rcd detects current flowing to earth (most actualy detect the imbalance between N & L)
short cct.
put it this way.
(for the purposes of this explanation the wires are bare)
imagine a lamp at the end of a wire
the wire is 30 feet long
a switch is at 29 feet
the supply is at 0 feet (the other end)
You drop a nail across at 1 foot
the electricty now goes back through the nail, instead of through the lamp
that is a short cct (we wont mention the bang)
now let us go into more precise detail
the lamp was drawing say 1 amp
now with the nail across the wires, the nail is bigger than the diameter of the wires, so for the sake of argument the nail can draw 1000 Amps
the cct is protected by a 6 amp mcb, as 1000 amps is now flowing, the mcb being only rated at 6 amps instantly switches off
if this was protected just my an rcd the rcd would have fried, becuse it only detects current to earth, but the mcb is for overload / short cct protection so it trips
same set up, but nail touches only the live wire and the other end is on the ground, this time the rcd will trip as there is more current flowing down the live than is coming back up the neutral (imbalance)
this is just a very basic explanation.
other things should be cnsidered
(like my speeling)
