What this RCD-operated circuit breaker does
The Siemens 5SU9354-8KK25 is a 2-pole (1P+N) RCD-operated circuit breaker from the SENTRON 5SU9 series, combining overcurrent protection and residual-current detection in a single 36 mm-wide module. It is rated 25 A at AC with a D tripping characteristic, meaning it tolerates the high inrush of motor or transformer loads without nuisance tripping while still clearing a fault. The Type A fault-current detection handles pulsating DC residual currents — common on single-phase rectified loads like VFDs and switched-mode supplies — so it's suited for mixed-load panels where standard AC-only RCDs would miss a fault. Breaking capacity is 10 kA per EN 60898 and IEC 60947-2, which covers the majority of domestic and light commercial service-entrance fault levels. The energy limiting class is 3, so it lets through less let-through energy than a Class 1 or 2 device — that matters when you want to protect downstream wiring and connected equipment during a short-circuit event.
Where it fits in the panel
Mounts on DIN rail in any position — no orientation restrictions. At 36 mm wide (2 width units) and 77 mm deep, it occupies the same footprint as a standard MCB/RCD combo, so it drops into existing SENTRON or third-party distribution boards without re-spacing. The 70 mm installation depth means it clears most enclosure back-panels even with conductors dressed behind it. Terminals accept solid or stranded conductors from 0.75 mm² up to 35 mm² — that covers everything from 1.5 mm² lighting circuits up to 10 mm² feeder tails. Supply can enter from top or bottom, which simplifies busbar routing in a crowded enclosure.
Thermal derating and ambient limits
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, and at 50 °C to 21.75 A. If the panel runs hot — say, a sealed outdoor enclosure in direct sun — factor the derating so the breaker doesn't trip on a warm day under a continuous load that's within the cable rating. Operating ambient range is -25 to +45 °C; storage range extends to -40 to +75 °C. The power dissipation at rated load is 1.5 W per pole — negligible for thermal budgeting in a multi-way board, but worth noting if you pack 24 of these into a small enclosure.