A Residual Current Device is an extremely fast acting switch that cuts the power when it detects a difference in the power going into the circuit against the power coming back out. If a difference is detected then the RCD trips almost instantly as the power must have found a way to go somewhere else other then back to the supply. This path may be through a faulty appliance, faulty wiring, or at worst – through you! These devices have saved countless lives by cutting the electricity supply before it can cause severe damage. An RCD does not sense overload (i.e. total amount of power drawn) which is why Ampfibian caravan adaptors do both.
Please note that RCDs do not offer protection when used with isolated supplies like small generators or inverters.