The Siemens 5SU9156-7KK25 is a SENTRON RCD-operated circuit breaker — an RCBO combining overcurrent protection and residual current detection in a single 36 mm wide (2 width units) package. The Type A fault-current detection catches pulsating DC residual currents, which standard AC-only RCDs miss — relevant for circuits feeding electronics, VFDs, or switching power supplies. The higher IEC rating applies when the device is used as a distribution circuit breaker per that standard; the EN rating governs domestic or similar installations. Overvoltage category III means it's rated for fixed-installation downstream of the utility meter, not for socket-outlet or equipment-level protection.
Thermal derating and conductor sizing
At 40 °C it derates to 23.5 A; at 45 °C to 22.75 A; at 50 °C to 21.75 A; at 55 °C to 20.25 A. If the panel internal ambient runs above 30 °C — common in a crowded enclosure — the load current must be reduced accordingly. The operating ambient range is -25 to +45 °C, so the 45 °C derating figure is the maximum continuous current at the top of the rated ambient band. That covers everything from a 1.5 mm² lighting circuit up to a 10 mm² feeder. The installation depth is 70 mm, with an overall depth of 77 mm including the front projection.