What it is and what it does
The Siemens 5SU9354-8KK20 is a 2-pole RCD-operated circuit breaker (RCBO) from the SENTRON family — a combined overcurrent and residual-current device that fits a single 36 mm wide (2 MW) DIN-rail module. It carries a 20 A rated current with a D tripping characteristic (high inrush tolerance, typical for motor or transformer circuits) and a 30 mA Type A residual-current trip for sinusoidal AC and pulsating DC fault currents. Breaking capacity is 10 kA per EN 60898 and IEC 60947-2, which covers most residential and light commercial panel fault levels.
What the ratings mean for fit
The 20 A rating is at 30 °C; at 40 °C it derates to 18.8 A, at 50 °C to 17.4 A — so if the panel ambient runs warm, size the branch circuit load accordingly. The D curve means magnetic trip occurs at 10–20× In (200–400 A), so it won't nuisance-trip on motor inrush or transformer magnetizing current. The 30 mA residual-current threshold meets standard personnel protection requirements (IEC 60364). Supply can enter from top or bottom, and mounting position is unrestricted — no orientation constraints in the enclosure. Touch protection is built in. The 77 mm depth plus 70 mm installation depth means it sits flush in a standard 80 mm deep distribution board.