What this RCCB does on the line
The Siemens SENTRON 5SV3644-8 is a 4-pole residual current circuit breaker (RCCB), type A, selective, rated 40 A with a 300 mA trip threshold at 230/400 V AC. The type A characteristic means it detects pulsating DC fault currents on top of sinusoidal AC — common in drives, rectifiers, and switching power supplies found on modern lines. The selective (S) design introduces a deliberate time delay so that downstream RCCBs trip first, keeping this unit as the backup ground-fault protection for the whole subdistribution.
Ratings that govern fit
The 300 mA residual sensitivity is a deliberate choice: it is above the 30 mA required for personnel protection, so this unit is used for fire protection or equipment protection where nuisance tripping from leakage in long cable runs or filters must be avoided. The 40 A continuous rating matches a 40 A busbar tap or feeder breaker feeding a subpanel.
The 72 mm width (4 modular units at 18 mm each) and 70 mm depth fit standard SENTRON distribution boards and enclosures. Silicon-free construction suits environments where silicone outgassing can foul contacts or optics.