Verification of safety in low-voltage power systems without nuisance tripping of residual current devices

Abstract

Low-voltage power systems require initial and periodical verification to check the effectiveness of protection against electric shock. As a protection in case of fault, automatic disconnection of supply is most often used. To verify such a protection measure, the earth fault loop impedance or resistance is measured. This measurement is easy to perform in circuits without residual current devices. When residual current devices are installed, their unwanted tripping occurs during the measurement, and the verification becomes problematic. The paper proposes a new method of testing the effectiveness of automatic disconnection of supply, without unwanted tripping of residual current devices, especially when their rated residual current is equal to 30 mA. The theoretical background of this method, along with mathematical analysis and experimental verification are presented. The proposed method uses the testing current which is many times higher than the rated residual current of commonly used residual current devices. This is an important advantage in terms of testing accuracy.