Sags, Swells, and Transients
Short-duration voltage disturbances can crash sensitive equipment, especially SCADA and instrumentation, even when actual power never fails.
A voltage sag (brief drop, typically a few cycles) commonly happens when a large motor starts on the same feeder; VFDs and UPS systems protect sensitive equipment from crashing during sags.
A voltage swell (brief rise) is less common but can come from sudden load shedding or transformer switching; surge protection devices absorb these for sensitive electronics.
Transient spikes from lightning or switching events can damage instrumentation and SCADA inputs; surge protectors and good grounding practices are the typical defense.
If your SCADA system loses inputs or reboots randomly, suspect power quality at the panel feeding it — a UPS at the panel is often a low-cost effective fix.
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