Definition
Inrush current is the instantaneous peak current that floods into a device the moment it is energized. In a switch-mode AC-DC power supply the type used by every ASIC miner the large input smoothing capacitor is fully discharged at switch-on and momentarily looks like a short circuit to the low-impedance mains. The result is a surge that can be several times the steady-state running current, lasting only a few milliseconds until the capacitor charges.
Why inrush matters when racking miners
A single miner's surge is brief, but energizing many power supplies at once multiplies the combined inrush. That stacked surge can trip a breaker that the running load would never bother, weld PDU relay contacts, or sag the supply voltage to neighboring equipment. This is why breaker selection for high-inrush loads is more involved than simply matching the steady current and why staggering power-on across a fleet, rather than flipping one master switch, protects your panel.
Taming the surge
Designers limit inrush with a soft-start circuit or an NTC (negative-temperature-coefficient) thermistor in series with the input. The cold thermistor presents tens of ohms at switch-on, capping the initial current; as it self-heats its resistance collapses to a fraction of an ohm and steady-state losses become negligible. Time-delay (Type D or motor-rated) breakers also ride through legitimate surges without nuisance tripping.
Account for inrush whenever you size breakers, PDUs, or a UPS. Read it together with soft start and total harmonic distortion for the full power-quality picture.
In Simple Terms
Inrush current is the instantaneous peak current that floods into a device the moment it is energized. In a switch-mode AC-DC power supply the type…