What the ratings mean for fit
The 5SJ4130-8HG40: At 40 °C ambient it holds the full 30 A; at 50 °C it derates to 28.2 A, and at 60 °C to 26.4 A. If this breaker lives in a hot panel — say next to a drive or a transformer — the 60 °C figure is the one that governs the continuous load, not the nameplate 30 A. The 1-pole format and 18 mm width (one modular unit) mean it snaps onto standard DIN rail and occupies one slot in a distribution board. Depth is 70 mm, so it clears most shallow enclosures. DC rating tops out at 60 V, so this is primarily an AC branch-circuit protector. For DC applications at higher voltages, look at a dedicated DC-rated MCB. The IP20 rating (IP40 in the handle area) is standard for panel interiors — not for washdown or outdoor exposure.