Summary: what is a voltage sag?
A voltage sag is when the RMS supply voltage suddenly drops below 90% of nominal (but stays above 10%) for 0.5 cycles to 1 minute and then recovers; below 10% counts as an outage. Most sags are caused by short circuits outside the plant, such as lightning, bird contact, excavation damage, or equipment failures at neighboring customers, and the factory can't prevent them. Typical sags in Taiwan's science parks last only about 0.1 s, but that's enough to make a tool's control power, PLC, and magnetic contactors drop out and reset, causing hours of downtime.
What is a voltage sag? Power stays on, voltage drops
Under the IEEE 1159 definition, a voltage sag is a decrease in RMS voltage to 10–90% of nominal lasting half a cycle to 1 minute. Taipower's definition is the same: RMS supply voltage suddenly falls below 90% of nominal for 0.5 cycles to 1 minute (Taipower FAQ: "What is a voltage sag?").
Chinese has many names for it, all meaning the same thing; in Europe and IEC standards it's usually called a voltage dip. The percentage refers to "how much voltage remains": a sag "to 70%" on a 220 V system means the voltage falls to about 154 V, not that it drops by 70%.
| Event | Voltage | Duration | What you see |
|---|---|---|---|
| Voltage sag | 10–90% of rated remains | Half a cycle to 1 minute | Lights flicker; sensitive gear trips |
| Interruption | Below 10% | ½ cycle to hours | Lights out; all stops |
| Swell | Above 110% | Half a cycle to 1 minute | Rare; can damage electronics |
| Undervoltage (sustained) | Below 90% | Over 1 minute | Motors overheat |
IEEE 1159 further divides sags by duration: 0.5–30 cycles is instantaneous, 30 cycles to 3 s is momentary, and 3 s to 1 minute is temporary (Power Monitors, Understanding IEEE 1159 Definitions).
One more term that often gets mixed up: "voltage drop" in electronics, such as a diode drop or line drop, is the voltage difference across a component or wire when current flows. It's a steady circuit phenomenon and has nothing to do with a voltage sag on the grid.
6 common causes of voltage sags
Most voltage sags come from a short circuit somewhere on the grid: voltage near the fault is pulled down instantly and recovers only when protection equipment isolates the fault. Every customer on the same grid feels it, so even if nothing went wrong at your plant, you still get the sag.
1. Lightning, weather
Strikes, falling trees and salt damage short lines and drag down the nearby grid.
2. Birds and animals
Birds and animals short supply equipment — a cause Taipower cited at STSP.
3. Digging damage
Roadworks cut or snag cables — another STSP cause.
4. A neighbor's fault
A neighbor's transformer fails and you sag too — as at Hsinchu in 2022.
5. Utility equipment
Substation or cable faults caused the 2023 STSP and 2025 Hsinchu and Kaohsiung sags.
6. Big motors starting
Inrush from motors, compressors and chillers dips plant voltage — fixable with soft starters or VFDs.
Inside or outside the plant?
Check two things. First, does the trip time line up with a large piece of equipment starting in the plant? If so, it's probably internal. Second, did several tools, or even the factory next door, have problems at the same time, with the lights flickering? If so, it's almost always a grid-side sag. For diagnosing trips during thunderstorms, see "Tools trip in thunderstorms: surge or sag?"
Why can a 0.1-second sag stop a tool for hours?
Pumps, heaters, and motors ride through 0.1 s on inertia. What can't hold on is the control system: AC control relays, magnetic contactors, control power supplies, and PLCs can drop out and reset within milliseconds. Once the control system resets, the whole tool alarms and stops, followed by restart, inspection, and requalification, and wafers or work in process may be scrapped.
So the loss from a sag usually isn't in that 0.1 s but in the recovery time afterward. It also means you don't need to protect the whole tool, just the few components that can't hold on. For how, see "Older equipment fails SEMI F47. How do you improve it?"
