How does the RCD work?

RCDs use the principles of Kirchhoff’s first law:

That is, the algebraic sum of the currents flowing to and from a node in a circuit should be equal to zero. In the event of an insulation fault to earth, a fault current (residual current) flows back to the source of the current via the earth and not via the current carrying conductors. This induce a current flow in the RCD trip circuit, i.e. the sum of the currents flowing through the RCD no longer equal zero.

If the effective value of the residual current exceed 50% of the designed operating level of the RCD (Rated residual current ), the RCD will trip and disconnect the load from supply.