What it is and what it does
The Siemens 5SU9304-7KK25 is a 1P+N RCD-operated circuit breaker (RCBO) from the SENTRON 5SU9 series, combining overcurrent protection and residual current detection in a single 18 mm wide module. It's rated 25 A at AC with a C tripping characteristic, meaning it handles moderate inrush from motor or lighting loads without nuisance trips. The Type A fault current detection covers sinusoidal AC and pulsating DC residual currents — the standard for most commercial and light industrial circuits. Breaking capacity is 10 kA per both EN 60898 and IEC 60947-2, so it safely clears faults up to that level without cascading upstream.
Thermal derating and conductor sizing
The 25 A rating holds at 30 °C ambient; at 40 °C it derates to 23.5 A, at 45 °C to 22.75 A, at 50 °C to 21.75 A, and at 55 °C to 20.25 A. If the panel runs warm — say near a transformer or drive — size the load accordingly. Terminals accept solid or stranded conductors from 0.75 mm² up to 35 mm², covering most branch-circuit wiring. Power dissipation is a modest 0.75 W at rated load, so heat buildup in a crowded enclosure is minimal.
Mounting and integration
Snaps onto standard DIN rail, occupies one width unit (18 mm), and the installation depth behind the panel is 70 mm (total depth 77 mm). Mounting position is unrestricted — horizontal, vertical, upside-down, it doesn't care. Supply can feed from top or bottom. IP20 with the distribution board installed and conductors connected; touch protection is built in. Overvoltage category III means it's rated for fixed-installation use downstream of the main distribution board.