What an RCCB does

It compares the current leaving on the live conductor with the current returning on the neutral. Any difference is leakage to earth, and the device trips when that leakage exceeds its sensitivity. It protects people and property from earth fault current — it does not protect the cable from overload.

RCCB or RCBO

An RCCB provides earth leakage protection only, so it must be backed by an overcurrent device on the same circuit. An RCBO combines both in one device width. Where space in the board is tight, or where each circuit needs its own earth leakage protection, the RCBO is usually the better answer.

Sensitivity and type

30 mA is the usual sensitivity for personnel protection on final circuits.

100 mA and 300 mA are used for fire protection and for larger installations, where the objective is to limit leakage current rather than to protect a person directly.

Type AC, A, F and B describe which waveform of residual current the device responds to. Electronic loads and drives produce residual currents that a Type AC device may not detect properly, so the type has to match the load.

Where selection goes wrong

Using a Type AC device on a circuit with a variable-speed drive or a single-phase inverter.

Putting the whole installation behind one 30 mA device, so a single fault trips everything.

Forgetting that leakage currents add up: many electronic loads on one protected circuit can collectively exceed the trip threshold and cause nuisance tripping.