Voltage Sag Analysis: Locating Faults in Industrial Networks

The Voltage Sag Problem

A voltage sag - a short reduction in voltage lasting from one half cycle to a few seconds - is the most common power quality disturbance affecting industrial production. Sags as small as 30% for 100 ms can drop out PLCs, contactors, and variable speed drives, causing line stoppages worth thousands of dollars per minute.

Capturing Sags Accurately

A Class A power quality analyzer with 1 ms resolution captures the full sag profile: pre-event voltage, residual voltage, duration, and recovery shape. The instrument timestamps each event with GPS-synchronized accuracy, allowing correlation across multiple measurement points.

The ITIC / CBEMA Curve

The ITIC curve maps acceptable voltage envelopes for IT equipment. Plot each captured sag on the curve - events below the lower bound are likely to disrupt operations. This single chart provides the strongest argument for installing UPS, ride-through capacitors, or dynamic voltage restorers.

Locating the Source

• Compare sag magnitudes at multiple measurement points - the deepest sag is closest to the fault.
• Examine current direction during the sag - inflow indicates an upstream source.
• Correlate timing with switchgear operations, motor starts, or weather events.
• Check utility data for matching transmission faults at the same instant.

Mitigation Recommendations

Once sources are identified, options range from coordination changes (faster upstream protection) to ride-through hardware. Document each event with the analyzer report and present a quantified loss figure to justify capital investment.