Single-phase SSR in a 22.5 mm footprint
The Siemens 3RF2320-1AA24 is a SIRIUS solid-state contactor — single-phase, zero-point switching, rated 20 A in AC-51 duty (resistive loads) and 17.6 A per UL 508. Control supply is 110-230 V AC, 50/60 Hz.
The AC-51 rating of 20 A is the resistive-heater or lighting load curve — the full current the SSR can carry continuously when switching resistive elements. For motor or inductive loads, you derate to the AC-53a curve (not listed here, but the part's 500 mA minimum operational current means it handles small loads cleanly without dropout). The 17.6 A UL 508 figure is the US-code equivalent for general industrial control panels; if your BOM specifies UL 508A, this is the number that governs the branch circuit rating. Zero-point switching means the thyristor fires only near the voltage zero-crossing, which minimizes inrush and conducted EMI — important if the contactor feeds heater banks or transformer primaries where soft-start behavior is preferred. The ON and OFF delays are both 40 ms plus a half-wave maximum, so the response is deterministic within one mains cycle.
The control terminals take 1x 0.5-2.5 mm² solid or 2x 0.5-1.0 mm². Dimensions are 22.5 mm wide × 95 mm high × 120 mm deep.