Energy Decision # 53 - Voltage Sags Explained: Find the Root Cause

Sags and swells are defined by how far voltage flows.

moves, and how long it stays out of range.

By the standard industry definition, a voltage sag or dip is a short drop to between 10 and 90% of nominal.

Below 10%, it's an interruption, not a sag.

A swell is a short rise to 110% of nominal or more, typically topping out around 180%.

Past one minute, a sag becomes undervoltage and a swell overvoltage.

A transient is a very short burst up to several thousand volts, lasting microseconds to milliseconds, most often from lighting or faulty contactors.

Sags are common on power lines.

Swells are rarer.

When voltage leaves equipment's rated range, you see it on the floor.

Lights flicker, sometimes to unsafe levels.

Equipment with electronic power supplies reset.

Motor-driven processes fail or throw errors because the motor loses torque.

Sensitive electronics shut down, restart, run at reduced performance, or suffer power supply damage and longer events can stop equipment entirely.

Left alone, sags and swells make out-of-warranty repairs routine, draining capital meant for other projects.

Momentary sags mainly cost you trips and resets.

ECM Web's troubleshooting guide adds that motors undergoing sag in swell conditions lose torque and efficiency, raising utility costs.

Now, that's just one source, so don't count on it as gospel truth, but this is directionally accurate.

Key number is cost per trip.

Scrap, restart labor, and lost throughput times events logged per year.

Compare that with spending on equipment damage and early replacement to know which problem to solve first.

Sags and swells are defined by how far voltage flows.

moves, and how long it stays out of range.

By the standard industry definition, a voltage sag or dip is a short drop to between 10 and 90% of nominal.

Below 10%, it's an interruption, not a sag.

A swell is a short rise to 110% of nominal or more, typically topping out around 180%.

Past one minute, a sag becomes undervoltage and a swell overvoltage.

A transient is a very short burst up to several thousand volts, lasting microseconds to milliseconds, most often from lighting or faulty contactors.

Sags are common on power lines.

Swells are rarer.

When voltage leaves equipment's rated range, you see it on the floor.

Lights flicker, sometimes to unsafe levels.

Equipment with electronic power supplies reset.

Motor-driven processes fail or throw errors because the motor loses torque.

Sensitive electronics shut down, restart, run at reduced performance, or suffer power supply damage and longer events can stop equipment entirely.

Left alone, sags and swells make out-of-warranty repairs routine, draining capital meant for other projects.

Momentary sags mainly cost you trips and resets.

ECM Web's troubleshooting guide adds that motors undergoing sag in swell conditions lose torque and efficiency, raising utility costs.

Now, that's just one source, so don't count on it as gospel truth, but this is directionally accurate.

Key number is cost per trip.

Scrap, restart labor, and lost throughput times events logged per year.

Compare that with spending on equipment damage and early replacement to know which problem to solve first.