The Siemens SENTRON 5SM2645-8 is a selective RCD unit (Type A) designed to pair with a 5SY series MCB in a 4-pole configuration. Rated at 63 A with a 300 mA trip threshold, it's built for branch circuits where you need fault discrimination — the 'S' curve delay lets a downstream RCD trip first, keeping this unit from dropping the whole submain on a nuisance ground fault. The 300 mA threshold is typical for fire protection or circuits where a lower mA trip would cause unwanted coordination issues.
63 A at 40 °C is the headline number, but the derating curve matters more in a warm enclosure: at 50 °C it's 56.7 A, at 60 °C it's 53.55 A, and at 70 °C it drops to 50.4 A. The 300 mA trip is fixed, non-adjustable, so it's not for variable leakage applications. Type A means it catches pulsating DC residual currents from single-phase rectifiers, which covers most VFD and switched-mode supply scenarios.
Snap-on DIN rail mount, 3 modular width units wide. The 77 mm depth and 124 mm width fit standard distribution board cutouts. Supply can land top or bottom, which helps when you're reworking an existing panel and the busbar arrangement is fixed. IP20 with the distribution board installed and conductors connected — fine inside a closed cabinet, not for wet environments. The selective design means it's intended as the upstream RCD in a coordinated system; it won't trip on transient leakage from downstream devices starting up.