RCD

Short for Residual Current Device. Trips when line current and neutral current are different. Protects against earth faults, leakage currents, and electric shocks.

Residual current

The difference between live and neutral currents.

Mechanism

Consists of a solonoid coil. There are 2 identical windings on the coil.

Under normal operation, the residual current is . Generated magnetic fields cancel each other out. There will not be any induced emf.

When some current leaks to earth (like when someone touch the live wire), there will be a residual current. Which causes an unbalanced flux. Trips when the residual current exceeds the rated sensitivity (usually ).