The Siemens SENTRON 5SJ6120-7 is a 1-pole miniature circuit breaker (MCB) from the 5SJ6 series, rated 20 A at 240 V AC with a C tripping characteristic. The 6 kA breaking capacity (rated per EN 60898 and IEC 60947-2) tells you it can safely interrupt a fault current up to 6 kA at the rated voltage; that's sufficient for most sub-distribution boards fed by a transformer of typical impedance, but if your available fault current exceeds 6 kA at the panel, you'll need to coordinate upstream or step to a higher-rated MCB. It occupies a single width unit (1 PU) on the DIN rail — 17.8 mm — so it fits tight enclosures where every slot counts. The 70 mm installation depth is standard for the 5SJ6 family, meaning it clears most 100 mm deep distribution boxes without issue.
Temperature range and environmental limits
Storage temperature spans -40 to +75 °C — that's the range for warehousing and transport, not operation. The operating ambient is -20 to +45 °C (the standard for this class), with periodic excursions to +55 °C at up to 95% humidity. If you're mounting this breaker in a non-air-conditioned enclosure near a furnace or in a Middle Eastern plant, the 45 °C ceiling is the number to watch: above that, you need to derate the 20 A rating or provide ventilation. The 75 °C max is strictly a non-operating limit for shelf life.
Voltage ratings and multi-phase use
That means a 1-pole breaker like this can be ganged with others on a 3-phase 400 V system (line-to-neutral on a 230/400 V supply), but each pole only switches its own phase. For DC circuits, the maximum rated voltage is 60 V — so it works on 24 V or 48 V DC control buses, but not on a 110 V DC battery bank.
Mechanical life and vibration resistance
Rated for 20,000 mechanical switching cycles — that's typical for an MCB in distribution duty where it's operated maybe a few times a year for isolation. If you're using it as a daily disconnect on a machine, the cycle count is fine for years of service. Vibration resistance is 50 m/s² at 25 to 150 Hz and 60 m/s² at 10 to 150 Hz per IEC 60068-2-6, which covers most industrial panel vibration from contactors and drives mounted on the same rail.
The closest functional sibling is the 5SJ6220-6 — same 20 A rating, same 6 kA breaking capacity, same 5SJ6 family, but 2-pole and B-curve instead of C-curve. The 2-pole version takes two width units on the DIN rail (35.6 mm) versus one for the 1-pole 5SJ6120-7. If your panel was wired for a 2-pole B-curve breaker, the 1-pole C-curve won't drop in without rewiring the neutral and checking the trip coordination. For a 1-pole 20 A C-curve application, the 5SJ6120-7 is the direct fit.