If you’ve ever watched a production line mysteriously trip, a building management system reboot for no apparent reason or LED lighting flicker like it’s trying to start a disco, there’s a solid chance you were dealing with sags and swells. They’re two of the most common and most misunderstood power quality disturbances. They can quietly chew through uptime, equipment life and maintenance budgets.
Let’s break down what sags and swells actually are, what causes them, how to measure them properly and what you can do about them.
What Are Voltage Sags and Swells?
In plain terms, sags and swells are short-duration changes in voltage level. A voltage sag is a temporary drop in RMS voltage. A voltage swell is a temporary rise in RMS voltage. They’re not the same as a blackout (interruption) or a long-term over-voltage or under-voltage condition. Think of them as quick punches to your electrical system.
Sometimes they’re so fast you don’t notice them but your equipment absolutely does. Most standards and meters describe these events using two characteristics: magnitude (how far the voltage moved from nominal) and duration (how long it lasted from cycles to seconds).
That’s why the same site can experience frequent tiny “nuisance” events or rare but brutal ones that take systems down.
Why Sags Happen
Sags are far more common than swells. They typically show up when something demands a large inrush of current or when the upstream network experiences a fault and voltage dips while protection devices do their thing.
Common causes include starting large motors such as HVAC systems, pumps, compressors and lifts. Welding equipment and other high inrush or rapidly varying loads also trigger sags. Short circuits and faults on the utility network or within a facility are frequent culprits. Transformer energisation and switching events create temporary voltage drops as do overloaded feeders or undersized conductors creating excessive voltage drop.
Here’s the kicker: a sag doesn’t need to be dramatic to cause trouble. Many modern devices including PLCs, variable speed drives, servers and control power supplies are sensitive to even brief dips. This is especially true if they’re already operating near their tolerance threshold.
Why Swells Happen
Swells are less frequent but they can be particularly problematic for insulation stress and component wear.
Typical causes include sudden load reduction, where voltage rises because the system is momentarily lightly loaded. Capacitor bank switching or incorrect reactive power compensation can create swells. Single-line-to-ground faults can cause voltage rise on the unfaulted phases in some systems.
Neutral issues, particularly in multi-tenant or mixed single-phase and three-phase environments, are another source. Distributed generation and solar interactions can also produce swells in certain network conditions.
Swells can push equipment beyond design limits long enough to degrade power supplies, trigger protective trips or cause overheating. The problem intensifies when swells combine with harmonics or poor earthing.
What Sags and Swells Do to Equipment and Operations
The impacts of sags and swells depend on your load types and how your site is designed but the patterns are familiar. You’ll see trips and resets affecting drives, PLCs, relays, BMS controllers and UPS transfers.
Data issues emerge such as server reboot loops, corrupted logs and communications dropouts. Process disruption follows, including product loss, rework, downtime and safety interlocks. Accelerated wear affects power supplies, contactors, insulation and capacitors.
Perhaps most frustrating are the mysterious “intermittent” faults that disappear when someone shows up with a clipboard. One of the most frustrating aspects is that facilities teams often only see the symptom (“it tripped”) rather than the electrical event that triggered it. That’s why measuring correctly matters.
Measuring Sags and Swells: What Good Looks Like
To diagnose sags and swells, you need monitoring that captures events accurately and consistently. In the real world, that means logging voltage RMS values at suitable intervals and capturing event waveforms when needed. You’ll want time-synchronised records so you can correlate cause and effect, plus event classification aligned to recognised standards.
For high-confidence compliance and forensic work, many organisations aim for Class A power quality measurement aligned with IEC 61000-4-30 Class A. This is commonly treated as the gold standard approach for consistent power quality measurement. Without that consistency, two different meters can “disagree” about the same event. This makes it harder to prove whether the issue is internal, upstream or tied to a specific asset.
Troubleshooting: How to Connect Events to Real Causes
If you’re seeing sags and swells, don’t jump straight to buying mitigation gear. Start with evidence. First, confirm what’s happening. Are you seeing sags, swells or both? On which phases? How deep? How long?
Next, correlate with site activity. Do events line up with motor starts, lifts, chiller staging, capacitor steps or tenant demand peaks? Check where the event originates. A meter at the main incomer tells you something very different from a meter at a critical distribution board.
Review protection and settings as well. Sometimes the “disturbance” is normal but your ride-through settings are too tight (for example, drives or control power supplies). A small amount of high-quality monitoring usually beats a large amount of low-quality guessing.
Mitigation Options
The best fix depends on whether the disturbance is coming from inside your facility or upstream. Options commonly include:
- Ride-through improvements such as UPS, DC bus support and control power conditioning.
- Soft starters or VSD tuning can reduce motor inrush and nuisance sags.
- Reactive power compensation tuning helps avoid capacitor-induced swells.
- Segregation of sensitive loads through dedicated feeders and improved distribution design can protect critical equipment.
If events originate upstream, you need solid records to escalate effectively through network investigation. The goal isn’t to eliminate every event (that’s rarely practical). It’s to reduce the events that cause cost: downtime, trips, damage and complaints.
How SATEC's Products Become the Metering Solution for Sags and Swells
To tackle sags and swells properly, you need two capabilities working together: accurate power quality metering at the edge and clear analytics and reporting that turns events into decisions.
SATEC’s portfolio is built around exactly that. SATEC offers dedicated power quality analysers and advanced meters designed to monitor voltage events and broader power quality metrics. This means you can move from “we think it dipped” to “here’s the event, time-stamped and quantified.”
Options include compact panel devices for switchboards, DIN-rail meters where space is tight and advanced revenue-grade meters with power quality features for deeper investigation. On top of that, SATEC provides NMI-approved electricity meters which support projects where compliant metering is non-negotiable.
When it’s time to turn raw data into action, Expertpower brings monitoring, analytics and reporting into one place. This means facilities teams, contractors and stakeholders can align on what happened, when it happened and what to do next.
Put simply: SATEC’s approach helps you detect, document and diagnose sags and swells. You can then use that evidence to justify the right mitigation, whether that’s internal changes or a network-level escalation.
Sags and Swells Key Takeaways
Sags and swells are common, costly and often invisible unless you measure them properly. Once you can see what’s happening (magnitude, duration, frequency and location), solutions become a lot more straightforward and a lot less expensive than trial-and-error.
If your site is dealing with nuisance trips, unexplained resets or tenants complaining about “power issues,” start with measurement you can trust and build your fix from there.
Talk to us about your power quality challenges.
FAQs - Sags and Swells
What’s the difference between sags and swells?
A sag is a short-term dip in voltage, while a swell is a short-term rise above normal voltage levels.
Are sags and swells caused by the utility or by my site?
Both are possible. Sags often come from motor starts or network faults, while swells can occur from load drops, capacitor switching or neutral issues.
Why do sags and swells trip equipment even when the power doesn’t go out?
Many devices (like PLCs, VSDs, and control power supplies) have limited voltage tolerance, so even brief voltage changes can trigger resets or protective trips.
How do I prove sags and swells are happening (and when)?
You need power quality monitoring that time-stamps and records voltage events so you can correlate them to plant operations or upstream disturbances.
