When does an RCD operate?

An RCD stands - Resdual Current Device. It is nothing but RCCB (Residual Current Circuit Breaker) works by constantly monitoring the current flow between the live and neutral wire.

In normal operation with the two lines in balance the circuit works. If for any reason, even a small amount of imbalance in detected (as small as 30 milliamps) the breaker will trip in no more than 200 milliseconds. This is well within the limits of preventing both fire and bodily harm. Regards - Ramadass