A voltage sag is a brief drop in electrical voltage, typically lasting anywhere from half a cycle (about 8 milliseconds on a 60 Hz system) to one minute. During a sag, the voltage falls to somewhere between 10% and 90% of its normal level, then recovers on its own. It’s the most common power quality disturbance in both homes and industrial facilities, and it’s the usual culprit when your lights dim for a split second or your router mysteriously reboots.
How Standards Define a Voltage Sag
The two major standards bodies use slightly different definitions. IEEE Standard 1159 defines a voltage sag as a decrease in RMS voltage to between 0.1 and 0.9 per unit (10% to 90% of normal) lasting from 0.5 cycles to 1 minute. The IEC, used widely outside North America, calls the same event a “voltage dip” and defines it more broadly as a temporary reduction below a threshold, with a duration from half a cycle to a few seconds. The terms “sag” and “dip” are interchangeable. IEEE prefers “sag,” IEC prefers “dip,” but they describe the same phenomenon.
The key distinction is that a voltage sag is not an outage. The power never fully disappears. It drops, holds at a reduced level for a fraction of a second to several seconds, then snaps back. That’s what separates a sag from a complete interruption, and it’s also what makes sags tricky: they’re brief enough that you might not notice them, yet deep enough to disrupt sensitive equipment.
What Causes Voltage Sags
The most common cause is a fault somewhere on the power grid. When a short circuit occurs on a transmission line or distribution feeder, it temporarily pulls voltage down across a wide area before protective equipment clears the fault. According to data from the EPRI Distribution Power Quality project, nearly one-third of the voltage sag problems experienced by one monitored industrial customer were caused by faults on the transmission system or on distribution feeders upstream from the substation serving them. In other words, a fault miles away on a completely different circuit can still sag your voltage.
The large majority of utility faults are single line-to-ground faults, often triggered by weather. Wind blows a tree branch into a power line, contamination builds up on insulators, an animal contacts equipment, or a vehicle strikes a utility pole. Three-phase faults are less common but produce more severe sags when they do occur. Sags also happen when a fuse blows to isolate a fault, briefly depressing voltage in the surrounding area.
Inside a building, the most typical cause is a large motor starting up. When a big air conditioning compressor or industrial motor kicks on, it draws a surge of current that can pull voltage down for a second or two. These motor-start sags usually last longer than 30 cycles but tend to be shallow if the electrical system is properly sized.
Signs of a Voltage Sag at Home
Voltage sags show up in everyday ways that are easy to miss or misidentify. The most common sign is lights dimming for a split second. LEDs are particularly sensitive, so a sag can look like your lights are “winking” rather than smoothly dimming. If you notice it happening when your air conditioner starts, that’s a classic motor-start sag.
The second telltale sign is electronic devices rebooting without explanation. Your TV, streaming box, or Wi-Fi router restarts as if someone had unplugged it for a moment. The sag dips low enough that the device’s internal power supply can’t sustain operation, so it cycles off and back on. If your router seems to drop your connection at random moments, especially during storms or when large appliances cycle, a voltage sag is the likely cause.
Why Industrial Equipment Is Especially Vulnerable
In factories and processing plants, voltage sags cause far more than flickering lights. They can shut down entire production lines. The equipment most vulnerable includes process controllers, programmable logic controllers (PLCs), control relays, and contactors. As industrial controls have grown more sophisticated, they’ve actually become more sensitive to power disturbances, not less. EPRI testing found that a newer model PLC was significantly more sensitive than an older model from the same manufacturer. The older unit could ride through a complete loss of voltage for nearly 11 cycles before tripping, while the newer model tripped at voltage levels between 55% and 60% of normal, even for very short sags.
This matters because a single tripped PLC can halt a process that takes hours to restart. In semiconductor manufacturing, food processing, or chemical production, a fraction-of-a-second voltage sag can ruin an entire batch of product. The financial cost of these disruptions often dwarfs the cost of the sag event itself.
Not all equipment fails at the same threshold. EPRI testing showed that some controllers had no problems with sags shorter than 10 cycles (the most common type), while others tripped on any sag below 60% of nominal voltage regardless of duration. This inconsistency makes it difficult to predict which devices will survive a given sag without testing them specifically.
How Deep and How Long Matters
A voltage sag has two defining characteristics: depth (how far the voltage drops) and duration (how long it stays low). A shallow sag to 85% of normal lasting a few cycles is very different from a deep sag to 20% lasting half a second, even though both technically qualify as sags.
Most residential and office electronics can tolerate brief, shallow sags without any visible effect. Problems typically start when voltage drops below about 70% of normal for more than a few cycles, or below 50% for even a single cycle. The industry uses a standard tolerance curve (called the ITIC curve, developed by the Information Technology Industry Council) that maps out the voltage and time combinations that typical computing equipment should be able to handle. Equipment that can “ride through” sags within those boundaries is generally considered robust enough for normal grid conditions.
Protecting Against Voltage Sags
The right protection depends on what you’re protecting and how severe your sag exposure is.
- Uninterruptible power supplies (UPS): The most familiar option. A UPS contains a battery that kicks in when voltage drops, covering sags, surges, and complete outages. This makes a UPS ideal for servers, network equipment, and any device where even a momentary interruption is unacceptable. For home users, a basic UPS on your router and computer handles the vast majority of sag-related annoyances.
- Active voltage conditioners (AVCs): Designed for larger-scale protection in commercial and industrial settings. An AVC corrects voltage sags and surges within a few milliseconds, handling loads from 150 kVA up to 2.4 MVA. The key difference from a UPS is that an AVC has no battery, so it cannot provide power during a complete outage. It’s a cost-effective choice when your main problem is sags and surges rather than full blackouts.
- Constant voltage transformers (CVTs): Also called ferroresonant transformers, these use magnetic energy storage to maintain a steady output voltage even when input voltage fluctuates. EPRI testing found that CVTs significantly improved sag ride-through for sensitive PLCs. They’re effective for protecting individual pieces of equipment with relatively small power demands.
For industrial facilities with frequent sag-related shutdowns, the first step is usually power quality monitoring to establish how often sags occur, how deep they are, and which equipment is actually tripping. That data determines whether the best approach is protecting individual sensitive devices, conditioning power at the panel level, or working with the utility to address fault sources on the grid.