Understanding Voltage Sags & Surges

When power fluctuates above and below the nominal voltage, your power supply is susceptible to voltage sags and surges. Take a look at the guide below to learn more about what they are and how they work.

What Are Voltage Sags?

A voltage sag is a sudden decrease in the flow of electricity through a power line by 10% or more below the nominal voltage. This power dip is typically brief, only lasting for one minute at most. Voltage sags are more commonplace than voltage surges.

Voltage sags are often caused by either manual operation switching or by single-line-to-ground fault (SLGF). SLGF is a type of electrical power transmission line fault that occurs when the neutral wire of an electric power system comes in contact with the ground. Besides damaging electrical appliances and devices, SLGF can cause power outages and blackouts.

Voltage sags decelerate phase converter operation, forcing the equipment to work harder. This process puts unnecessary strain on the equipment and produces an unreliable power supply.

What Are Voltage Surges?

A voltage surge is a sudden increase in the flow of electricity through a power line by 10% or more above the nominal voltage. For reference, most U.S. households run on about 120 volts. Like voltage sags, each voltage surge only lasts for a minute or less. Although they occur less frequently than voltage sags, voltage surges still impact households across the country.

By suddenly accelerating phase converter operation, voltage surges overwhelm electrical equipment with the power demand. A voltage surge can cause the equipment to shut down altogether. Voltage surges slow equipment operation, decrease equipment longevity, and can cause temporary or permanent equipment damage.

Voltage Sag Causes

When the electrical demand suddenly decreases, the voltage drops or “sags.” What causes a voltage sag in the first place? Below are the seven most common causes of voltage sags.

1. Lightning

Lightning is one of the leading causes of residential voltage sags. Lightning draws a lot of power from the closest power supply, which puts excess strain on the power grid and leaves less power available for household use.

2. Wind

Strong winds can loosen or even knock down power lines during a storm. This can create fluctuations in electrical demand, frequency, and voltage levels that cause voltage sags.

3. Trees Falling Onto Power Lines

When trees fall onto power lines, they can disconnect the power lines from the breaker box. This interrupts the flow of electricity and puts additional strain on the remaining power lines, causing the voltage to dip temporarily.

4. Construction Workers Digging Into Buried Cables

While digging buried power cables out of the ground to install new ones, construction workers can accidentally cut the cables and cause voltage sags in the surrounding area.

5. Squirrels & Rodents

We’ve all seen squirrels and other rodents make the brave journey across power lines only to chew through electrical wires or get caught in the breaker box. Chewed-up wires cause short circuits, whereas the latter problem causes the fuse to blow. In both cases, a voltage sag in the local area may occur and the issue will need to be addressed by the electrical company to restore proper function.

6. Equipment Failures

Failing equipment causes a sudden drop in voltage, as nearby equipment adjusts to carrying a greater electrical load. This process can involve the failure of one piece or multiple pieces of equipment.

7. Traffic Accidents

Cars involved in traffic accidents often damage or knock down power lines in the process. Either situation can cause voltage sags and even total power outages.