The Siemens 3RT1647-1AL21 is a 3-pole capacitor contactor in the S3 frame, designed specifically for switching power-factor-correction capacitor banks under AC-6 duty. Its headline rating is 50 kVAr at 400 V — that's the reactive power it can switch without exceeding the capacitor inrush current limits. The 230 V AC coil pulls from a standard control transformer at 50/60 Hz. This is a phased-out product. The official Siemens successor is the 3RT2636-1AL23.
The 90 A at AC-1 at 400 V rating tells you the contactor's thermal current capacity for resistive loads — it's the ceiling for the main poles. For capacitor switching (AC-6), the relevant figure is the 50 kVAr at 400 V; that's the bank size this contactor is designed to switch, with the series resistors or reactors handling the initial surge. The 3-pole set (3 NO main contacts) handles three-phase banks. One auxiliary NO contact is built in for status feedback or interlocking. The IP00 body (IP20 on the front with a cover or box terminal) means it's intended for enclosure mounting — the panel provides the environmental seal. The S3 frame dimensions (167 x 70 x 183 mm) fit the standard S3 footprint — check your panel depth against the 183 mm projection.
The manufacturer lists this as a phased-out product. The preferred successor is the 3RT2636-1AL23. No last-time-buy window is stated, so availability is confirmed per inquiry. RoHS compliance date is noted as 05/01/2012, meaning the part meets the EU RoHS substance restrictions from that date forward.