A fieldbus system comprises a switching device, a control device and a fieldbus module connected to a first electrical supply voltage, wherein a value of the first electrical supply voltage is ascertained and transmitted to a control device by the fieldbus module, wherein the ascertained value of the first electrical supply voltage is compared with a predetermined value of the first electrical supply voltage by the control device, wherein, if the value of the first electrical supply voltage that is transmitted to the control device is not concordant with the predetermined value of the first electrical supply voltage, shutdown of a second electrical supply voltage by the switching device is initiated by the control device.