Selective RCD for downstream coordination
The Siemens 5SM2646-8 is a selective RCD unit from the SENTRON family, designed to provide time-delayed ground-fault protection so a downstream RCD trips first — the selective design keeps this unit from nuisance-tripping on transient faults and ensures the upstream device stays closed during a downstream fault. Rated at 63 A AC and 300 mA Type A residual fault current, it handles the main feeder or sub-feed in a distribution board where coordination with branch RCDs is required. Four-pole construction (4 poles) covers three-phase plus neutral, with a 50/60 Hz operating frequency and an insulation voltage (Ui) of 460 V.
What the ratings mean for fit
The 300 mA tripping threshold is higher than a standard 30 mA RCD — this is intentional for a selective (time-delayed) device. It allows downstream 30 mA RCDs to clear a fault first, while the 5SM2646-8 waits through the delay and only trips if the fault persists. That selectivity is what protects the rest of the panel from a full blackout on a branch fault. Type A detection means it responds to AC sinusoidal residual currents and pulsed DC residual currents up to 6 mA smooth DC — covers the typical rectified waveforms from single-phase power supplies and VFDs you'd find in a mixed-load panel. Temperature derating is published: 58.6 A at 40 °C, dropping to 50.4 A at 70 °C. If the panel ambient runs hot — near a furnace line or a crowded enclosure — that 63 A nameplate effectively becomes a 50 A device. Factor that into the load schedule. IP20 protection with connected conductors means it's for indoor, dry-location distribution boards only — not for washdown or outdoor exposure.
Panel integration and mounting
Occupies 3 modular width units on a DIN rail, with a depth of 70 mm and a width of 123 mm. Fastening method is standard REG (regular) snap-on. Mounting position is any, so vertical or horizontal rail orientation is fine. Supply connection can be top or bottom. The unit is not sealable and has no adjustable time delay or adjustable fault current — it's a fixed-setting selective device, no field calibration required. Overvoltage category III and pollution degree 2 confirm it's rated for the fixed-installation distribution board environment.