note - on a split load unit, if ALL the load mcb are switched of on one side, that RCD won`t trip
As stated above, the RCD basically measures the current drawn through the phase, and ensures the neutral is carrying (within typ. 30mA) the same current.
Suppose we`ve a 4 cct board, split into a 2+2 configuration...ok?
Each pair of ccts is protected by a 30mA RCD.
To make things easier:
cct3... kitchen sockets.
You`re working on the cooker cct, so you isolate the MCB, and begin works. "your" RCD is also supplying the general sockets, which are still energised.
ANY item plugged into one of those sockets is "returning" the current down the neutral; which is electrically connected to the neutral of your cooker cct.
When you cut the cable, you short neutral to earth. Unless you`re on TT, the resistance path is (substantially) the same, therefore 50% of the returning current which is currently being drawn by all items plugged into the general sockets will flow via the earth path. If the total load was in excess of 60mA, more than 30mA will flow via earth.