Definition
A voltage sag (also called a dip) is a short-duration decrease in RMS voltage. Under IEEE 1159, a sag is defined as a reduction to between 0.1 and 0.9 per unit of nominal voltage, lasting from half a cycle up to one minute. It is the single most common power-quality disturbance, and it is the one that most often causes mining hardware to trip offline without an obvious fault.
What causes a sag
Sags are usually caused by large loads starting up or by faults elsewhere on the distribution network. When a motor, transformer, or another miner energizes, the inrush current pulls voltage down briefly across shared wiring. A distant short circuit can also depress voltage across a wide area for a few cycles until protective devices clear it.
Effect on miners
An ASIC power supply rides through shallow, brief sags using stored energy in its capacitors. Deeper or longer sags push the input below the supply's undervoltage threshold, causing the unit to drop the load and the miner to reboot. Repeated sag-induced restarts add thermal cycling to the hashboards and can corrupt in-progress work. Voltage sags are distinct from a voltage swell, which is the opposite event, and from a sustained low-voltage power quality condition.
Logging sag frequency and depth helps decide whether a site needs conductor upgrades, staggered start sequencing, or ride-through equipment before adding hashrate.
In Simple Terms
A voltage sag (also called a dip) is a short-duration decrease in RMS voltage. Under IEEE 1159, a sag is defined as a reduction to…