How does a differential current monitor detect earth leakage in an electronic device?

• A. By measuring the leakage current on the neutral conductor only
• B. By comparing current in the live and neutral conductors; any imbalance indicates leakage and triggers protection
• C. By detecting voltage fluctuations on the supply line
• D. By measuring the total current of the circuit

Answer: (B)

Earth-leakage protection works by watching the current that leaves on the live conductor and the current that returns on the neutral conductor. When everything is normal, these two currents are the same, so the total current shown by the sensing element is zero.

The monitor uses a current transformer around both conductors so their magnetic fields cancel out. If some current leaks to earth, not all of the live current returns on the neutral, so a small residual current flows through the sensing circuit. That imbalance is detected, and the protection device trips to disconnect power.

This approach is more reliable for detecting earth leakage than trying to sense leakage on the neutral alone, or watching voltage fluctuations, or measuring the circuit’s total current, because those other methods wouldn’t reliably indicate a leakage path to earth.