What it is and what it does
The Siemens 5SU1674-7CK81 is a 4-pole RCBO (residual current operated circuit breaker with integral overcurrent protection) from the SENTRON 5SU1 series, Super Resistant K design. It combines overcurrent protection (C characteristic) with Type B residual current detection in a single 11-module-wide unit for 277/480 V AC networks at 50/60 Hz.
Thermal derating — the real current rating
The headline 100 A rating holds only at 30 °C ambient. At 40 °C it derates to 92.5 A, at 45 °C to 89 A, at 50 °C to 85 A, at 55 °C to 81 A, and at 60 °C to 77 A. For a panel that runs warm — typical in a crowded enclosure — the 40 °C or 50 °C figure is the one that governs the load you can actually protect.
Breaking capacity and standards
Rated short-circuit breaking capacity is 10 kA per both EN 60898 and IEC 60947-2. That means it can safely interrupt a fault current up to 10 kA at the rated voltage — sufficient for most commercial and light industrial distribution boards where the prospective fault current stays under that threshold. Overvoltage category III and pollution degree 2 align with fixed-installation downstream of the main distribution panel.
Physical fit and mounting
Width is 198 mm (11 width units at 18 mm per unit), depth 77 mm, installation depth 70 mm, height 90 mm. Mounts on DIN rail in any position. Supply can enter from top or bottom. IP20 rating applies once the distribution board is installed with conductors connected — standard for enclosed panel mounting.