FI-Protector, Type AC — 25 A, 4-pole, 30 mA, 400 V
The Siemens 5SM1342-0FD is a Type AC FI-Protector (residual-current device / RCCB), rated 25 A, 4-pole, with a 30 mA rated residual operating current (IΔn) at 400 V. The 55 mm (4 TE) housing snaps onto a DIN rail for panel integration. The 30 mA sensitivity is the common threshold for personal protection against direct-contact faults (≤ 30 mA per IEC 60364). At 25 A rated current, this device covers sub-distribution and final circuits feeding socket outlets, lighting, and small motor loads up to that continuous rating.
The 4-pole (3P+N) configuration suits three-phase four-wire systems (TN-S, TN-C-S, TT) where the neutral conductor must be switched and monitored for residual current. The 400 V rated voltage matches standard low-voltage distribution in 230/400 V networks. The 30 mA IΔn is the maximum allowed for supplementary protection against electric shock in most IEC-based codes; circuits with higher leakage (heating elements, long cable runs) may need a higher threshold or time-delayed type to avoid nuisance tripping. Type AC detection covers sinusoidal residual currents only. If the downstream load includes rectifiers, VFDs, or switched-mode power supplies that generate pulsed DC or mixed-frequency fault currents, a Type A or Type B device is required. This part is not suitable for those applications.
Panel Integration
The 55 mm (4 TE) width matches standard modular DIN-rail enclosures. The FI-Protector occupies four adjacent 18 mm mounting spaces. Terminals accept copper conductors up to the cross-section specified in the datasheet; torque settings and strip lengths follow Siemens installation instructions for the 5SM series.