Which device would immediately interrupt power on earth leakage, typically rated at 30 mA for domestic use?

• A. RCBO
• B. MCB
• C. RCD
• D. RCCB

Answer: (C)

The key idea is protective devices that respond to earth leakage. A residual current device continuously compares the current flowing in the live conductor with the current returning in the neutral. Under normal conditions these two are equal. If some current leaks to earth—such as through a fault or a person touching a live part—the return current is reduced, creating a difference (residual current). When this difference reaches the standard domestic sensitivity, typically about 30 mA, the device trips instantly to disconnect the circuit, reducing the risk of electric shock.

An MCB protects against overcurrent or short circuits, not leakage to earth, so it wouldn’t reliably trip on earth leakage alone. An RCCB is simply an older term for a residual current device and functions the same way, while an RCBO combines leakage protection with overcurrent protection but isn’t the basic device described by the 30 mA domestic leakage standard. The option described here is the residual current device, which is designed to interrupt power for earth leakage at the common 30 mA sensitivity.