Abstract: Zinc oxide surge arresters are used to replace capacitive elements for voltages below the pick-up voltage of the surge arrester. Advantageously, the surge arresters may be provided in an alternating current network to provide a high degree of protection against transient disturbances without increasing cost.

Abstract: The present invention relates to a protective device for a load connected to a power supply line of an electric power system, comprising an LC network having low-pass characteristics, connected into the power supply line on the system side preceding the load. So that disturbances with frequencies below the cut-off frequency of the LC network can also be kept away from the load, a switching element controlled by a monitoring device is arranged according to the invention in series with the LC network in the power supply line. The monitoring device is connected to the power supply line on the system side at a junction preceding the switching element and keeps the switching element closed only for as long as the instantaneous value of the time derivation of the voltage on the power supply line deviates from the instantaneous value of a nominal value which is predeterminable in its variation with time, by no more than an equally predeterminable amount.

Abstract: A monitoring device for semiconductor rectifiers which are located in a three-phase current bridge circuit, in which the bridge circuit is provided with two semiconductor rectifiers per rectifier branch, including a current-sensitive detector circuit connected to each pair of semiconductor rectifiers of each branch through a resistor, wherein when the semiconductor rectifiers are intact, the sum of all current flowing through the resistors is zero, as detected by the current-sensitive detector circuit.