Adjustable trip class sets the motor protection curve
The 3RB3123-4NB0 is a SIRIUS solid-state overload relay — no bimetal drift, no heater coil swaps. The trip class is field-adjustable across CLASS 5E, 10E, 20E, and 30E, so one relay covers fast-acting (pump) to slow-start (fan, flywheel) loads without stocking four separate units. The motor current window is bounded by the setting range: IMotor must sit above the lower setting value and below upper setting value × 3.5. For a 1.25 A relay, that means the motor FLA lands within the adjustment pot range; the 3.5× ceiling handles the inrush without nuisance tripping.
Panel fit — S0 frame, contactor-mount, 45 mm wide
This is a S0-size overload relay, 45 mm wide and 87 mm tall, depth 84 mm. It mounts directly onto the contactor (fastening method: contactor mounting), not on the DIN rail itself — so the panel layout needs the contactor footprint, not a separate relay slot. The Pozidriv PZ 2 screwdriver tip is what you want in the pouch — a #2 Pozidriv, not a Phillips, to avoid cam-out on the M4 main-contact screws.
Tripped message output and auxiliary contact ratings
The relay outputs a signal contact for the "tripped" message — useful for tying into a PLC input or a remote annunciator. The auxiliary contact is rated 2 A at 24 V, 4 A at 120 V, and 3 A at 230 V, so it handles standard control voltages without an interposing relay. Shock resistance is 15g / 11 ms on the main body; the signaling contact in the tripped position derates to 9g / 11 ms. Vibration per IEC 60068-2-27: 1-6 Hz at 15 mm amplitude, 6-500 Hz at 20 m/s², 10 cycles — suitable for most industrial panel environments.