The effect of earth fault current harmonics on tripping of residual current devices

The most popular means of protection against indirect contact in low voltage systems is automatic disconnection of supply, since it is a simple and economically attractive way to achieve safety in an electrical installation. As a protective device, residual current devices (RCDs) are commonly used. In certain conditions using the RCDs is obligatory. The widespread application of static converters, especially frequency converters, results in installing RCDs in circuits where nonsinusoidal earth fault current flows. The earth fault current in the output circuit of frequency converters comprises harmonics and interharmonics, whose order mainly depends on the PWM frequency. The level of higher frequency components in the earth fault current may exceed the fundamental one. The nonsinusoidal earth fault current influences tripping of the residual current devices. In the paper, results of the theoretical analysis and the laboratory tests of residual current devices operation are presented and discussed. They prove that in some cases higher frequency components cause improper tripping of the residual current devices and a real hazard of electrocution exists.