RCD (diferencial, residual current device)

How it works

Inside the RCD, the live (phase) and neutral wires pass through a small magnetic core. While the outgoing and returning currents are equal, their effects cancel out. If part of the current escapes to earth, an imbalance appears, the core detects it and the device opens the circuit. That is why it protects you against indirect contact, such as touching the casing of a faulty washing machine.

Sensitivity and types

• 30 mA (high sensitivity). Compulsory on the circuits of a Spanish home under ITC-BT-25 of the Low Voltage Regulations (REBT).
• 300 mA. Used in industry or as general fire protection, not to protect people.
• Types AC, A and B. They indicate which leakage waveforms the device detects. Type A also detects pulsating DC leaks typical of electronics. For EV chargers, ITC-BT-52 requires at least a type A with 6 mA DC detection or a type B.
• Super-immunised (superinmunizado). An RCD reinforced against nuisance tripping from network disturbances, useful in homes with lots of electronics.

RCD and MCB are not the same

The MCB (magnetotérmico) protects the cables against overloads and short circuits; the RCD protects people against leaks. A home fuse box needs both.

How to check it works

Every RCD has a test button marked T. Press it once a month: it should trip. If it does not, it is not protecting you and must be replaced. In Seville, Spain we deal with it through RCD keeps tripping.