What it detects
An RCD compares currents flowing through a circuit’s live conductors and operates in response to residual current according to its characteristics. It can contribute to protection against electric shock and certain fault conditions. It does not directly measure body resistance or recognise every hazardous electrical contact.
Related concept: Electrical risk.
RCDs and overcurrent circuit breakers
These are different functions. An overcurrent circuit breaker protects against excessive current; an RCD responds to a difference in currents. Some devices combine both functions, but not every RCD provides overload or short-circuit protection on its own. Selection and coordination form part of designing the complete circuit.
Sensitivity and type
Besides rated residual operating current, relevant features include detectable waveform, time delay, rated current and coordination with other devices. Electronic loads may generate residual currents requiring an appropriate type. Selecting only “30 mA” leaves these conditions unresolved. The Spanish guide distinguishes, among others, types AC, A, F and B.
Testing and maintenance
Operating the test button checks a function specified by the manufacturer, but does not certify the whole installation or replace instrument-based verification. Follow instructions and record anomalies. Repeated tripping requires investigation: increasing the rated residual operating current or disabling the device to avoid inconvenience can remove necessary protection.
Hypothetical example
Equipment containing power electronics is added and tripping begins. Analysis must distinguish leakage, an incompatible RCD type, accumulated residual currents or a fault. Leaving the RCD bypassed is not the answer. It is also necessary to verify that the chosen protection operates for the fault currents that this equipment can produce.
What it does not guarantee
An RCD does not remove the need for insulation, enclosures or earthing where required. It may not operate when contact current flows between live conductors without sufficient imbalance. Its presence does not authorise live work. High-sensitivity additional protection reduces certain risks, but does not make every installation, extension lead or operation safe.
