Voltage Sag (Dip) and Voltage Swell (Surge)

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Introduction and Explanation[edit]

Voltage sags or dips 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 or surges 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 the most common power disturbances. 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.

Figure – 1 below depicts a typical Voltage sag;


Figure - 1: A Typical Voltage Sag

[Voltage Sags (Dip) and Swells, Power Standards Lab, Available at http://powerstandards.com/tutorials/sagsandswells.htm]

Causes of Voltage Sags and Swells[edit]

Voltage Sags are caused by abrupt increases in loads such as short circuits or faults, motor 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.

Factors Contributing to the Causes of Voltage Sags or Swells[edit]

1. Rural location remote from power source

2. Long distance from a distribution transformer with interposed loads

3. Unreliable grid system

4. Power distributor’s tolerances not suitable for voltage sensitive equipment

5. Switching of heavy loads

6. Unbalanced load on a three phase system

7. Equipment not suitable for local supply

Symptoms of Sags and Swells[edit]

1. Production rates fluctuates

2. Equipment does not operate correctly

3. Dimming of lighting systems

4. Variable speed drives close down to prevent damage

5. Relays and contactors drop out

6. Unreliable data in equipment test

Methods of Dealing with Sags and Swells[edit]

1. Transformer with a tap changer

2. Constant voltage (ferro-resonant) transformer

3. Servo controlled voltage stabilizer

4. Switch mode power supply

5. Saturable reactor

6. Soft starters on larger electrical equipment

7. Connect larger loads to points of common coupling

8. Choose equipment with dip resilience

Economic Implications[edit]

In a business environment, the equipment in use has to be resilient to the normal characteristic defects of the supply and this is not the case with off-the-shelf equipment. The cost of correction is much lower if corrective action is taken at the design stage of the equipment and this requires knowledge of the nature and probability of defects. This is the most cost effective approach.

Some equipment makers are recognizing the problem but the competitive market means that manufacturers will only respond to customers’ requirements. Until customers understand the problems and realize that equipment suppliers can provide a solution, they will not specify improved performance. The exception is the variable speed drive market where manufacturers are actively promoting products with enhanced dip ride through.


1. Voltage Sags (Dip) and Swells, Power Standards Lab, Available online at http://powerstandards.com/tutorials/sagsandswells.htm

2. “Effects of Voltage Sags, Swell and Other Disturbances on Electrical Equipment and their Economic Implications” by Norbert Edomah at 20th International Conference on Electricity Distribution [Prague, 8-11 June, 2009], Available online at