What it is and what it does
The Siemens 5SU9306-0KK20 is a 2-pole RCD-operated circuit breaker (RCBO) from the SENTRON family — a single module that combines overcurrent protection and residual current detection in one 18 mm-wide DIN-rail package. It's rated 20 A at 30 °C with a Type B tripping characteristic (instantaneous magnetic release for short circuits) and a 30 mA residual-current sensitivity for earth-fault protection. The breaking capacity is 6 kA per EN 60898 and IEC 60947-2, which covers most residential and light commercial distribution boards.
DIN-rail fit and panel integration
This RCBO occupies one modular width (18 mm) on a standard DIN rail, with a depth of 77 mm and height of 125 mm. The installation depth is 70 mm, so it clears a standard 80 mm-deep enclosure back wall. Mounting position is any orientation, and the supply can enter from either top or bottom — no forced busbar orientation. Touch protection is built into the design, and the IP20 rating applies once installed with conductors connected inside a distribution board.
Temperature derating — what the panel builder needs to know
The 20 A rating holds at 30 °C ambient. At 40 °C it derates to 18.8 A, at 45 °C to 18.2 A, at 50 °C to 17.4 A, and at 55 °C to 16.2 A. If you're packing this into a warm enclosure alongside other heat sources, use the 40 °C or 50 °C column for your load calculation — don't just read the 30 °C number off the front. The unit is rated for ambient down to -40 °C and up to 75 °C storage, so cold-storage or outdoor panels are fine.