The Siemens 5SY3240-6 is a 2-pole miniature circuit breaker from the SENTRON 5SY3 series, rated 40 A at 400 V AC with a B tripping characteristic. Breaking capacity is 4.5 kA per EN 60898 for residential/commercial panels, and 6 kA per IEC 60947-2 for industrial distribution — the higher figure is the one to use when coordinating with upstream switchgear in a plant environment. It also carries a 5 kA rating under UL 1077 and CSA C22.2 No.235, so it's accepted in North American supplementary protection applications.
At 36 mm wide (2 width units) and 76 mm deep, it leaves room for wiring in a crowded enclosure. Installation depth is 70 mm, meaning the breaker body sits flush with the rail; the extra 6 mm of overall depth covers the terminal screws and arc chamber.
Halogen-free and silicon-free construction — important for panels in clean rooms, semiconductor fabs, or areas where outgassing could contaminate optics or contacts. Humidity tolerance is specified up to 95% at 55 °C, derating to 35% at 75 °C. Energy limiting class 3 means it restricts let-through energy well — good for protecting downstream equipment during a fault.