What is the purpose of an RCD in a household circuit?

• A. To regulate voltage in the home
• B. To detect an imbalance between live and neutral currents and quickly disconnect the circuit to prevent electric shocks.
• C. To measure total current
• D. To provide extra lighting

Answer: (B)

An RCD protects people by sensing a difference between the current in the live wire and the current returning in the neutral wire. When everything is working normally, those currents are the same. If some current leaks to earth—for example due to a fault or if a person touches a live conductor—the currents are no longer equal. The RCD detects this imbalance and disconnects the circuit within milliseconds, greatly reducing the risk of electric shock. Domestic RCDs typically trip at about 30 mA, which is enough to protect you without causing unnecessary interruptions for small harmless faults. It’s not about regulating voltage, measuring total current, or providing extra lighting; protection against overload is handled by other devices like fuses or circuit breakers.