The Siemens 5SU9156-7KK20 is a SENTRON RCD-operated circuit breaker (RCBO) — a combined overcurrent and residual-current device in a single 2-module-wide unit (36 mm). It protects a 1P+N circuit at 20 A rated current with a 10 mA residual-current trip threshold, Type A (pulsating DC and AC fault currents).
The 20 A rating at 30 °C derates to 18.8 A at 40 °C and 16.2 A at 55 °C — factor the ambient temperature inside the enclosure when sizing for continuous load. The 10 mA residual-current sensitivity is lower than the standard 30 mA, so it catches smaller leakage currents; that makes it suitable for circuits where a higher threshold could mask a developing fault, but it also means the device may trip on normal capacitive leakage from long cable runs or surge suppressors. Rated insulation voltage is 264 V, overvoltage category III, pollution degree 2 — standard for fixed-installation distribution boards. IP20 with connected conductors, so it lives inside a closed panel.