What this RCBO is and what it does
The Siemens 5SU1354-7VK40 is a SENTRON 5SU1 RCBO — a combined residual-current circuit breaker with overcurrent protection in a single 2-module-wide (36 mm) DIN-rail package. It protects a 1P+N circuit at 40 A rated current with a C tripping characteristic, meaning it handles moderate inrush from motor-start or capacitive loads without nuisance trips. The Type A fault-current detection catches pulsating DC residual currents from rectifiers or VFDs, which a plain Type AC unit would miss. Breaking capacity is 10 kA per EN 60898 — enough for most commercial and light-industrial distribution boards where the prospective short-circuit current stays under that threshold. Overvoltage category III means it's rated for fixed-installation downstream of the main panel.
Thermal derating — the number that actually decides your load
The 40 A rating is only valid at 30 °C ambient. At 40 °C it's 37.6 A; at 50 °C it drops to 34.8 A; at 60 °C it's 32.4 A; and at 70 °C it's 29.2 A. If your panel runs hot — say 50 °C inside the enclosure — you need to treat this as a 35 A device, not 40 A. The mounting position is unrestricted, so you can stack these vertically in a crowded rail without derating further for orientation.
Terminal capacity and wiring
Solid and stranded conductors from 0.75 mm² up to 35 mm² fit the terminals. The 77 mm depth plus 70 mm installation depth.