The Siemens SENTRON 5SV3344-6KK03 is a 4-pole, Type A residual current circuit breaker (RCCB) rated 40 A at 400 V AC, with a 30 mA trip threshold and instantaneous response. It's built to protect against earth faults in circuits where non-sinusoidal AC and pulsating DC residual currents can occur — think variable-speed drives, washing machines, or any gear with rectifiers that spit out chopped waveforms.
Snaps onto DIN rail (REG) in any orientation, so you're not fighting gravity in a tight enclosure. It's 4 modular width units wide — 72 mm — and 70 mm deep, which is standard for this class. The depth is the same as its installation depth, meaning nothing sticks out the back to foul the gland plate or busbars. Silicon-free construction, which matters if you're potting or conformal-coating nearby boards.
Thermal derating — the real-world rating
Push it to 45 °C and it's good for 38.1 A; at 50 °C it's 35.59 A; at 55 °C it's 32.29 A; at 60 °C it's 28.19 A; at 65 °C it's 23.3 A; at 70 °C it's 17.6 A. That derating curve is the one that matters when you're stuffing four of these side-by-side in a non-ventilated enclosure.