RCD (Residual Current Device) is a life-saving device, designed to prevent you from getting a fatal electric shock if you touch something live, such as a bare wire.
In a Passive or Latching unit the RCD will remain closed, once set, and will only trip when a hazardous earth fault occurs. If the unit loses supply – perhaps in a power cut – it will stay latched, making it particularly useful in commercial applications, such as controlling refrigeration units & freezers