What is a residual current device (RCD)/RCCB and what is its purpose in LV protection?

• A. It detects overcurrent and trips on overload
• B. It detects imbalance between live and neutral and trips
• C. It measures insulation resistance
• D. It monitors power factor

Answer: (B)

A residual current device protects people by sensing leakage to earth. It does this by continuously comparing the current flowing in the live conductor with the current returning in the neutral conductor. Under normal operation these currents are equal, so no net current goes anywhere else. If current leaks to earth—such as through a person who touches a live part or a fault in an appliance—the returning current on the neutral is reduced, creating an imbalance. The RCD detects this imbalance and trips the circuit quickly to cut off power, minimizing the risk of electric shock.

This is different from overcurrent protection, which trips on overload or short-circuit conditions; insulation resistance testing, which measures how well parts are insulated, and power factor monitoring, which looks at how effectively current and voltage waveforms align. So the device’s purpose is specifically to detect earth leakage and disconnect the supply to prevent shock.