An old sparkie....just an ember now! Apologies to the others with newer knowledge for the simplified explanation.
Normal earthing relies on a low resistance to draw a high current and blow the fuse.
ELCB - Earth leakage cicuit breaker measures the current going to earth. If it rises above a certain level (30mA?? grey matter failing) it trips out.
RCD -Residual current device. Compares the current going in and out (active and neutral) and earth. If more current is going in than going out it must be going somewhere and again trips out.
This is why some resistive or capacitive loads like motors or fridge compressors or large banks of fluro lights would trip these devices out.
EDIT: ELCB and RCD trip out very fast typically 10mS, a full alternating current cycle (sine wave) takes ~16mS. So the electricity is disconnected much faster than a heartbeat so you don't go into defib.




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