What it is and where it fits
The Siemens SENTRON 5SY8240-7 is a 2-pole miniature circuit breaker (MCB) with a C tripping characteristic, designed for branch-circuit protection in industrial control panels. It snaps onto a DIN rail via the quick assembly system and occupies 2 modular width units (36 mm). The IP20 rating (with connected conductors) means it's intended for enclosed panel mounting — not for wet or outdoor locations. Rated for 400 V AC (60 V DC per pole), it delivers a 25 kA breaking capacity per IEC 60947-2 at 400 V AC — enough to handle high-fault-current industrial feeds. Under UL 1077 it's rated 5 kA, so it qualifies as a supplementary protector for North American panels. The DC rating drops to 15 kA at 60 V, which covers most control-circuit DC buses.
Key ratings and what they mean for fit
The C-curve trips between 5 and 10 times rated current — standard for motor-starting and transformer inrush where you need to avoid nuisance trips on short-duration surges. The 25 kA IEC breaking capacity means it can safely interrupt a fault of that magnitude without welding contacts or cascading failure upstream. The 15 kA DC rating at 60 V covers most 24 V and 48 V DC control circuits, but note the 72 V DC maximum operating voltage — don't exceed that on a DC bus. The 76 mm depth and 70 mm installation depth mean it fits standard 80 mm deep enclosures with room for wiring. The combined top and bottom terminals accept both busbar and cable — no need for separate feed-through lugs. Vibration resistance per IEC 60068-2-6 covers ±1 mm at 5–25 Hz and 50 m/s² at 25–150 Hz, so it's fine on machine-mounted panels near conveyors or compressors. Temperature range spans -40 °C to 75 °C, with humidity derating: 95% RH up to 55 °C, 55% RH at 70 °C, 35% RH at 75 °C. That's broad enough for unheated outdoor cabinets in cold climates or near hot machinery. The sealable option allows locking the handle in the OFF position for lockout/tagout compliance.
Standards and compliance
Designed to IEC / EN 60947-2 and UL 1077, plus CSA C22.2 No. 235. Overvoltage category III and pollution degree 3 suit it for fixed industrial installations where transients and conductive dust are present. Halogen-free and silicon-free construction matters for clean-room or high-reliability environments where outgassing can't be tolerated.