What it is and what it does
The Siemens SENTRON 5SU1324-7KX40 is a 2-pole (2P) RCBO — a combined residual-current circuit breaker with overcurrent protection — rated 40 A at 30 °C, with a Type A residual-current characteristic and a C tripping curve. It breaks fault currents up to 10 kA per EN 60898 and 15 kA per IEC 60947-2, so it covers both domestic-distribution board and light industrial panel applications where the prospective short-circuit current stays under those limits.
Derating and thermal limits
The 40 A rating holds at 30 °C ambient. At 40 °C it derates to 37.6 A, at 50 °C to 34.8 A, and at 70 °C to 29.2 A. If the panel runs hot — say next to a drive or transformer — size the upstream protection for the actual temperature, not the nameplate 40 A.
Mounting and footprint
Occupies 3 modular width units (54 mm wide) on a DIN rail, with a depth of 77 mm and an installation depth of 70 mm. Mounting position is any, so it fits vertical or horizontal busbars. The IP20 rating applies when the distribution board is installed with conductors connected — standard for enclosed panels.