Voltage Swell

A voltage swell is a temporary increase in RMS voltage above nominal — the mirror image of a sag. IEEE 1159 defines a swell as a rise to between 1.1 and 1.8 per unit (110 to 180 percent of nominal), lasting from half a cycle to one minute. Swells occur less often than sags, but they are frequently more damaging, because they push components past their voltage ratings rather than merely starving them: insulation, capacitors, and semiconductor junctions all age faster — or fail outright — when driven above spec.

What causes a swell

The most common trigger is a large load switching off abruptly. A heavy load pulls the local voltage down through feeder and transformer impedance; remove it in an instant and the voltage rebounds high until upstream regulation catches up — seconds for fast regulators, longer for mechanical tap changers. On three-phase systems, a single-line-to-ground fault raises the voltage on the two healthy phases while the faulted one sags, so a fault somewhere else on the feeder can arrive at your panel as a swell. Utility capacitor-bank switching for power-factor correction can overshoot momentarily, and lightly loaded feeders at night run chronically high even without an event. Notably, a mining operation can inflict swells on itself: dropping an entire container of ASICs offline at once — a mass curtailment, breaker trip, or emergency stop — removes megawatt-scale load in milliseconds and sends the local bus voltage climbing for everything still connected.

What a swell does to mining hardware

A modern ASIC power supply is a switch-mode design with meaningful input tolerance, and its protection circuits are the first line of defense: the APW-family supplies implement overvoltage protection that shuts the unit down immediately when limits are exceeded, requiring a power cycle to recover. A fleet that mysteriously drops out and needs manual restarts after a grid event is often reporting a swell through its protection behavior. The damage mode, though, is usually cumulative rather than instant — repeated or sustained swells stress input bulk capacitors, MOVs, bridge rectifiers, and EMI-filter components, quietly shortening PSU life until a failure appears months later with no obvious cause. Distinguish the swell from its neighbors: a transient voltage spike rises and falls in microseconds and is a surge-protection problem, while a sustained overvoltage lasting minutes or more is a regulation problem to raise with your utility. The swell sits between them, cycles to a minute, and calls for its own countermeasures.

Practical defense for miners

Start with measurement: a power-quality logger, or even the voltage telemetry many miners and PDUs expose, will show whether swells correlate with specific times, equipment cycles, or utility events. Fix what you control — stagger large load transitions instead of dropping them simultaneously, verify transformer tap settings if your site runs chronically high, and ensure solid grounding and bonding so protective devices behave predictably. Surge protection helps at the margins but is aimed at transients; frequent swells justify line conditioning or a conversation with the utility, armed with your logs. Home miners on a shared panel should note that their own well pump, HVAC compressor, or workshop equipment switching off can produce the small local swells their miner's PSU keeps riding through. Tracking swells alongside voltage sag events gives the fuller picture of feeder health — and turns "the miner keeps tripping" from a mystery into a diagnosis.

Document what you find. A dated log of swell events — magnitude, duration, what tripped and what rode through — is worth more than any single gadget: it distinguishes a utility problem from a site problem, backs a warranty claim when a batch of supplies fails early, and tells you whether the money should go to conditioning equipment or to rewiring a shared circuit. Power quality is one of the few variables in mining you can actually control; treating it with the same rigor as hashrate telemetry pays for itself the first time it catches a failing connection before the smoke does.