What it is and what it does
The Siemens 5SU1374-8AK81 is a 4-pole RCBO (residual current circuit breaker with overcurrent protection) from the SENTRON Super Resistant K series. It combines a 100 A MCB with D tripping characteristic and a 30 mA Type B residual current device in one 11 modular-width unit. Type B residual current detection means it catches smooth DC fault currents, AC pulsating currents, and high-frequency AC — the full spectrum you get from variable-speed drives, UPS systems, and PV inverters. That 30 mA trip threshold protects personnel against direct-contact faults. The D-curve (10-20x In) handles high inrush loads like motor starters, welding transformers, and large capacitor banks without nuisance tripping on startup. Rated breaking capacity is 10 kA per EN 60898, which covers most commercial and light industrial distribution boards.
Thermal derating — the real current rating
At 30 °C ambient the device carries its full 100 A. That drops to 92.5 A at 40 °C, 89 A at 45 °C, 85 A at 50 °C, 81 A at 55 °C, and 77 A at 60 °C. In a sealed distribution board running hot, the 40 °C or 50 °C figure is the one that governs your load — not the 30 °C headline. Mounting position is any, and supply can enter from top or bottom, which simplifies busbar layout in a crowded enclosure. The 77 mm depth plus 70 mm installation depth means it fits standard flush-mount consumer units and distribution boards without a deep can.
Panel fit and compliance
At 11 modular width units (198 mm) this is a wide device — plan for it in the DIN rail layout before you start wiring. The halogen-free and silicon-free construction matters for high-reliability installations (rail, marine, data centers) where outgassing in a fire damages downstream electronics. Rated insulation voltage is 440 V, overvoltage category III, pollution degree 2. IP20 when installed in a distribution board with conductors connected — standard for enclosed panel mounting, not for wet-area standalone use.