Voltage Sag

By R.W. Hurst, Editor

Voltage sag is a significant power quality problem resulting in significant economic losses and equipment damage. Electrical equipment is vulnerable to voltage sags, and their impact can be severe. Understanding the causes and effects and implementing proper mitigation techniques can help reduce the risk of power quality problems, increase equipment reliability, and reduce downtime. It is crucial to work with experienced professionals to ensure that electrical systems are designed and installed correctly and to provide the necessary maintenance and monitoring to prevent sags and other power quality problems.


Voltage sag is a temporary reduction in voltage, typically lasting for a few cycles, that occurs when a large load is connected to an electrical power system. Voltage sag, a dip or a momentary interruption can cause significant problems for sensitive electronic equipment. Power quality problems can result in unexpected downtime, reduced efficiency, and equipment damage, leading to substantial economic losses.

Power quality measures how well electrical power is delivered to equipment, and voltage sag is one of the most common power quality problems. It occurs when the supply drops below the nominal voltage level, resulting in a temporary power loss or electrical service interruption. Voltage sags can happen for various reasons, including sudden changes in the load, the line to ground fault in the power line distribution system, or short circuits.

The negative effect on electrical equipment can be severe. Electronic equipment, such as computers, servers, and medical equipment, can malfunction or shut down entirely. Large loads like motors or compressors can experience decreased efficiency and power output. The source of voltage sags can also cause unnecessary wear and tear on equipment, leading to a shorter lifespan and increased maintenance costs.

The difference between sag and dip is that sag refers to a more significant reduction, typically between 10-90% of the nominal voltage level. In contrast, a dip is a small reduction in voltage, normally less than 10% of the minor level. The duration of a voltage sag can vary depending on the cause, but it usually lasts for a few cycles or seconds.

Voltage sag can be prevented by installing proper electrical design and equipment that can handle fluctuations. One way to mitigate them is to use Uninterruptible Power Supplies (UPS) to provide backup power during an interruption. Additionally, regulators and transient suppressors can help stabilize the voltage magnitude levels and prevent voltage sag.

Mitigation techniques can involve improving the power distribution system to reduce the risk of voltage sag, upgrading the electrical equipment to handle fluctuations, or implementing advanced monitoring systems that detect and respond to sags in real time. The use of protective devices, such as overvoltage protection, can also help reduce the impact of voltage sag on electrical equipment.

In addition to voltage sag, other power quality problems include flicker, rms voltage swell, interruption, and variation. Voltage flicker is a variation of voltage that can cause visual flickering in lights or other devices. A swell refers to a temporary increase in levels above the nominal level. Interruption is a complete loss of power, typically lasting for several seconds or minutes. Finally, variation refers to a change in voltage levels, either sudden or gradual, that can affect the performance of electrical equipment.

To avoid power quality problems, electrical systems must be designed and installed correctly, with proper consideration given to the potential impact of fluctuations. Maintenance and monitoring are also essential to prevent power quality problems from occurring or to respond quickly when they do. Regular testing and analysis can help identify potential issues before they cause significant damage or downtime.

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