What it is and what it does
The Siemens 5SJ4311-7HG42 is a SENTRON miniature circuit breaker (MCB) from the 5SJ4 series, designed for branch-circuit overcurrent protection in industrial control panels and distribution boards. Its 3-pole construction and 5 A rated current at 30 °C and 40 °C make it a fit for lighting, small motor, or control-transformer circuits where the load current sits at or below 5 A per phase. The 3-modular-width footprint — 54 mm wide — snaps onto standard DIN rail in any mounting position, so it integrates into existing panel layouts without re-drilling or adapter plates.
Ratings that matter for fit
The 5 A rating holds at 30 °C and 40 °C ambient, then derates to 4.9 A at 45 °C, 4.8 A at 50 °C, 4.6 A at 55 °C, and 4.5 A at 60 °C (–). If your panel runs hot — say, a sealed enclosure near a motor — the effective trip threshold drops; size the breaker for the actual ambient, not the nameplate. The DC rating maxes at 60 V, so it can handle a 48 VDC control bus but not a 125 VDC battery bank. Overvoltage category III and pollution degree 3 mean it's rated for fixed-installation mains with non-conductive dust and occasional condensation — typical for industrial switchgear, not clean-room electronics.