The Siemens 5SV3644-8MJ is a Type A residual current operated circuit breaker (RCCB) rated 40 A with a 300 mA trip sensitivity and 10 kA short-circuit capacity. The 3+N-pole configuration covers three-phase plus neutral on a 400 V system. The selective (S-type) designation means it delays tripping to coordinate with downstream RCCBs — useful on distribution subfeeds where you want the faulted branch to clear before the main.
The 300 mA rating is a common threshold for fire protection in commercial and industrial circuits (EN 60364-4-42). It's above the 30 mA required for personnel protection, so this device guards against insulation faults and arcing that could start a fire, not against direct shock risk. The Type A waveform detection covers pulsating DC residual currents from rectifiers and switching power supplies — typical in modern panels with VFDs, UPSs, or LED drivers.
Type A selective RCCBs of this rating go into distribution boards feeding mixed loads — lighting, socket outlets, small motor circuits, and electronic equipment. The selective delay (typically 40–150 ms) lets downstream 30 mA RCCBs trip first on a ground fault, keeping the main board live for the rest of the installation. Common in commercial buildings, industrial control panels, and infrastructure sub-distribution.