The Siemens 5SU1356-7KV40 is a combined RCD/MCB — an FI/LS protector — in a Type A (PSE/SSF) configuration. It packs a 40 A C-curve overcurrent trip and a 30 mA residual-current detection into a single 1+N-pole, 70 mm-wide module. The 6 kA breaking capacity suits it for final-subcircuit protection in residential and light commercial distribution boards where you need both earth-leakage and overcurrent protection in one DIN-rail slot.
What the ratings mean for fit
Rated residual current IΔn = 30 mA for personal protection against direct contact. Type A detects AC sinusoidal and pulsed DC residual currents up to 6 mA smooth DC superimposed. Rated current In = 40 A with C-curve characteristic: the magnetic release trips at 5–10× In (200–400 A), so it handles motor inrush and capacitive loads without nuisance tripping. The 40 A rating matches a 7.5 kW resistive load at 230 V or a 9 kVA inductive load — typical for a cooker circuit, a large shower, or a submain feeding a small workshop. The 1+N-pole form switches the phase and disconnects the neutral; the neutral pole is switched but not protected against overcurrent, which is standard for single-phase final circuits in TN-S and TT systems. Breaking capacity Icn = 6 kA per IEC 61009-1: this is the prospective short-circuit current the device can interrupt at its rated voltage. For a domestic final circuit with a 16 A or 20 A MCB upstream, the prospective fault current at the board is usually well under 6 kA — so this rating is adequate for most residential and small commercial boards. If your installation's upstream transformer or service entrance can deliver more than 6 kA at the fault point, you'd need a current-limiting upstream device or a higher-rated breaker.