The Siemens 5SM9645-6KK is a SENTRON RC unit electronic, a 4-pole residual current device (RCD) with type A fault current sensitivity. It is rated for 63 A at 230/400 V AC, 50 Hz, with a tripping residual current of 300 mA. The instantaneous design means it trips without intentional delay when the residual current exceeds the threshold, making it suitable for direct protection against earth faults in TN and TT systems.
Current rating and thermal derating
The 5SM9645-6KK carries a rated current of 63 A at AC, but the actual continuous current it can handle depends on ambient temperature. At 40 °C and 45 °C it holds the full 58.59 A; at 50 °C it derates to 56.7 A, at 55 °C to 54.81 A, at 60 °C to 53.55 A, at 65 °C to 52.29 A, and at 70 °C to 50.4 A. This derating curve is essential for panel builders sizing the device in a warm enclosure — the 63 A label is a reference condition, not a blanket rating for all installations.
Panel integration
As a 4-pole DIN-rail mounted RC unit, the 5SM9645-6KK snaps onto a standard 35 mm DIN rail in the distribution board or sub-panel. The 300 mA sensitivity is typical for general-purpose earth leakage protection where nuisance tripping from capacitive leakage currents (e.g., from long cable runs or electronic loads) must be avoided, while still providing personnel protection for indirect contact.