When did you last press the 'TEST' button on your RCD, or Residual Current Device? Not sure, well RCD's should be tested quarterly. Next you will be telling me you haven’t even got one, wait, you haven't? Well in that case, you need to know just how vital they are to the safety of an electrical system, especially the one serving you and your family.
Far from ethical or legal experiments by the Nazis during WW2 has given us important data on how human beings react to electricity passing through their bodies. It was discovered that up to 30 mA, or 0.003 of an amp is safe to humans, beyond that level is potentially hazardous to one’s health.
So guess what rating a domestic RCD has, yes it's 30mA. So the RCD monitors the outgoing current in a circuit, and compares it to the amount of current returning. The RCD allows up to 30mA to leave the circuit, i.e. to earth, but trips when a greater amount of current goes missing – i.e. to a person.