The Siemens 5SM1642-6 is a 4-pole residual current circuit breaker (RCCB) from the SENTRON 5SM1 series, rated at 25 A and 300 mA sensitivity, 400 V AC, Type A, instantaneous trip. It mounts on DIN rail (REG) and occupies 4 width units (roughly 72 mm on the rail). The Type A designation means it detects sinusoidal AC fault currents plus pulsating DC fault currents — common in modern equipment with rectified loads like VFDs, switching power supplies, and LED drivers. The 300 mA trip threshold is a standard for general protection in industrial and commercial distribution boards where nuisance tripping from leakage currents must be avoided.
De-rating and ambient limits
The 5SM1642-6 carries its full 25 A rating at 40 °C and 45 °C ambient. Above that, it de-rates linearly: 24 A at 50 °C, 23 A at 55 °C, 22 A at 60 °C, 21 A at 65 °C, and 20 A at 70 °C. In a crowded enclosure with multiple devices shedding heat, check the internal ambient — if it pushes past 45 °C, the available current drops. The part is finger and back-of-hand safe (IP20 with distribution board installed and conductors connected).
Fault current and coordination
This is an instantaneous RCCB (no short-time delay), so it trips within milliseconds of a fault exceeding 300 mA. The permissible I²t value is 70 000 A²·s, which governs the let-through energy the device can handle during a fault. Surge current resistance is rated at 1 kA (8/20 µs waveform typical). For selectivity with upstream devices, note the instantaneous characteristic — it will trip on any fault above threshold, so coordinate with the upstream MCB or fuse to avoid nuisance blackouts on a branch fault.
Installation notes
Mounts on standard DIN rail (DIN 43880 size 1). Installation depth is 55 mm. Mounting position is any. The device is silicon-free, which matters for certain potting or conformal-coating environments. The test circuit requires a minimum of 100 V to operate the test button. No auxiliary contacts or shunt trip are integrated; if you need remote tripping or status feedback, you'll need an external accessory.