Science park sag timeline
Below are science-park sag incidents with clear dates and locations from Taipower statements and news reports. These are only the ones that made the news; many more sags go unreported.
13 sags in a year at STSP
According to Taipower, the Southern Taiwan Science Park had 13 voltage sags in 2020 and 3 more by mid-April 2021, caused by bird contact, customer equipment failures, and excavation damage. In the same period, one park company said its own count was "18 in one quarter," so the figures don't match.
Sources: Commercial Times, 2021/4/16; Liberty Times, 2021/4/15Hsinchu Science Park
A main transformer owned by a 161 kV extra-high-voltage customer failed inside its plant, disturbing voltage on the nearby grid; the sag lasted about 0.1 s. Taipower's supply was not interrupted, but some customers' equipment tripped as protection kicked in.
Source: Taipower press release, 2022/3/2Southern Taiwan Science Park
A 161 kV GIS disconnect switch failed at Taipower's Fenghua substation, causing voltage sags for some companies in the Tainan and Kaohsiung parks. UMC confirmed that some wafers were scrapped and some tools had to be restarted.
Source: CNA, 2023/6/1Hsinchu (New Year's Eve)
An equipment fault at Taipower's Longtan substation tripped a 345 kV transmission line, causing a brief voltage sag but no outage for some Hsinchu Science Park customers. Tool alarms went off at several semiconductor fabs, and engineers were called back on Lunar New Year's Eve.
Source: TVBS News, 2025/1/29Kaohsiung parks
A tie cable at Taipower's Gaogang extra-high-voltage substation failed during energization, causing voltage sags across Kaohsiung. The Qianzhen and Nanzi technology industrial parks and the STSP Kaohsiung Park were affected, and equipment tripped at major customers including the high-speed rail, the Kaohsiung MRT, and the water utility.
Source: TVBS News, 2025/5/9
Compiled by DEYly. For event details, refer to the cited reports and Taipower statements; new public incidents will be added.
Can sags be prevented?
Factories can't stop grid-side sags; what they can do is help equipment ride through. From lowest to highest cost, common approaches include adjusting undervoltage protection settings, hardening control components, adding sag protectors on control circuits, installing an online UPS, and requiring SEMI F47 for new equipment. For how to choose, see "How to prevent voltage sag trips in a factory" and "How to choose voltage sag protection."
FAQ
Are voltage sag, voltage dip, and momentary sag the same thing?
Yes. All refer to a short drop in voltage to 10–90% of nominal followed by recovery; "voltage sag" is common in the US, and "voltage dip" in Europe. On the shop floor, people also say "the power blinked."
What's the difference between a voltage sag and an outage?
Whether the power is cut. During a sag, 10–90% of the voltage remains and supply continues; only below 10% does it count as an outage (interruption). Sags usually last under 1 second; outages can last seconds to hours.
How long does a voltage sag usually last?
Under IEEE 1159, anything from half a cycle to 1 minute counts. Typical sags in Taiwan's science parks last only about 0.1 s; for example, Taipower said the 2022 Hsinchu Science Park sag lasted about 0.1 s.
What causes voltage sags?
There are six common causes: lightning and weather, bird or animal contact, excavation damage, equipment failure at a neighboring customer, utility equipment failure or aging, and large equipment starting inside the plant. The first five come from outside the plant; the factory can't prevent them, only help equipment ride through.
Is a voltage sag the same as a "voltage drop" in electronics?
No. In electronics, a voltage drop is the voltage difference across a diode, resistor, or wire when current flows, such as "a 0.7 V diode drop"; it's a steady phenomenon. A voltage sag is a power quality event in which grid supply voltage briefly drops and then recovers.
Can sags be prevented?
Factories can't prevent grid-side sags; they can help equipment ride through: require SEMI F47 for new equipment, add sag protectors or a UPS on control circuits that reset, and use power quality monitoring to record every sag.