What this RCBO does in the panel
The Siemens 5SU1656-1KV40 is a combined residual-current and overcurrent protective device (RCBO) — what Siemens calls an FI/LS protector. It packs a Type AC residual-current unit rated at 300 mA trip threshold and a C-curve 40 A miniature circuit breaker into a single 1+N-pole module, 70 mm wide on the DIN rail. That 6 kA breaking capacity handles fault currents up to that level at the service entrance or sub-distribution board where the prospective short-circuit current stays within that range.
Ratings that decide fit
The C-curve means the magnetic trip fires at 5 to 10 times rated current — 200 A to 400 A — which suits motor-starting and transformer inrush loads that would nuisance-trip a B-curve. The 40 A thermal element carries continuous loads up to that current. The 300 mA residual threshold is a deliberate choice: it is higher than the 30 mA required for personal shock protection, so it is typically specified for circuits where the natural leakage from filters, VFDs, or long cable runs would cause unwanted trips on a 30 mA device. This makes it a fit for distribution sub-mains, lighting circuits with electronic ballasts, or equipment circuits where the earth leakage budget is tight. Type AC detection means it responds to sinusoidal AC residual currents only. Pulsating DC residuals (from rectifiers, half-wave devices, or VFDs) will not reliably trip this device — that is the limitation of Type AC. If the downstream load includes any power electronics, a Type A or Type F RCD would be the correct choice. The 6 kA breaking capacity is the rated short-circuit withstand; for installations with higher prospective fault current, upstream current-limiting fuses or a higher-rated breaker would be needed to keep the let-through energy within the device's limits.