RCBO with D-curve and 300 mA residual protection
The Siemens 5SU1644-2RC63 is a 4-pole RCBO (residual current circuit breaker with overcurrent protection) rated 63 A, occupying 8 module widths on a DIN rail. The D characteristic means it handles high inrush currents — typical for motor, transformer, or welding loads — without nuisance tripping. The Type AC 300 mA residual current protection covers sinusoidal AC earth faults.
What the ratings mean for your panel
63 A at 4-pole covers three-phase loads up to that current — think 30-40 kW motor circuits, large distribution subfeeds, or industrial machine supplies. The D-curve (10-20x In magnetic trip) is the key decision point: it avoids tripping on motor starting current or transformer inrush that would nuisance-trip a B or C curve breaker. The 300 mA AC residual element protects against line-to-ground faults, not personnel shock (which needs 30 mA or lower). This is a common spec for distribution boards feeding fixed equipment where arc-fault protection is required but not direct human contact. At 8 modules wide on a DIN rail, it takes up about 144 mm of rail space. Plan for that fill factor when laying out the enclosure — a 4-pole RCBO at this rating is a wide device.