What this RCBO is and what the ratings mean for fit
The Siemens 5SU1653-6KV20 is a combined residual-current circuit breaker with integral overcurrent protection (RCBO) — a single module that replaces a separate RCD and MCB in a DIN-rail distribution board. The 20 A B-curve thermal-magnetic trip protects against overload and short-circuit on the line conductor, while the 300 mA Type A residual-current element detects sinusoidal AC and pulsating DC fault currents to ground. The 4.5 kA breaking capacity (Icn) at 230/400 V is sized for domestic and light commercial sub-distribution boards where the prospective short-circuit current stays below that threshold; above that, upstream back-up protection is needed. The 1+N-pole construction switches and protects the line conductor only; the neutral is switched but not monitored for overcurrent. The 70 mm width (4 module spaces) is standard for a 1+N RCBO in the Siemens 5SU series, so it occupies four 17.5 mm DIN-rail positions in the enclosure.