What it is and what the ratings mean
The Siemens 5SY5440-7 is a SENTRON 4-pole miniature circuit breaker rated 40 A at AC with a C tripping characteristic and a 10 kA breaking capacity per EN 60898. The C-curve means it trips between 5 and 10 times rated current — standard for motor-starting and moderate inrush loads like pumps and small compressors out here on the route. The 10 kA interrupting rating tells you it can safely clear a fault up to that level without welding its contacts shut or cascading upstream; that's the figure the panel designer needs for selectivity coordination. The 4-pole design (4P) switches all three phases plus the neutral, which is what you want for a three-phase motor feeder where you need to isolate the neutral for maintenance. The 4 MW width (72 mm) takes up four 18 mm modular slots on the DIN rail — standard for a 4-pole MCB in this class.
Temperature derating — the real-world current
The 40 A rating holds at 35 °C ambient. At 40 °C it derates to 38.4 A; at 50 °C it's 36.76 A; at 60 °C it's 35.04 A. If this breaker sits in a hot panel next to a drive or a transformer, use the 50 °C or 60 °C figure for your load calculation — not the 40 A sticker. The humidity limits are 95% RH up to 55 °C, then dropping to 35% RH at 75 °C, so a sealed outdoor enclosure that traps moisture could be a problem.
Mounting and integration
Snaps onto a standard DIN rail in any mounting position. The quick-assembly system means you can gang multiple breakers on a busbar without extra hardware. Combined terminals top and bottom accept copper conductors; the touch-protection feature meets IP20 with conductors connected — safe for an open panel during commissioning. Depth is 76 mm, which fits standard 80 mm deep enclosures with room for wiring.