What it is and what it does
The Siemens 3RB2025-2NB0 is a SIRIUS solid-state overload relay — no bimetals, no heater packs to swap. It protects motors against overload and phase loss by monitoring current through the main circuit and tripping a CLASS 20E curve. That trip class means it allows a 20-second overload at 7.2× FLA before opening, which matches standard NEMA-design motors and most IEC general-purpose induction motors on pumps, fans, and conveyors. Rated for AC-3 duty up to 690 V, it handles the switched motor load directly. The front panel carries IP20 protection — safe for enclosed panels, but not for washdown areas. Mounts directly onto a contactor (contactor mounting), saving DIN rail space in a crowded enclosure. Screw-type terminals on the main circuit accept 0.5 to 4 mm² solid or 2× 0.5 to 2.5 mm² stranded — standard panel wire sizes for motor circuits up to this class.
Where it goes and what it guards
This relay lives inside a motor control center or a standalone panel, snapped onto the contactor. It's sized for a 45 mm wide footprint, 73 mm tall, 75 mm deep — fits standard 45 mm contactor profiles across the SIRIUS family. The ATEX approval (PTB 06 ATEX 3001 Ex II (2) GD) means it can be used in the safe-area side of a motor starter that feeds equipment in a Zone 1 or Zone 2 gas or dust atmosphere — the relay itself sits outside the hazardous area but is certified for the associated apparatus role. Operating temperature range of -25 to +60 °C covers most indoor industrial environments. Storage and transport range of -40 to +80 °C handles cold warehouses and hot shipping containers without issue.
What the ratings mean for fit
CLASS 20E is the trip curve — it's the standard for most general-purpose motor protection. If your motor starter was designed around a CLASS 10 or CLASS 30 relay, this one will trip at a different speed during a stall. CLASS 20 is the middle ground: slower than CLASS 10 (which protects short-time-rated motors) and faster than CLASS 30 (for high-inertia loads like centrifuges). For a standard pump or fan motor, CLASS 20 is what you want. The 690 V AC-3 rating covers 480 V and 600 V class systems with headroom. No derating worries on a 400 V or 480 V line. Shock resistance of 15g for 11 ms means it survives the mechanical slam of a contactor closing and the vibration of a running motor starter. Mounting position is unrestricted — any orientation works.