How does a residual current device (RCD) detect faults and trip?

How does a residual current device (RCD) detect faults and trip?

• A. It detects overcurrent in the live conductor and trips.
• B. It trips when voltage exceeds a preset level.
• C. It monitors the current balance between live and neutral; any imbalance above a threshold indicates leakage to earth and triggers a trip.
• D. It uses a temperature sensor.

Answer: (C)

RCDs protect you by watching the balance of current in the live and neutral conductors. Under normal use, all the current that leaves on the live returns on the neutral, so the two currents cancel in the sensing core. If some current leaks to earth—through a fault or a person—the return current is reduced, creating an imbalance. The RCD senses this residual current (often with a toroidal current transformer around both conductors) and, if the imbalance exceeds a small safety threshold (typical domestic devices around 30 mA), it activates a trip mechanism to disconnect the circuit almost instantly. This immediate disconnection greatly reduces the risk of electric shock. It’s not about overcurrent or voltage level or temperature—it's specifically about detecting leakage by equal currents no longer summing to zero.

RCDs protect you by watching the balance of current in the live and neutral conductors. Under normal use, all the current that leaves on the live returns on the neutral, so the two currents cancel in the sensing core. If some current leaks to earth—through a fault or a person—the return current is reduced, creating an imbalance. The RCD senses this residual current (often with a toroidal current transformer around both conductors) and, if the imbalance exceeds a small safety threshold (typical domestic devices around 30 mA), it activates a trip mechanism to disconnect the circuit almost instantly. This immediate disconnection greatly reduces the risk of electric shock. It’s not about overcurrent or voltage level or temperature—it's specifically about detecting leakage by equal currents no longer summing to zero.