Summary: preventing sag trips
Preventing voltage sag trips has to start on the equipment side, because lightning, line faults, and equipment failures at neighboring customers can't be avoided. We recommend five steps in order: 1) identify which equipment trips on sags; 2) adjust protection settings on undervoltage relays, VFDs, and similar devices; 3) harden control components; 4) add voltage sag compensators only on control circuits that reset, and use a UPS only for loads that must survive outages; 5) require SEMI F47 compliance for new equipment and monitor and record every sag.
Why can't voltage sags be "prevented," only ridden through?
A voltage sag is a short drop in voltage to 10–90% of nominal followed by recovery; in Taiwan's science parks, typical sags last only about 0.1 s. Most originate outside the plant: lightning and high winds, bird contact, excavation damage, equipment failures at neighboring customers, and substation faults. According to Taipower, the Southern Taiwan Science Park had 13 voltage sags in 2020 alone (Commercial Times, 2021/4/16).
A factory can't control the grid, only its own equipment. So "preventing voltage sags" really means this: in the 0.1 s when the voltage drops, your equipment doesn't stop with it. For the definition, causes, and real incidents, see What is a voltage sag.
Step 1: Find out which equipment trips on sags
You don't need to protect the whole plant from day one. When a sag hits, usually only a few kinds of equipment trip, and often the same model trips together. Starting with those costs the least and shows the clearest results.
No power quality measurements? That's fine. You can line up the timing from equipment alarm logs, downtime reports, maintenance work orders, the facility power monitoring system, and input voltage event logs from existing UPS units. If the lights flickered and tools tripped at the same moment, it was almost certainly a voltage sag.
5 ways to prevent voltage sag trips (from lowest to highest cost)
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Adjust protection settings
Many trips happen not because equipment "can't hold on," but because protection settings "react too fast." An undervoltage relay trips the moment it detects low voltage; a VFD stops as soon as its DC bus voltage dips. After confirming with the equipment maker, adding a suitable delay to the undervoltage relay and enabling the VFD's momentary-power-loss restart is often enough to ride through a 0.1 s sag.
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Harden control components
Magnetic contactors drop out when voltage is low, and control power supplies have very short hold-up times; these two are common sources of tool resets. You can add coil hold-in devices to contactors, or switch to control power supplies with a wider input voltage range. For how to improve each component, see "Older equipment fails SEMI F47. How do you improve it?"
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Add sag compensators to controls
A voltage sag compensator (also called a sag protector; common names include VSP and DySC) stays out of the way normally. When it detects a sag, it switches in about 1 ms and uses stored energy to hold up the voltage for several seconds. The key is to connect only the control circuits that reset. Pumps, heaters, and motors ride through on inertia, so the capacity can be small: a 100 kVA tool usually needs only a few kVA (how to size it).
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Add an online UPS
An online UPS supplies power continuously through double conversion, so it covers both sags and outages, the most complete protection available. The cost: running 24 hours a day loses about 3–6% of power continuously, lead-acid batteries need replacing every 3–5 years, and it takes up a lot of space. It suits loads that must keep running even during an outage, such as servers and critical process data systems.
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Require SEMI F47 for new equipment
SEMI F47 requires semiconductor equipment not to stop when voltage drops to 50% for 0.2 s, 70% for 0.5 s, or 80% for 1 s. Writing it into specs for new purchases builds ride-through in at the source. But note: F47 covers only the main equipment. When voltage drops below 50%, phases are unbalanced, or sags occur back to back, F47-compliant tools can still stop (for the test procedure, see "What is SEMI F47").
Compensator, UPS or settings?
| Method | Covers sags | Covers outages | Standby power use | Maintenance | Best for |
|---|---|---|---|---|---|
| Tune settings | Partly | No | None | None | Undervoltage relays, VFDs |
| Harden controls | Partly | No | None | Low | Contactors, power supplies |
| Sag compensator | Yes; ~1 ms, holds seconds | No | Very low (bypass) | No batteries (supercaps) | Control circuits |
| Online UPS | Yes, 0 ms | Yes (minutes+) | About 3–6% | Batteries every 3–5 yrs | Must-run loads |
| SEMI F47 equipment | Within spec | No | None | None | New fab tools |
For a full technical comparison of protection equipment, see "How to choose voltage sag protection." Sag compensator figures are based on a supercapacitor-type sag protector; switching and compensation times vary by product, so check the datasheet when buying.
Which to choose?
- Must keep running during outages→Online UPS or generator
- Only brief sags; settings adjustable→Adjust settings first
- Only brief sags; only controls reset→Compensator on controls
- Critical loads already on online UPS→No compensator needed
- Buying new tools→Specify SEMI F47
Final step: set up monitoring and record every sag
Prevention isn't a one-time job. Record when each sag happened, on which phase, how deep, and for how long; only then can you tell whether protection works and which tool to protect next. You can install a power quality analyzer at the main switchboard; if you already have sag compensators, many models log the time and waveform of every sag themselves. With records in hand, you can also ask your local Taipower district office about supply events at the time.
FAQ
Besides a UPS, what else can stop sag-related downtime?
Besides a UPS, there are four approaches, and most cost less: 1) adjust undervoltage relay and VFD protection settings; 2) harden control components such as contactors and control power supplies; 3) add voltage sag compensators (sag protectors) only on control circuits that reset; in normal operation they run in bypass, draw almost no power, and need no battery replacement; 4) require SEMI F47 compliance for new equipment. A UPS is best for loads that can't go down even during an outage.
Why do undervoltage relays make tools trip during voltage sags?
An undervoltage relay's job is to cut the circuit when voltage is too low. Without a delay setting, even a 0.1 s sag makes it act immediately, and the tool stops. Adding a suitable delay, after confirming with the equipment maker, is the cheapest prevention. But the delay can't be stretched indefinitely, or the relay loses its protective purpose.
How is a voltage sag compensator different from a UPS?
A sag compensator is built for momentary sags lasting a few seconds and runs in bypass normally, drawing almost no power. A UPS covers both sags and outages, but an online UPS's 24-hour double conversion loses about 3–6% of power continuously, and its batteries need replacing every 3–5 years. For control circuits that only need sag protection, not outage protection, a compensator is the better value.
Do homes or offices need voltage sag prevention?
Homes usually don't; a sag at most makes the lights flicker. Office servers and network gear can't afford data interruptions, and a UPS is generally enough. What really needs systematic prevention is factory production lines, because one trip takes hours to recover from and can scrap work in process.
Which equipment should I start with?
Start with the tool that tripped most often over the past year when "the power blinked." Cross-check equipment alarm logs, downtime reports, and UPS event logs to find the models that trip repeatedly, protect one first, observe the results, and expand once it's proven.