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Sources of Voltage Sags

What are Voltage Sags?

A typical voltage sag, graphed as an RMS voltage vs. time. This sag affected a three-phase system, dropping the voltage to 22.5% of nominal for 0.236 seconds. (Captured and displayed by a PSL PQube)

Voltage sags are brief reductions in the voltage on ac power systems. (The American “sag” and the British “dip” have exactly the same meaning, and may be used interchangeably.) How brief? Between 1/2 cycle and a few seconds. Disturbances that last less than 1/2 cycle are commonly called “low frequency transients”; voltage reductions that last longer than a few seconds are commonly called “undervoltage.”

What about Power Systems?

Power systems have non-zero impedances, so every increase in current causes a corresponding reduction in voltage. Usually, these reductions are small enough that the voltage remains within normal tolerances. But when there is a large increase in current, or when the system impedance is high, the voltage can drop significantly. So conceptually, there are two sources of voltage sags:

  • Large increases in current
  • Increases in system impedance

As a practical matter, most voltage sags are caused by increases in current.

It is convenient to think of the power system as a tree, with your sensitive load connected to one of the twigs. Any voltage sag on the trunk of the tree, or on a branch leading out to your twig, will cause a voltage sag at your load. But a short circuit out on a distant branch can cause the trunk voltage to diminish, so even faults in a distant part of the tree can cause a sag at your load.

What are the Main Characteristics of Voltage Sags

Voltage sags have two main characteristics: depth and duration. Do a scatter plot of depth vs. duration, and you get a CBEMA or ITIC graph.


Where do Voltage Sags Originate?


The most common cause of voltage sags:starting a large load, such as a motor or a computer room. The motor might be a pump, a compressor, a fan, part of an HVAC system.

Most voltage sags originate within your facility. The three most common causes of facility-sourced voltage sags are:

  • Starting a large load, such as a motor or resistive heater. Electric motors typically draw 150% to 500% of their operating current as they come up to speed. Resisitive heaters typically draw 150% of their rated current until they warm up
  • Loose or defective wiring, such as insufficiently tightened box screws on power conductors. This effective increases your system impedance, and exaggerates the effect of current increases
  • Faults or short circuits elsewhere in your facility. Although the fault will be quickly removed by a fuse or a circuit breaker, they will drag the voltage down until the protective device operates, which can take anywhere from a few cycles to a few seconds


What are the Sources of Voltage Sags?

Occasionally, a voltage sag will originate on the utility grid. But in-plant causes are far more common.

Experts can identify the specific source of a voltage sag with an advanced power quality monitor, such as those found at

Voltage sags can also originate on your utility’s electric power system. The most common types of utility-sourced voltage sags are:

  • Faults on distant circuits, which cause a corresponding reduction in voltage on your circuit. Typically, these faults are removed by “reclosers”, or self-resetting circuit breakers. These reclosers typically delay 1 to 5 seconds before self-resetting. If the fault is still present when the recloser resets, you may see a series of voltage sags, spaced 1 to 5 seconds apart. Faults on utility systems may be phase-to-phase, or phase-to-earth; depending on the transformers between you and the fault, you will see different levels of voltage reduction
  • Voltage regulator failures are far less common. Utilities have automated systems to adjust voltage (typically using power factor correction capacitors, or tap switching transformers), and these systems do occasionally fail

It is important to understand the source of the voltage sags before trying to eliminate them, because the wrong solution can actually make the problem worse. For example, if you install a ferro-resonant transformer as a voltage regulator, or a battery-operated UPS (a reasonable and common approach), but inadvertently install it upstream from the motor that is causing your voltage sags, the voltage sags will get worse, not better. In most cases, the correct solution is to adjust the equipment so that it is less sensitive to voltage sags.

Sag sensitivity – five ways equipment fails during voltage sags
CBEMA curve – voltage sag depth and duration at world-wide semiconductor plants
Sag immunity – inexpensive, simple ways to increase sag immunity
Semiconductor sag standards – industry standards, SEMI F47, F42

Power Standards Lab
980 Atlantic Ave.
Alameda, California 94501
TEL: ++1.510.522.4400
US: 1.888.SEMI.F47 or 1.888.736.4347

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