What it is and what it does
The Siemens 5SV3645-8 is a 4-pole residual current operated circuit breaker (RCCB) from the 5SV family, type A with selective characteristic. It is rated for 125 A nominal current and 300 mA residual sensitivity, on a 400 V AC supply. The type A designation means it detects sinusoidal AC fault currents plus pulsating DC fault currents — the standard choice for circuits with single-phase rectifiers, switched-mode power supplies, or electronic loads that produce pulsed DC leakage. The selective (S-type) characteristic introduces a deliberate time delay so this unit coordinates with downstream RCCBs: in a fault, the closer device trips first, keeping supply to healthy circuits.
Where it fits in the panel
Packaged as a single unit, 78 x 97 x 85 mm, this is a DIN-rail mounted device sized for distribution boards and sub-distribution panels. The 4-pole format covers three-phase plus neutral, so it protects the entire outgoing circuit. At 125 A it suits main incoming protection or heavy sub-feed duty — think large commercial kitchen equipment, HVAC plant, or industrial machine supply where the full load current sits above 100 A.