Residual current devices are widely used in low-voltage electrical installations to protect against electric shock and fire. The increasing use of power electronic equipment, such as photovoltaic systems, electric vehicle chargers and variable-speed drives, may result in direct current components in the residual current, which can affect the operation of type A residual current devices. This paper investigates the influence of a superimposed smooth direct current component on the tripping characteristics of twopole and four-pole type A residual current devices with rated residual operating currents of 30 mA, 100 mA and 300 mA. The results show that the direct current component alters the tripping behaviour of the tested devices and, at sufficiently high values, may lead to a loss of sensitivity. The two-pole 30 mA device exceeded the tripping-current limit specified by the standard, while one pole of the fourpole 30 mA device no longer tripped at a direct current component of 60 mA. The results demonstrate that smooth direct current components can significantly affect the reliable operation of type A residual current devices and should therefore be considered when selecting residual current protection for installations in which such currents may occur.
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