Definition
A voltage swell is the inverse of a sag: a temporary increase in RMS voltage above nominal. IEEE 1159 defines a swell as a rise to between 1.1 and 1.8 per unit, lasting from half a cycle to one minute. Swells are less frequent than sags but can be more damaging to equipment because they push components past their rated voltage.
What causes a swell
The most common cause is a large load suddenly switching off, which removes the voltage drop it was creating and lets the supply rebound high until regulation catches up. On three-phase systems, a single-line-to-ground fault can raise voltage on the two unfaulted phases. Switching of capacitor banks for power-factor correction can also momentarily overshoot.
Effect on miners
Switch-mode ASIC supplies clamp and absorb modest overvoltage, but sustained or repeated swells stress input capacitors, MOVs, and rectifier diodes, accelerating failure. Where swells are frequent, surge protection and proper grounding matter as much as they do for transient spikes. A swell differs from a slow, sustained overvoltage and from the momentary transient voltage spike, which rises and falls in microseconds rather than cycles.
Operators tracking swells alongside voltage sag events get a fuller picture of feeder health and can justify investment in conditioning equipment. See also power quality for the broader framework.
In Simple Terms
A voltage swell is the inverse of a sag: a temporary increase in RMS voltage above nominal. IEEE 1159 defines a swell as a rise…