Influence of the Design of a Residual Current Device on Its Break Time

Residual current devices (RCDs) are devices that can provide very good protection measures against electric shock. Yet, under certain circumstances, they can cause unnecessary and unexpected switching off of power in the protected circuits. The main component that determines the properties of an RCD is the summation current transformer. In recently produced RCDs, whose operation is independent from the installation voltage, the summation transformer, apart from detection of the residual current, has yet another task: it must also provide an appropriate energy value to the electromagnetic release in order to carry out mechanical disconnection of the contacts. In this type of RCD, the core of the summation transformer should be made of magnetic material with very high permeability and appropriate geometric dimensions. Manufacturers of RCDs, in order to reduce production costs and to promote miniaturization of the devices, use cores for summation transformers made of amorphous or nanocrystalline materials quite often. In such RCD designs, the impedance-matching circuit is used in the secondary circuit of the summation transformer to ensure proper sensitivity to the residual current. The paper describes the impact of using additional impedance-matching elements in the secondary circuit on the RCD break time. The paper presents the results of tests and measurements of the RCD break times of various internal structures.