What it is and what it does
The Siemens 5SJ4208-7HG42 is a 2-pole miniature circuit breaker from the SENTRON 5SJ4 series, with a C-curve tripping characteristic and an 8 A rated current at 30 °C. It snaps onto standard DIN rail and fits a 36 mm wide slot — two modular width units. The C-curve means it trips between 5 and 10 times rated current, so it's sized for general-purpose loads like lighting, small motor circuits, and distribution subfeeds where moderate inrush is expected.
What the ratings mean for fit
Rated 8 A at 30 °C, but the thermal derating curve is published: at 40 °C it still holds 8 A, at 45 °C it drops to 7.8 A, at 50 °C to 7.6 A, at 55 °C to 7.4 A, and at 60 °C to 7.2 A. If your panel ambient runs above 40 °C, the breaker's effective ampacity is lower than the nameplate — size the upstream protection accordingly. The DC side is rated 60 V maximum, so it can switch DC control circuits up to that voltage, but not higher. IP20 with connected conductors, IP40 in the handle area — fine for enclosed distribution boards where the front face is accessible but the terminals are behind a cover. Degree of pollution 3 and overvoltage category III mean it's rated for the typical industrial panel environment with conductive dust and transient overvoltages from the mains.