Re: How does a residual current detector work?

An RCD uses a special current transformer, wound in such a way that when 
the current in the 2 conductors is equal, there is no net magnetic field 
as the fields from the 2 conductors cancel each other out. 
When there is an imbalance due to earth leakage, this produces an output 
signal (either electrical, to operate a relay, or magnetic, to directly 
operate a mechanical trip mechanism).