This thesis examines residual current devices (RCDs) as a key element of safety in low-voltage electrical installations, with a focus on their operating principles, correct application, typical installation errors, and the influence of leakage currents and DC components on their reliability.
The theoretical part introduces the different types of RCDs (AC, A, B, B+), their applications depending on the intended protection, and their implementation in various earthing systems (TN, TT, IT). The core of the thesis addresses the most frequent wiring faults involving RCDs, with particular attention to the unintended reconnection of the neutral (N) and protective earth (PE) conductors. Numerical analysis is used to explain the impact of this fault on current distribution and RCD tripping behavior.
In the experimental part, leakage currents of several household appliances were measured. The results confirmed that leakage currents from multiple devices add up and can cause nuisance tripping of RCDs if too many appliances are connected to a single device. Based on the measurements, guidelines for the design of RCD-protected circuits were developed, including limiting the number of appliances connected to one RCD according to their leakage characteristics. The experiments were conducted at ETI, a company specializing in the production of protective devices and related electrotechnical equipment.
In addition, tests were carried out to investigate the effect of DC components on the performance of type A RCDs. The analysis showed that the presence of direct current causes saturation of the RCD’s magnetic core and reduces its operational reliability. Type A devices proved unsuitable in the presence of permanent DC components; therefore, the use of type B or B+ RCDs is recommended in such cases.
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