What it is and what it does
The Siemens 3VM1120-5MH32-0AA0 is a SENTRON molded case circuit breaker (MCCB) — the piece that sits in the distribution panel protecting a feeder or a big motor branch from overload and short circuit. Three poles, rated for a maximum of 320 A, with a TM120M thermal-magnetic overcurrent release. That TM120M means the thermal element handles the overload curve and the magnetic element trips instantaneously on a hard fault; no electronic trip unit, no communication module, no ground-fault monitoring built in — this is a straight electromechanical breaker.
Breaking capacity — what it interrupts and where
Rated breaking capacity varies sharply with system voltage. At 240 V it clears 187 kA — enough for most low-voltage industrial services. At 415 V it still holds 121 kA. At 440 V it drops to 75 kA, and at 500 V it's 17 kA. That last number matters: on a 480/500 V system (common in North American and some Asian plants), the available fault current has to be coordinated with this 17 kA ceiling, or the breaker won't interrupt a high-energy fault safely. Insulation voltage is rated at 800 V, so the internal clearances are there for 690 V class systems — the limitation is the arc extinction at higher voltages, not the dielectric.
Thermal derating — continuous current vs panel temperature
Continuous current rating holds at 20 A from 40 °C through 55 °C ambient. At 60 °C it drops to 19 A, and it stays at 19 A through 70 °C. That's a shallow derating curve — useful if the breaker sits in a hot panel near other heat sources. Operating range is -25 °C to 70 °C; storage range is -40 °C to 80 °C. Front protection is IP40 — sealed against tools and wires over 1 mm, but not against water ingress. Mount it inside a panel, not in a washdown zone.
What it doesn't have — and why that matters for your panel
No trip indicator, no undervoltage release, no phase-failure detection, no communication function, no ground-fault monitoring. This is a bare electromechanical MCCB — if you need remote signaling or a shunt trip for emergency-off circuits, you'll add external accessories or pick a different variant. The 3-pole form factor fits standard SENTRON panel-mounting footprints; dimensions are 70 mm deep, 76.2 mm wide, 130 mm high. Width is exactly 3 inches — matches the standard MCCB cell width in most distribution boards.