RCD (Residuall Current Device)
The RCD is a 'trip' device that has been designed to disconnect the electricity supply before a fatal electric shock is sustained.
When installing new circuits or replacing old fuse boards, the installation of these devices is now obligatory.
However, as these devices monitor different perameters than those of fuses, when installed into an existing installation they can often highlight latent faults within that system, and start tripping when before there seemed to be no issues at all.
Therefore, before using these in an existing electrical system, it's a very good idea to have the installation checked to make sure everything is ok.