The Siemens 5SJ4130-7HG42 is a SENTRON 5SJ4 miniature circuit breaker, 1-pole with C tripping characteristic, rated 30 A at AC. It snaps onto standard mounting rail and occupies one modular width (18 mm). The C curve handles moderate inrush — motor starters, small transformers, lighting banks — without nuisance tripping on startup, while clearing faults fast enough for branch-circuit protection.
Rated 30 A at both 30 °C and 40 °C ambient; derate to 29.1 A at 45 °C, 28.2 A at 50 °C, 27.2 A at 55 °C, and 26.4 A at 60 °C. That means in a warm panel — say 50 °C — you still get 28.2 A continuous, so a 30 A breaker on a 24 A load leaves headroom. DC rating is 60 V max, so it works for 48 VDC control circuits but not 125 VDC railway auxiliaries.