What it is and what it does
The Siemens 5SU1354-1LB25 is a 2-pole RCBO — residual current circuit breaker with overcurrent protection — from the SENTRON family. It combines a 25 A thermal-magnetic overcurrent trip (C-curve) with a 30 mA Type AC residual current detection in a single 36 mm wide (2 MW) DIN-rail module. The 10 kA breaking capacity per EN 60898 covers the majority of final-subcircuit fault levels in commercial and light industrial panels. The short-time delayed G, super resistant K design means it tolerates brief inrush and surge currents without nuisance tripping — useful on circuits with small motor drives or capacitive loads that a standard RCBO might false-trip.
Ratings and what they mean for fit
The 25 A rating at 30 °C is the headline thermal current. Derate it as the panel ambient climbs: 23.5 A at 40 °C, 22.75 A at 45 °C, 21.75 A at 50 °C, 20.25 A at 55 °C and 60 °C, 19.25 A at 65 °C, and 18.25 A at 70 °C. If the panel runs hot — say 50 °C inside a sealed enclosure — this RCBO is good for 21.75 A continuous, not 25 A. The 30 mA residual trip threshold is standard for personnel protection per IEC 60364. Type AC means it detects sinusoidal AC fault currents only; if the circuit has rectified or pulsed DC fault paths (variable-speed drives, UPS, LED drivers), a Type A or Type B RCBO would be the correct choice. The C-curve (5–10× In magnetic trip) is the common choice for general-purpose and moderate motor-start loads.
Panel integration and mounting
Two modular-width units (36 mm) snap onto standard DIN rail. Depth is 77 mm, installation depth 70 mm — check the enclosure depth before committing; a shallow 80 mm deep box leaves only 3 mm clearance behind the conductors. Supply can enter from top or bottom. The IP20 rating applies only when the distribution board is installed with connected conductors — this is a panel-interior device, not for standalone surface mounting. Pollution degree 2, overvoltage category III, mounting position any. Halogen-free and silicon-free construction matters for clean-room or high-reliability environments where outgassing can contaminate contacts or optics.