Define an RCD and its typical domestic rating.

• A. A fuse that melts at 100 A
• B. A device that stabilizes voltage to 230 V
• C. A residual current device detects imbalance between live and neutral and trips; typical rating is 30 mA.
• D. A protective device that reduces energy use

Answer: (C)

An RCD, or residual current device, monitors the current flowing in the live and neutral conductors and trips if there is any leakage to earth. This imbalance means electricity could be escaping through a person or faults to earth, so the device automatically disconnects the circuit to reduce the risk of electric shock. In typical domestic wiring, the protective rating is 30 milliamps (0.03 A), which provides fast fault detection while minimizing nuisance trips. This is its main job—unlike a fuse, which protects against overcurrent, or a voltage stabilizer or energy-saving device, which serve different functions. So the standard domestic rating is 30 mA because it offers a good balance of safety and practicality for everyday life.