What the D-curve and breaking capacities mean for your panel
The Siemens 5SY8440-8 is a 4-pole miniature circuit breaker from the SENTRON family, rated with a D tripping characteristic — meaning it tolerates inrush currents 10 to 20 times the rated current before tripping magnetically, which makes it the right choice for motor circuits, transformers, and other inductive loads that draw a heavy startup surge. Its breaking capacity is 25 kA at 400 V AC per IEC 60947-2, so it can safely interrupt fault currents up to that level without upstream fuses needing to clear first — that's the figure that governs selectivity coordination in a distribution board. Under UL 1077 the same part carries a 5 kA interrupting rating, which is the number a panel builder in a North American spec needs for the SCCR label. For DC circuits it's rated 15 kA at 60 V, covering battery-backed control buses and solar combiner boxes.
DIN-rail footprint and panel integration
The 5SY8440-8 occupies 4 modular width units (72 mm) on a standard DIN rail, with a depth of 76 mm and height of 90 mm — the 70 mm installation depth leaves clearance behind a standard 120 mm deep enclosure gland plate. Mounting position is unrestricted, and the quick-assembly fastening system lets you snap it onto the rail without tools. Combined terminals top and bottom accept both busbar and conductor termination, and the sealable design allows the panel builder to lock the cover after commissioning.
Environmental limits and derating
Ambient temperature range spans -40 °C to 75 °C, but the breaker's thermal-magnetic trip curve is influenced by surrounding temperature: relative humidity must be derated above 55 °C — max 95% RH up to 55 °C, then 55% RH up to 70 °C, and 35% RH up to the 75 °C ceiling. Vibration resistance per IEC 60068-2-6 is ±1 mm at 5 to 25 Hz and 50 m/s² at 25 to 150 Hz, which covers typical industrial panel vibration from nearby motors and conveyors. Mechanical service life is rated at 10,000 operating cycles.
Compliance and documentation
The 5SY8440-8 is certified to IEC / EN 60947-2 and UL 1077, with pollution degree 3 and overvoltage category III — the standard specification for fixed-installation industrial distribution panels. Construction is halogen-free and silicon-free, which matters for clean-room or semiconductor-adjacent installations where outgassing can contaminate optics or contacts. Protection class is IP20 with conductors connected — standard for a DIN-rail component inside a locked enclosure; no washdown rating.