The Siemens 3RB2133-4UX1 is a SIRIUS solid-state overload relay for three-phase motor circuits. It protects against overload and phase imbalance by monitoring current through straight-through transformers, and its trip class is field-adjustable among 5E, 10E, 20E, and 30E — so you can match the relay's response to the motor's starting characteristics without stocking multiple fixed-class units.
The trip class adjustment (5E through 30E) is the main selection parameter: a 5E setting trips fast for short-start motors (e.g., centrifugal pumps), while 30E accommodates high-inertia loads like fans or conveyors that take longer to accelerate. The relay carries one NC auxiliary contact for the control circuit, wired via spring-loaded terminals that accept 2x (0.25 to 1.5 mm²) solid or finely stranded wire — no screwdriver needed for termination, which speeds panel wiring. It is rated for surge voltage withstand of 6 kV and passes burst immunity at 2 kV on power ports and 1 kV on signal ports per IEC 61000-4-4 severity level 3, so it holds up in electrically noisy industrial panels without nuisance tripping. Shock resistance is 15g for 11 ms, enough for mounting on vibrating machinery.
The straight-through transformer design means the main power conductors pass directly through the relay body, keeping wiring simple and reducing panel clutter.