What the ratings mean for fit
The Siemens 5SL4240-6 is a 2-pole SENTRON miniature circuit breaker with a B tripping curve, rated 40 A at 400 V AC and a 10 kA breaking capacity per EN 60898. The B-curve trips between 3 and 5 times rated current — standard for residential and light commercial lighting circuits where inrush is low. The 10 kA SCCR at 400 V means it safely interrupts a fault up to that level without upstream damage, provided the prospective short-circuit current at the panel doesn't exceed that figure. It also carries a 10 kA rating per IEC 60947-2, covering industrial installations where the standard differs. The part is rated for 40 A at 30 °C ambient but derates to 37.89 A at 40 °C and 34.47 A at 55 °C — a factor to check if the breaker sits in a warm enclosure or near other heat sources. The 2-pole design (2 MW units wide, 36 mm) snaps onto DIN rail in any mounting position, with an installation depth of 70 mm plus the 76 mm overall depth. IP20 with connected conductors means it's protected against finger contact but not moisture — standard for enclosed panel use; no washdown rating here.
Where it's used
This MCB is designed for residential buildings and infrastructure — think distribution boards in apartments, offices, or light commercial spaces. The B-curve and 40 A rating suit general-purpose lighting and socket circuits where loads are resistive or slightly inductive. The 2-pole format handles single-phase 230 V or two-wire 400 V line-to-line circuits. The sealable design (tamper-proof cover option) is useful for utility-metered installations.