The Siemens 5SJ4201-8HG42 is a SENTRON 5SJ4 miniature circuit breaker with 2 poles, rated 1 A at AC and a D tripping characteristic. At 1 A, this is sized for small loads: a fractional-horsepower motor, a control transformer, or a single lighting branch. The 2-pole configuration handles single-phase 240 V or two legs of a three-phase system. Rated for 60 V DC maximum per pole, so it also works in DC control circuits up to that voltage.
Thermal derating — what the 1 A rating actually delivers in a warm panel
At 45 °C it derates to 0.97 A, at 50 °C to 0.9 A, at 55 °C still 0.9 A, and at 60 °C to 0.88 A. A 1 A D-curve MCB in a 60 °C environment will carry only 0.88 A continuously without tripping. That's a 12 % reduction; plan your load accordingly.
Width is 36 mm (2 MW), depth 70 mm, height 121 mm. Installation depth matches the overall depth — no extra clearance needed behind the rail. Touch protection is built in, so accidental contact with live terminals is prevented when the breaker is installed in an enclosed panel.