RCBO for mixed-load final circuits
The Siemens 5SU1353-7KL20 is a 2-pole RCBO (residual-current circuit breaker with overcurrent protection) from the SENTRON family, rated 20 A with a C tripping characteristic and Type A residual-current sensitivity. The C curve (5–10× In magnetic trip) suits mixed loads with moderate inrush — lighting banks, small motorised valves, or socket circuits in commercial panels. Type A RCD detects pulsating DC fault currents, covering electronics-fed loads like switched-mode supplies and variable-speed drives, which a plain AC type would miss. Breaking capacity is 4.5 kA per EN 60898 and 5 kA per IEC 60947-2, sufficient for most final sub-distribution boards in residential or light commercial installations where the prospective fault current stays under those thresholds. Overvoltage category III and pollution degree 2 confirm it is rated for fixed-installation mains connection, not plug-in portable use.
Thermal derating — real-world current
The 20 A rating holds only at 30 °C ambient. At 40 °C it derates to 18.8 A; at 50 °C to 17.4 A; at 60 °C to 16.2 A; at 70 °C to 14.6 A. If the panel runs warm — typical inside a packed distribution board — size the upstream protective device and load accordingly. The full derating table runs from 30 °C through 70 °C in 5 °C steps.
Mounting and integration
Two modular-width units (36 mm) snap onto standard DIN rail. Depth is 77 mm, installation depth 70 mm — it clears most 80 mm-deep enclosures. Supply can enter from top or bottom, which simplifies busbar routing in retrofit panels. Touch protection is built in, IP20 when the distribution board is closed. Halogen-free and silicon-free construction matters in environments sensitive to outgassing — clean rooms, medical facilities, or areas with silver contacts that corrode under silicone vapour.