RCD
A residual-current device (RCCB, IEC 61008) with no overcurrent function, protecting the circuits fed from its bus. The classic split-load board has two RCDs, each covering half the circuits, arranged so lighting upstairs and sockets downstairs are not on the same one — a trip leaves you a lit route to the panel.
RCBO
A residual-current breaker with overcurrent protection (IEC 61009): RCD and MCB in one module, one per circuit. A fault trips only its own circuit. Dearer per way, and the arrangement the UK's 18th Edition (BS 7671:2018) pushed installers toward, since it requires 30 mA additional protection for socket outlets up to 32 A and for cables buried in walls, and discourages a single RCD covering everything.
Sensitivity and type
30 mA Type A is the default for final circuits; 100 or 300 mA time-delayed (S-type) upstream for fire protection and selectivity; Type B on EV-charger and drive circuits (the types).
Portable RCDs
Where the installation cannot be counted on — an older building, a rented workshop, a site abroad — a portable RCD (PRCD, IEC 61540) in the plug or the cord gives the user 30 mA protection regardless. That is the device in our export RCD cords (RCD cords).
The North American reader
The same choice exists at home: a GFCI breaker protects a circuit from the panel, a GFCI receptacle protects a room from the wall (breakers vs receptacles). What differs is the 30 mA threshold and the panel-first habit.