What it is and what it does
The Siemens 5SU1356-1KK40 is a SENTRON RCBO — a combined residual-current device and miniature circuit breaker in one 2-module-wide (36 mm) package. It protects a single-phase final subcircuit against overload, short-circuit, and earth leakage in one unit, saving a DIN-rail position over separate RCD and MCB. Rated 40 A with a C tripping curve, it handles motor-starting inrush and moderate transformer loads without nuisance trips. The residual element is Type AC — sensitive to sinusoidal AC fault currents only, not pulsed DC or smooth DC — so it's suited for general-purpose socket outlets and lighting circuits where no electronic loads are downstream. Breaking capacity is 6 kA per EN 60898 (the household standard) and 10 kA per IEC 60947-2 (the industrial standard), giving it headroom for most commercial and light-industrial distribution boards with a prospective fault current under 10 kA.
Mounting and environment
Snaps onto a 35 mm DIN rail in any orientation — top or bottom feed accepted. The 77 mm depth fits standard distribution boards; the 70 mm installation depth leaves room for wiring behind the busbar. IP20 with the distribution board installed and conductors connected — not for wet or dusty environments. Rated for pollution degree 2 and overvoltage category III, which is the normal condition inside a dry indoor panel. Ambient temperature range from -40 °C to +75 °C covers cold storage and hot machine cabinets. Derate the 40 A rating above 30 °C: at 40 °C it's 37.6 A, at 50 °C it's 34.8 A, at 70 °C it's 29.2 A. That derating matters if the RCBO sits next to heat-generating contactors or drives.