The Siemens 5SU1656-6KV40 is a combined residual current device and miniature circuit breaker (FI/LS protector) in a single 70mm-wide module. It packs Type A residual current detection at 300mA with a B-curve 40A overcurrent trip and 6kA breaking capacity into a 1+N-pole format — one protected phase plus switched neutral.
What the ratings mean for fit
The Type A classification means it detects pulsating DC residual currents in addition to sinusoidal AC — essential on circuits feeding single-phase electronics, VFDs, or switching power supplies where a pure AC-sensitive (Type AC) device would miss fault current. The 300mA residual threshold is sized for fire protection rather than personal shock protection (30mA is the touch-safe limit); this unit goes into sub-distribution or feeder circuits where the goal is arc-flash and cable-fire prevention, not direct personnel protection. The B-curve 40A overcurrent characteristic trips between 3x and 5x rated current (120-200A magnetic release), suited for resistive and general-purpose loads — lighting, socket outlets, small motor circuits — where inrush is moderate. The 6kA breaking capacity (Icn) at the rated voltage is adequate for domestic and light commercial distribution boards with a typical prospective short-circuit current under 6kA; verify the fault level at the point of installation before specifying. The 1+N-pole configuration switches the phase and disconnects the neutral, leaving the neutral pole unswitched in the off state on some variants — check the wiring diagram for your specific board layout. The 70mm width (4 module spaces on a DIN rail) is wider than a standard single-pole MCB (17.5mm) because the RCD core and trip electronics occupy the extra space.
Where it goes in the panel
Snaps onto a 35mm DIN rail in a distribution board or sub-panel. The 70mm width means it occupies four 17.5mm module positions — plan the rail fill accordingly, especially when combining with other RCBOs or accessories. Supply-side connection is typically from the busbar; load-side terminals accept up to 25mm² conductor. No separate RCD block needed upstream — this is a single-device solution for combined overcurrent and residual protection.