The Siemens 3RB2906-2BG1 is a SIRIUS current measuring module, designed as a stand-alone unit that uses straight-through transformers for the main circuit.
The adjustable current response range of 0.3 to 3 A covers small motor and transformer protection — set the dial to match the full-load current of the load, and the matching 3RB relay will trip within its Class 10 curve. Rated for AC-3e duty up to 690 V, with a 3 A operational current at 400 V, so it handles the control-circuit side of the motor branch, not the main power path.
Lifecycle reality — NRND
This module carries an NRND (Not Recommended for New Design) status. That means Siemens will continue to manufacture it for existing BOMs and service requirements, but new projects should select the current-generation alternative. For an existing line or a repair, the part is still available through the supply channel — quote to order.
Stand-alone installation — no DIN-rail clip required. The module mounts directly to the panel or enclosure backplate via its own fixing points, with the straight-through transformer aperture accepting the main conductor. Keep the wiring paths short to the associated 3RB relay to avoid picking up coupled noise.