Testing Sensitivity of A-Type Residual Current Devices to Earth Fault Currents with Harmonics.

Stanislaw Czapp

doi:10.3390/s20072044

Abstract

In many applications, modern current-using equipment utilizes power electronic converters to control the consumed power and to adjust the motor speed. Such equipment is used both in industrial and domestic installations. A characteristic feature of the converters is producing distorted earth fault currents, which contain a wide spectrum of harmonics, including high-order harmonics. Nowadays, protection against electric shock in low-voltage power systems is commonly performed with the use of residual current devices (RCDs). In the presence of harmonics, the RCDs may have a tripping current significantly different from that provided for the nominal sinusoidal waveform. Thus, in some cases, protection against electric shock may not be effective. The aim of this paper is to present the result of a wide-range laboratory test of the sensitivity of A-type RCDs in the presence of harmonics. This test has shown that the behavior of RCDs in the presence of harmonics can be varied, including the cases in which the RCD does not react to the distorted earth fault current, as well as cases in which the sensitivity of the RCD is increased. The properties of the main elements of RCDs, including the current sensor, for high-frequency current components are discussed as well.

Highlights

Electrical and electronic systems from their nature introduce an electric shock hazard
The aim of this paper is to present and comment on the results of the wide laboratory test of the A-type residual current devices (RCDs)’s sensitivity to the non-sinusoidal earth fault currents
Especially high-order harmonics, increase the tripping threshold of the RCDs; in some cases, the RCDs may not react to the earth fault currents having a relatively high content of high-order harmonics

Summary

Introduction

Electrical and electronic systems from their nature introduce an electric shock hazard. The fundamental rule of electrical safety, both in high- and low-voltage systems, is included in standard EN 61140 [1]: Hazardous-live-parts are not allowed to be accessible, and accessible conductive parts are not allowed to be hazardous-live. For low-voltage systems, detailed rules referring to the protection against electric shock are described in the multipart standard. The most common method of protection against indirect contact is the automatic disconnection of supply. In the case of a line-to-earth fault, dangerous voltage may appear at the metallic enclosure of the current-using equipment, and risk of electrocution, due to indirect contact, exists (Figure 1). To prevent a fatal accident, the power supply has to be disconnected within the time specified in standard HD 60364-4-41 [2]

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