time is used to drive the active switching elements (AH - EL), characterized in that - after the bridge rectifier has been operated at least once in the short-circuit operating mode in the first method phase, it is determined on the basis of at least one feature of the signal (VB+) whether the signal (VB+) exceeds the upper threshold value due to a cable break in a cable at at least one of the DC voltage terminals (B+, B-), wherein a duration of the at least one short-circuit operating mode in which the bridge rectifier was operated in the first method phase is used as the at least one feature of the signal (VB+), and if that is the case, - the bridge rectifier then is operated in the short-circuit operating mode for a prolonged period of time in a second method phase by presetting a corresponding short-circuit operating mode time or lowering the lower threshold value, and/or in the second method phase, a driving signal with a second switching time which is shorter than the first switching time is used to drive the active switching elements (AH - EL). is operated in the short-circuit operating mode for a prolonged period of time in a second method phase by presetting a corresponding short-circuit operating mode time or lowering the lower threshold value, and/or in the second method phase, a driving signal with a second switching time which is shorter than the first switching time is used to drive the active switching elements (AH - EL).