Siemens 5SJ4303-7HG42 — 3 A, 3-Pole MCB for DIN-Rail Panels
The Siemens 5SJ4303-7HG42 is a 3-pole miniature circuit breaker from the SENTRON 5SJ4 series, rated at 3 A at both 30 °C and 40 °C ambient. The 3 A rating holds steady up to 40 °C, then derates linearly to 2.6 A at 60 °C — a curve worth checking if the breaker sits near transformers or other heat sources inside a sealed enclosure. The 70 mm depth (installation depth also 70 mm) keeps it flush with standard busbar systems and terminal blocks on the same rail.
Thermal Performance and Derating
Rated current is 3 A at 30 °C and 40 °C, dropping to 2.9 A at 45 °C, 2.8 A at 50 °C, 2.7 A at 55 °C, and 2.6 A at 60 °C. This derating profile means the breaker's real-world capacity in a warm panel is lower than the nameplate 3 A — factor the ambient temperature into your load calculation. The 75 °C maximum covers internal heating from the load plus ambient; the -40 °C floor suits unheated warehouses or outdoor enclosures in cold climates.
Protection class is IP20 with conductors connected, IP40 in the handle range — suitable for enclosed panels where accidental contact is prevented. The breaker is sealable, allowing lockout/tagout provisions on the handle. Vibration resistance is 50 m/s² at 25–150 Hz and 60 m/s² at 35 Hz (4 s), adequate for machine-mounted subpanels.
DC Rating and Application Notes
Maximum DC rated voltage is 60 V. This is a thermal-magnetic MCB designed primarily for AC circuits (per EN 60898 / IEC 60947-2), but the DC rating allows use in 24 V or 48 V control circuits — though verify the DC breaking capacity against your fault current. The 3-pole design (3P) does not switch the neutral; neutral conductor switching is not provided.