RCDs provide protection against leakage currents by quickly switching off the electrical circuit. They act like an electrical security guard, constantly monitoring the current flowing through the circuits they protect. 

In a typical circuit, electricity flows “in” via the live supply line and flows “out” through the neutral line, returning to the source. If the circuit lacks a return path, it is incomplete (open), and current won’t flow.  

For the circuit loop to maintain safe operation, the same amount of current leaving the source must return. 

Therefore, when your shore power lead is plugged into an outlet, the 15 amps travelling through the hot wire must equal the amount returning via the neutral line. When the amount of current returning through the neutral wire is less than 15 amps, there is a current leakage (residual current). 

If your electrical wiring is done correctly, the leaking current flows through the dedicated earth conductor back to the source. In other cases, it may escape through other paths, such as suppressor capacitors in your devices or you (or any other person). 

When you have an RCD along your circuit, sufficient residual current will trigger the built-in safety switch and cut off the power supply. We’ve said “sufficient” here because residual currents below the RCD’s safety threshold will not activate the tripping mechanism.