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Voltage Sags (Dips) and Swells

Voltage sags – or dips which are the same thing – are brief reductions in voltage, typically lasting from a cycle to a second or so, or tens of milliseconds to hundreds of milliseconds. Voltage swells are brief increases in voltage over the same time range. Longer periods of low or high voltage are referred to as “undervoltage” or “overvoltage”.

Voltage sags are caused by abrupt increases in loads such as short circuits or faults, motors starting, or electric heaters turning on, or they are caused by abrupt increases in source impedance, typically caused by a loose connection.

Voltage swells are almost always caused by an abrupt reduction in load on a circuit with a poor or damaged voltage regulator, although they can also be caused by a damaged or loose neutral connection.


A typical voltage sag

Voltage sags are the most common power disturbance. At a typical industrial site, it is not unusual to see several sags per year at the service entrance, and far more at equipment terminals.

Voltage sags can arrive from the utility; however, in most cases, the majority of sags are generated inside a building. For example, in residential wiring, the most common cause of voltage sags is the starting current drawn by refrigerator and air conditioning motors.

Sags do not generally disturb incandescent or fluorescent lighting. motors, or heaters. However, some electronic equipment lacks sufficient internal energy storage and, therefore, cannot ride through sags in the supply voltage. Equipment may be able to ride through very brief, deep sags, or it may be able to ride through longer but shallower sags.


1996 Version of the IT Industry Tolerance Curves (update from original CBEMA curve). The vertical axis is percent of nominal voltage

“Well-designed” equipment should be able to tolerate any power event that lies in the shaded area. Note that the curve includes sags, swells, and transient overvoltages. The semiconductor industry developed a more recent specification (SEMI F47) for tools used in the semiconductor industry in an effort to achieve better ride through of equipment for commonly occurring voltage dips and therefore improving the overall process performance. It is basically the same as the ITI Curve but specifies an improved ride through requirement down to 50% retained voltage for the first 200 msec. Many short voltage dips are covered by this additional requirement. IEC 61000-4-11 and IEC 61000-4-34 provide similar voltage dip immunity standards.

Many utilities have benchmarked performance of the supply system for voltage dips but it has not been the general practice to specify any required performance levels for the system. Performance is often specified using the SARFI index that provides a count of all events with magnitudes and durations outside of some specifications. For instance, SARFI-70 would provide a count of all voltage dips with a retained voltage less than 70% (regardless of duration). SARFI-ITIC would provide a count of all voltage dips that exceeded the ride through specifications of the ITI Curve.

The table below provides a summary of voltage dip performance levels from a few major benchmarking efforts. Note that these are average performance levels and it would not be reasonable to develop limits based on an average expected performance (although these are the correct values to use when evaluating the economics of investments in ride through solutions).


Example of average voltage dip performance from major benchmarking projects. These values represent voltage dip performance on medium voltage systems

The voltage dip performance can vary dramatically for different kinds of systems (rural vs urban, overhead vs underground). It may be important to include some of these important factors in the specification of the power quality grades.

It will also be important to specify the performance for momentary interruptions. These events can be a particular problem for customers and are not included in most assessments of reliability.

A previous CEA Technologies report prepared by Electrotek Concepts recommended that the SARFI indices be calculated for the following magnitude and duration categories:


Recommended magnitude and duration categories for calculating voltage dip performance

The reasons for these categories were explained as follows:

  • The 90% level provides an indication of performance for the most sensitive equipment
  • The 80% level corresponds to an important break point on the ITI curve and some sensitive equipment may be susceptible to even short sags at this level.
  • The 70% level corresponds to the sensitivity level of a wide group of industrial and commercial equipment and is probably the most important performance level to specify
  • The 50% level is important, especially for the semiconductor industry, since they have adopted a standard that specifies ride through at this level
  • Interruptions affect all customers so it is important to specify this level separately. These will usually have longer durations than the voltage sags
  • The first range of durations is up to 0.2 seconds (12 cycles at 60 Hz). This is the range specified by the semiconductor industry that equipment should be able to ride through sags as long as the minimum voltage is above 50%
  • The second range is up to 0.5 seconds. This corresponds to the specification in the ITIC standard for equipment ride through as long as the minimum voltage is above 70%. It is also an important break point in the definition of sag durations in IEEE 1159 (instantaneous vs. momentary)
  • The third duration range is up to 3 seconds. This is an important break point in IEEE 1159 and in IEC standards (momentary to temporary)
  • The final duration is up to one minute. Events longer than one minute are characterized as long duration events and are part of the system voltage regulation performance, rather than voltage sags

As a final note, remember that voltage sags are voltages, and therefore always occur between two conductors – there is no such thing as a “sag on phase A” — it must be a sag between phase A and phase B, or a sag between phase A and Neutral.

Contact us for more information about:

  • Voltage sag/swell standards
  • Effects of sags on equipment
  • Testing equipment tolerance to voltage sags
  • Improving equipment immunity
  • General solutions to sag problems

Alex McEachern, 1/2004based in part on work done with Mark McGranaghan

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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