Which device provides earth-leakage protection by sensing residual current?

• A. Overcurrent protection
• B. Earth-leakage protection by residual current
• C. Voltage regulation
• D. Surge protection

Answer: (B)

This protection is achieved by a residual current device. It watches the current in the live and neutral conductors and only trips when there’s an imbalance between them. Under normal operation the two currents cancel out, so no trip occurs. If some current leaks to earth—through a person or a fault—the balance is disrupted, creating a residual current. When this is detected, the device trips quickly to disconnect the circuit, preventing electric shock and reducing fire risk.

This is different from overcurrent protection, which responds to excessive current levels but not to leakage. Voltage regulation keeps the voltage steady, and surge protection clamps transient voltage spikes. Neither of those protect against leakage paths, which is why sensing residual current is the function that delivers earth-leakage protection.