The Siemens 5SU1353-1KK20 is a SENTRON RCBO — residual current breaker with integrated overcurrent protection — in a 1P+N configuration, meaning it switches and protects the line conductor only, with the neutral passing through but not switched. The breaking capacity is rated 4.5 kA per EN 60898 and 15 kA per IEC 60947-2, so it's sized for sub-distribution boards where the prospective fault current stays under 4.5 kA at the service entrance — the higher IEC rating gives headroom for industrial back-up protection coordination. Operates at 50 Hz only.
In a crowded enclosure, the effective rating drops: 18.8 A at 40 °C, 18.2 A at 45 °C, 17.4 A at 50 °C, 16.2 A at 55 °C and 60 °C, 15.4 A at 65 °C, and 14.6 A at 70 °C. For a panel running at 50 °C — common in a sealed cabinet near a machine — the breaker is good for 17.4 A continuous, not 20 A. Factor this into the load schedule.