The Siemens SENTRON 5SM3644-0MJ is a Type AC residual current operated circuit breaker (RCCB) from the 5SM3 design series. It's rated 40 A, 3+N-pole, 300 mA sensitivity, 400 V, with a 10 kA breaking capacity at 50 Hz. This is the core earth-leakage protection device for a three-phase panel — catches ground faults on the line side of the load, not the branch circuit.
What the ratings mean for fit
40 A continuous rating means this RCCB sits upstream of loads drawing up to that current — sized for a sub-distribution board feeding several smaller circuits. The 300 mA residual sensitivity is for fire protection or equipment protection, not personnel shock protection (that's 30 mA or lower). It trips on sinusoidal AC earth faults only (Type AC); if the load has rectifiers, VFDs, or switching supplies that generate pulsed DC fault currents, you'd need Type A or Type B instead. The 10 kA breaking capacity at 400 V means it can safely interrupt a fault current up to that level — check the prospective short-circuit current at the installation point; if it exceeds 10 kA, you need an upstream current-limiting device.
Deployment context
This RCCB snaps onto a DIN rail in the main or sub-distribution panel. 4 MW (modular width units) means it occupies four 18 mm slots — plan the rail fill accordingly. The 3+N-pole design switches all three phases plus the neutral; the neutral must pass through the device for the residual current sensing to work. Rated 50 Hz only — not suitable for 60 Hz installations or variable-frequency supplies without verifying the core's frequency response.