The Siemens 5SY4525-8CV is a single-pole-plus-neutral miniature circuit breaker (MCB) with a D25 trip characteristic, rated for a short-circuit breaking capacity (Icu) of 15 kA at both 230 V and 400 V. In a 1P+N configuration, the neutral pole switches but does not provide overcurrent protection — only the live pole does. This is the standard arrangement for single-phase final subcircuits in TN and TT systems where you need to isolate the neutral on fault, common across European distribution boards. The D25 curve means the magnetic trip threshold sits at 10 to 20 times the rated current (250–500 A instantaneous trip), designed for loads with high inrush — motor starters, small transformers, discharge lighting, or capacitor banks. A C-curve breaker would nuisance-trip on those inrush events; the D-curve holds through them.
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What the 15 kA Icu rating means for panel integration
The 1P+N form factor occupies the same DIN-rail width as a single-pole breaker (one module, 18 mm). That keeps the panel fill factor tight — you get neutral switching without sacrificing a slot. The D25 rating at 25 A continuous handles typical single-phase loads up to 5.75 kW at 230 V, so it fits lighting subcircuits, small pump feeds, or control transformer primaries in the same enclosure.