Mark Atherton wrote:

I understand the principle operation of an rcd, phase and neutral both
connected through the rcd and if one becomes unbalanced by a fault to earth
the rcd trips BUT what about the rcd's I got from Screwfix.They are plug in
types to replace re-wirable fuses on old ccu, only plug into phase conductor
so how are they balanced with the neutral?I am studying my 236 part II at
present so be as technical as you want.

Are you sure you have an RCD? The mini-trips that Screwfix sell are just
MCBs.

How do MCBs work? I can't find it on google.

Ben.