The Siemens 5SM1344-0FD is a Type AC FI-Protector residual current device (RCD) rated for 40 A, 4-pole operation at 400 V with a 30 mA sensitivity.
The 40 A current rating sets the maximum continuous load the device can carry — sized for a 40 A submain or feeder circuit. The 30 mA sensitivity (IΔn) is the standard threshold for personal protection against indirect contact; it will trip on leakage currents above 30 mA. Type AC detection means it responds only to sinusoidal AC residual currents — not pulsed DC or smooth DC. That makes it suitable for circuits with purely resistive or inductive AC loads (heaters, lighting, motors without VFDs), but not for loads with rectifiers or variable-speed drives that generate DC fault components. The 4-pole (4P) construction switches all four conductors — three phases plus neutral — and requires a neutral connection to function. The 400 V rated voltage matches standard three-phase low-voltage distribution.