What it is and what it does
The Siemens 5SU9304-7KX20 is a SENTRON RCBO — an RCD-operated circuit breaker combining overcurrent protection and residual current detection in a single 18 mm wide module. It's rated 20 A at 30 °C with a C tripping characteristic, Type A fault current sensitivity, and a 10 kA breaking capacity certified to both EN 60898 and IEC 60947-2. That dual certification means it's accepted across residential, commercial, and light industrial panel builds where the local standard may reference either norm. Type A detection handles pulsating DC residual currents up to 6 mA superimposed on AC — common from rectified loads like LED drivers, switched-mode power supplies, and variable-speed drives. If your panel feeds any electronics, Type A is the correct choice over Type AC.
Thermal derating and real-world current
The 20 A rating holds at 30 °C ambient. Above that, the continuous current must be derated: 19 A at 40 °C, 18.2 A at 45 °C, 17.6 A at 50 °C, and 17.2 A at 55 °C. If the panel interior runs hot — and most enclosed panels do — size the load circuit for the derated value at your expected ambient, not the headline 20 A.
Mounting and wiring constraints
The 18 mm width (1 width unit) fits standard DIN-rail enclosures. Installation depth is 70 mm, overall depth 77 mm — check the enclosure depth before committing; a shallow 80 mm box leaves only 3 mm clearance for wiring. Accepts solid or stranded conductors from 0.75 to 35 mm², which covers everything from lighting circuits to sub-main feeders. Mounting position is any orientation, so it works in vertical or horizontal busbar arrangements without derating.
Compliance and approvals
Rated for overvoltage category III, protection class IP20 (IP40 with connected conductors in a distribution board), halogen-free and silicon-free construction. The 10 kA breaking capacity is listed under both EN 60898 and IEC 60947-2, covering the two main MCB/RCBO standards globally. Touch protection is included on the terminals.