An RCD is an acronym for Residual Current Device.
This device works in a simple principle that whatever amount of current entered must be equal to the amount of current that will return.
If not, the unbalance current develops changing magnetic that enable the electromagnet to activate. The electromagnet pulls the switch open and the circuit is switched off.
Having leakage current, this device will interpret as if there is a current flowing to another path such as a body of an electrocuted person in the load end. In this way, an electrical shock can be avoided.
Sensitivity ratings of this device range from 30mA up to 300 mA used depending on purpose or code requirements.