What earth leakage protection actually detects
Earth leakage protection — implemented as an RCD (Residual Current Device), RCCB, or an earth-leakage trip unit built into a larger breaker — monitors the current flowing out through the live conductor(s) against the current returning through neutral. Under normal operation those two figures match; if current is leaking to earth through a fault (damaged insulation, a person contacting a live part), the imbalance trips the device, disconnecting the circuit. This is a fundamentally different protection function from an overcurrent breaker, which responds to too much current flowing, not current flowing somewhere it shouldn't.
Rated residual current: personnel protection vs fire protection
The rated residual current (commonly written as IΔn) sets the trip sensitivity. Devices rated around 30mA are the standard for protecting people against electric shock, since that level is broadly recognised as the threshold below which a shock is unlikely to be lethal for a healthy adult. Higher ratings (100mA, 300mA and above) are used for fire protection or selective/discrimination purposes on larger distribution circuits, where the goal is preventing an earth fault from starting a fire rather than protecting a person in direct contact.
Type (AC, A, B) and pole count
RCD "type" describes what shape of fault current the device can actually detect: Type AC detects standard sinusoidal AC residual current only; Type A additionally detects pulsating DC residual currents, which is increasingly required wherever variable-speed drives, EV chargers or similar electronics are on the circuit; Type B extends detection to smooth DC residual currents and is specified for demanding applications such as larger three-phase drives and EV charging infrastructure. Pole count (commonly 2-pole for single-phase, 4-pole for three-phase circuits — a 4-pole, 160A unit is a typical three-phase distribution-board specification) has to match the circuit it protects.