What this 4-pole C-curve 40 A MCB does on the rail
The Siemens 5SY7440-7 is a 4-pole miniature circuit breaker from the SENTRON family, rated 40 A at AC with a C-curve trip characteristic — meaning it handles moderate inrush from motor starters or transformer loads without nuisance tripping, while still clearing short circuits fast. Breaking capacity is 15 kA per EN 60898 and 20 kA per IEC 60947-2, so it fits distribution boards where fault levels stay under those thresholds. At DC (IEC 60947-2) it's rated 15 kA. Four modular width units (72 mm wide) snap onto a DIN rail; mounting position is any, and the quick-assembly system means it clips in without tools. IP20 with conductors connected — standard for enclosed panel use.
Thermal derating and ambient limits
Rated current holds at 40 A up to 35 °C ambient. Above that it derates: 38.4 A at 40 °C, 37.6 A at 45 °C, 36.76 A at 50 °C, 35.92 A at 55 °C, and 35.04 A at 60 °C. The surrounding temperature influence curve caps at 95 % RH up to 55 °C, then drops to 55 % RH at 70 °C and 35 % RH at 75 °C. Operating temperature range is -40 to 75 °C, so it works in unheated enclosures or near ovens as long as the derating is applied.
Compliance and environmental specs
Halogen-free and silicon-free construction, sealable, with a degree of pollution 3 rating and overvoltage category III. Suitability listed for mechanical engineering and industry applications. Energy limitation class 3; neutral conductor is not switched. Combined terminal top and bottom are present.