Momentary Power Outage: Causes, Damage, and Factory Countermeasures

In Korea, the term “momentary power outage” (순간정전) commonly refers to a short voltage sag: the supply voltage falls below its normal level for a fraction of a second up to a few seconds, then recovers. Engineers also call it a voltage sag or momentary voltage dip. The power is never fully lost, but sensitive equipment can still stop.

Consider a night shift on a production line. A robot halts and the HMI goes dark, yet the utility log shows no outage. A lightning strike on the upstream network caused a brief fault, and a breaker opened and reclosed. People barely noticed the flicker. The control electronics did not miss it.

Causes of momentary power outages

Most momentary outages begin outside the plant, on the utility network. Knowing the source helps identify which equipment is most exposed.

• Lightning strikes on transmission or distribution lines cause a sudden voltage collapse.
• Earth and short-circuit faults create fault current that pulls the voltage down until the fault is cleared.
• Breaker and recloser operation isolates a fault, then restores supply, and the voltage dips during each step.
• Large load starts, such as high-capacity motors or welders, can depress voltage on the shared feeder.

Most of these events are never recorded. The utility log shows no outage, and the plant logs an “unexplained trip.” Once voltage is actually measured, repeated dips on the same feeder often come to light.

Little visible damage, real downtime

The damage is easy to miss. Lighting may flicker, but people rarely register it. The real impact falls on power-electronic equipment.

• Inverters trip when the DC bus voltage leaves its allowed window.
• Servo drives and PLCs can reset or raise alarms when the control supply drops briefly.
• Restart time adds up. Re-energizing, homing axes, verifying the line, and restoring process settings can take tens of minutes.

In fabs and display lines, where an interrupted process can scrap work in progress, restart time becomes the larger loss. Equipment tolerance varies. Per SEMI F47, semiconductor equipment should ride through a voltage at 80% for 1 second, 70% for 0.5 seconds, and 50% for 0.2 seconds. The ITIC (CBEMA) curve shows the published tolerance envelope many equipment types are designed to meet. Without knowing a machine’s tolerance, you cannot size a countermeasure.

Start countermeasures with load classification

A common mistake is trying to protect the entire plant at once. Costs rise sharply, and the benefit is concentrated in the sensitive loads anyway. A staged approach keeps the design focused.

1. List the loads. Record ratings, control supply, and restart time for each machine.
2. Classify by sensitivity. Separate loads that stop on a brief dip from those that do not.
3. Group only the sensitive loads. Put inverters, servo drives, PLCs, and control panels on dedicated circuits.
4. Size compensation to the event. Match capacity and ride-through duration to the expected depth and length of the sag.

The WESCO TSP® is a voltage sag compensator designed to fit this kind of load-classified layout. It targets low-voltage (LV, 1 kV and below) circuits, typically the branch feeding sensitive loads. It uses EDLC ultracapacitors, so it is battery-free, and its energy storage is designed for a 10-year service life. Transfer time is within 2 ms, and compensation lasts 1 to 3 seconds. It is a battery-free alternative to battery UPS systems for this purpose, not a UPS itself.

Key takeaways

• A momentary power outage is a short voltage sag that recovers without a full blackout. It is the same phenomenon as a voltage sag.
• Lightning, earth and short-circuit faults, breaker operation, and large motor starts are the main causes.
• The first casualties are inverters, servo drives, and PLCs, and restart can take a long time.
• Classify loads by sensitivity first, then apply voltage sag compensation only where it matters.

Frequently Asked Questions

Q. Is a momentary power outage the same as a voltage sag?

Yes. Momentary power outage, voltage sag, and momentary voltage dip describe the same event: the voltage falls below its normal level for a short time and then recovers. Unlike a sustained outage, the supply is never fully lost.

Q. What causes a momentary power outage?

The main causes are lightning strikes, earth and short-circuit faults, circuit breaker and recloser operation, and large load starts. A fault anywhere on the network can affect every facility sharing that supply line.

Q. How should a factory plan countermeasures for voltage sags?

Start by classifying loads by sensitivity. Identify the equipment, such as inverters, servo drives, and PLCs, that stops during a brief dip, then apply compensation only to those loads. This keeps both cost and effort proportionate.