1. Definition
Voltage sag, also called voltage dip, is a condition where voltage drops temporarily from its normal value and then returns after the disturbance clears. Unlike a power interruption, the supply is still present, but the voltage is low enough that equipment may not operate normally.
2. Common Causes
Common causes include faults or short circuits on the network, large motor starting, transformer energizing, large load switching, feeder disturbances, and weak distribution systems. A sag may originate from inside the plant or from the utility network outside the plant.
3. Impact on Equipment
Equipment such as variable speed drives, PLCs, industrial computers, contactors, relays, UPS units, and control systems can be sensitive to voltage reduction. The impact may include trips, resets, stopped production lines, rejected products, or long restart time.
4. Simple Comparison
| Disturbance | Voltage Condition | Duration | Typical Impact |
| Voltage Sag | Voltage drops | Brief to < 1 minute | Drive trip / PLC reset |
| Interruption | Voltage is lost | Brief or long | Total equipment shutdown |
| Voltage Swell | Voltage rises | Brief | Overvoltage risk |
| Transient | Very fast spike | us-ms | Electronic disturbance |
5. Detection and Mitigation
Analysis is carried out using a power quality analyzer to record voltage, current, event time, duration, and sag depth. Mitigation can include power quality monitoring, protection coordination, soft starter/VFD for large motors, UPS/DVR/sag compensator for sensitive loads, and evaluation of transformer, cable, and internal distribution capacity.
Summary: Voltage sag is a temporary voltage drop that can cause sensitive equipment to trip, reset, or stop operating. This disturbance is usually brief, but its impact can be significant in industrial processes.
Conclusion: Voltage sag is not a complete power outage, but a temporary voltage drop that can disturb sensitive equipment. With proper monitoring and mitigation, the risk of trips and operational disturbance can be reduced.