A leakage protection plug is also called a PRCD (portable residual current device). In normal operation the current flowing out on the live conductor equals the current returning on the neutral, so the magnetic fluxes cancel inside the zero-sequence current transformer and there is no output on its secondary winding.
If an appliance leaks, part of the current returns through a person or through earth. Live and neutral current no longer match; the transformer detects the difference and, once it exceeds the rated residual operating current, the mechanism trips and opens the circuit within 0.1s.
The ZJLB series is rated below 10mA residual operating current and below 0.1s trip time. Test and reset buttons are provided; pressing test once a month confirms the trip still works.
Note that a PRCD protects people and equipment — it does not replace a proper earth connection at the socket.