What it is — and what the ratings mean for your panel
The Siemens 3RB2025-2SB0 is a SIRIUS soft starter with a 24 V DC control supply that draws its power from L+ minus 4 V DC minimum — meaning it needs a clean 20 V DC rail at minimum to hold in. That matters for panels with long DC runs or shared supplies where voltage drop eats into the headroom. It's rated for an I2t of 0.8 A²·s, which governs the thermal stress the device can handle during a start cycle; keep your motor's starting profile under that curve or you'll trip the overload before the motor gets up to speed. The part lives in an IP20 enclosure — fine for a clean control cabinet, but it won't survive washdown or outdoor dust. Mount it inside a panel with the gland plate sealed, and it'll handle ambient temps from -40 °C to 70 °C, which covers most heated or unheated electrical rooms.