What it is and where it fits
The Siemens 3RH1921-1FA22-ZW97 is a SIRIUS auxiliary switch block — a 4-pole block with 2 normally-open and 2 normally-closed contacts, designed to snap onto 3RT1 contactors and contactor relays. It carries a 10 A maximum rating and switches auxiliary circuits across a wide voltage range: 6 A at 24 V, 6 A at 125 V, 6 A at 230 V, and 3 A at 400 V, among other ratings. The block uses screw-type terminals and accepts 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) wire with core end processing, or 2x (20 to 16) or 2x (18 to 14) AWG. Front protection is IP20, insulation voltage is 690 V at pollution degree 3, and surge voltage resistance is 6 kV. Mechanical life is 10 million switching cycles typical. Contact reliability is rated at 1 faulty switching per 100 million operations at 17 V, 1 mA.
What the ratings mean for fit
The 10 A maximum is the thermal current (Ith) through the contacts, but the real-world switching capacity depends on voltage and load type. For a 24 V DC control circuit, the contacts handle 6 A — that's the figure to use when sizing for a PLC output or relay coil. At 110 V, derate to 1 A; at 220 V, 0.3 A. The AC-15 rating at 690 V is 1 A, which covers inductive loads like contactor coils at higher line voltages. The 2 NO + 2 NC configuration (EN 50005 layout) gives flexibility for interlocking, status feedback, or auxiliary circuits without an extra block. The snap-on mounting means it clips directly onto the 3RT1 contactor or contactor relay — no tools needed for installation, but verify the contactor frame size (S00/S0/S2/S3) matches the block's intended fit.