What this part is and what it does
The Siemens SIRIUS 3RH1921-1LA02 is an auxiliary switch block designed to snap onto 3RT1 contactor relays and power contactors, adding two normally-closed (NC) contacts. It carries a maximum thermal current of 10 A and is rated for AC-15 and DC-13 switching duties across a range of control voltages: 6 A at 24 V, 6 A at 125 V, 6 A at 230 V, and 3 A at 400 V, among others. The screw-type terminals accept 2x (0.5…1.5 mm²) or 2x (0.75…2.5 mm²) with core-end processing, or 2x (20…16) and 2x (18…14) AWG. Mechanical life is rated at 10 million switching cycles, with contact reliability of one faulty switching per 100 million operations at 17 V, 1 mA.
Mounting and integration
Snap-on mounting onto the contactor body — no DIN rail required for the block itself, though the base contactor typically mounts on a 35 mm rail. The block adds 44 mm width, 38 mm height, and 51 mm depth to the assembly. Front-side IP20 protection means it is safe for finger contact inside an enclosed panel but not for exposed washdown areas. Rated impulse withstand voltage is 6 kV, with insulation voltage of 690 V AC at pollution degree 3.
Switching ratings — what they mean for your circuit
The AC-15 ratings (6 A at 230 V, 3 A at 400 V, 1 A at 690 V) govern solenoid and contactor coil switching — the most common load for an auxiliary block. The DC-13 ratings (10 A at 24 V, 4.7 A at 60 V, 3 A at 110 V, 1.2 A at 220 V, 0.5 A at 440 V, 0.26 A at 600 V) cover DC electromagnet loads with a time constant. If your control circuit operates at 24 VDC and pulls more than 10 A inrush, this block is not a fit. The 1 faulty switching per 100 million operations at 17 V, 1 mA ensures reliable dry-contact sensing in PLC inputs.