Arc fault detection unit for an electrical low-voltage AC circuit, having an evaluation unit, at least one voltage sensor assigned to the circuit, for periodically determining electric voltage values of the electrical circuit, at least one current sensor assigned to the circuit, for periodically determining electric current variables of the circuit, both of which are connected to the evaluation unit, wherein the evaluation unit is configured in such a way that the electric voltage values are fed to a first arc fault detection function, which carries out a first arc fault detection on the basis of the signal profile of the voltage and, if a threshold value (SW1, SW4), which relates to the change in the voltage over the time, is exceeded in magnitude, outputs a first arc fault detection signal, that the electric voltage values and current variables are fed to a second arc fault detection function, which carries out a second arc fault detection on the basis of the voltage values and current variables, wherein the second arc detection function continuously calculates an arc voltage (U Lb) with the aid of the determined voltage values and the determined current variables, and, if a second threshold value, which relates to the arc voltage, is exceeded in magnitude, outputs a second arc fault detection signal, that the two arc fault detection signals are fed to an OR function so that, in the presence of a first or second arc fault detection signal, an OR-linked arc fault detection signal is output as an output-side arc fault detection signal, that the evaluation unit is configured in such a way that the electric current variables are fed to a third arc fault detection function, which continuously compares the current variable or the equivalent thereof with a third threshold value and, if said third threshold value is exceeded, outputs a third arc fault detection signal, and that the OR-linked arc fault detection signal and the third arc fault detection signal are fed to an AND function so that the output-side arc fault detection signal is output by the arc fault detection unit only in the presence of the third and the OR-linked arc fault detection signal. an evaluation unit, at least one voltage sensor assigned to the circuit, for periodically determining electric voltage values of the electrical circuit, at least one current sensor assigned to the circuit, for periodically determining electric current variables of the circuit, both of which are connected to the evaluation unit, wherein the evaluation unit is configured in such a way that the electric voltage values are fed to a first arc fault detection function, which carries out a first arc fault detection on the basis of the signal profile of the voltage and, if a threshold value (SW1, SW4), which relates to the change in the voltage over the time, is exceeded in magnitude, outputs a first arc fault detection signal, that the electric voltage values and current variables are fed to a second arc fault detection function, which carries out a second arc fault detection on the basis of the voltage values and current variables, wherein the second arc detection function continuously calculates an arc voltage (U Lb) with the aid of the determined voltage values and the determined current variables, and, if a second threshold value, which relates to the arc voltage, is exceeded in magnitude, outputs a second arc fault detection signal, that the two arc fault detection signals are fed to an OR function so that, in the presence of a first or second arc fault detection signal, an OR-linked arc fault detection signal is output as an output-side arc fault detection signal, that the evaluation unit is configured in such a way that the electric current variables are fed to a third arc fault detection function, which continuously compares the current variable or the equivalent thereof with a third threshold value and, if said third threshold value is exceeded, outputs a third arc fault detection signal, and that the OR-linked arc fault detection signal and the third arc fault detection signal are fed to an AND function so that the output-side arc fault detection signal is output by the arc fault detection unit only in the presence of the third and the OR-linked arc fault detection signal.