The Siemens SIRIUS 3RV2742-5CD10 is a Size S3 circuit breaker rated 20 A operational current, designed for system protection per UL 489 and CSA C22.2 No. 5. It delivers 100 kA breaking capacity at 240 V AC and 50 kA at 400 V AC — the 400 V figure is the one that governs most industrial panel applications where line-to-line faults are the real event. The thermal overload release handles sustained overcurrents; the magnetic short-circuit trip clears fast faults.
The 3RV2742-5CD10 occupies a 70 mm wide footprint on the DIN rail, 170 mm tall, 176 mm deep. Those clearances — 70 mm above and below, 10 mm to the side — are the minimum for free air circulation and arc-quenching; crowding them derates the interrupting rating. Main terminals accept 2x (2.5 to 50 mm²) solid or stranded, or a single 70 mm² conductor. Screw-type terminals with a 4 mm hex socket driver; torque to the value on the nameplate. Auxiliary switch slot is populated or left for field addition.