Power quality problems are disturbances in your electrical supply that damage equipment or stop it without warning. The two most common are harmonic distortion and voltage sags. They rarely look electrical. They show up as burnt motor windings, hot transformers, bulging capacitors, nuisance breaker trips and drives that fault with "no fault found."
Harmonics cause slow heat damage that shortens asset life. Sags cause trips that last a fraction of a second but can halt a line for hours. This guide shows how each one appears on the plant floor, how to diagnose it and how to stop it from coming back.
Key Takeaways
Power quality is how closely your electrical supply matches a clean sine wave at the right voltage and frequency. When the wave changes shape or size, sensitive equipment overheats, misreads signals or shuts down.
Harmonic distortion is the bending of the voltage or current wave by currents at whole multiples of the base frequency. On a 50 Hz system, the 5th harmonic runs at 250 Hz. Drives, UPS units, LED lighting and computers create them. They draw power in short pulses, not in a smooth wave.
A voltage sag is a short drop to between 10% and 90% of normal voltage that lasts from half a cycle to one minute. Most sags start with faults on the utility network, such as lightning or a tree on a line. Large motor starts inside the plant cause many others.
Reliability programs based on reliability-centered maintenance (RCM) and FMEA should list power quality as a failure cause on every electrical asset. Most sites skip it, so the same failures keep returning as unplanned maintenance.
Harmonics and voltage sags are the two most common power quality problems, and they work on very different timescales. Over months, harmonics wear assets out, while sags stop them in milliseconds.
| Attribute | Harmonics | Voltage Sags |
|---|---|---|
| How it works | Bends the shape of the voltage or current wave all the time | Cuts the size of the voltage for 0.5 cycles to 1 minute |
| Main source | VFDs, UPS units, LED lighting and IT loads inside the plant | Utility faults, lightning and large motor starts |
| Type of damage | Slow heat build-up and insulation aging | Sudden trips and process stops |
| Typical symptom | Hot motors, transformers and neutrals; blown capacitor fuses | Several machines stop at once with no fault found |
| Key metric | THD and TDD (%) | Depth (% of voltage kept) and duration |
| Main standard | IEEE 519 | IEEE 1159, ITIC curve, SEMI F47 |
Knowing which pattern you face tells you where to look first. Slow, repeated failures point to harmonics, while sudden, shared trips point to sags.
Use the free MTBF calculator to measure the time between failures on your electrical assets.
Harmonics cause equipment failures by adding heat, vibration and electrical stress that age parts early. A common rule of thumb says every 10 °C above an insulation's rating roughly halves its life.
The giveaway is repetition. When the same asset type fails again and again on the same supply, well before its expected life, harmonics are a likely cause.
Voltage sags cause sudden stops rather than slow wear. A sag to 70% voltage for 100 milliseconds barely flickers the lights, yet it can drop out a whole production line.
Each machine usually restarts fine, so the event gets logged as "tripped, reset, no fault found." The real signature is several unrelated machines stopping at the same moment, often during storms.
Accurate downtime tracking exposes this pattern. When every stop carries a timestamp and an asset name, shared trips stand out at a glance.
Use this matrix when a failure repeats and you suspect power quality problems. If the symptom matches, check power quality before you replace the part again.
| Symptom on the Floor | Likely Cause | Equipment Affected | First Check |
|---|---|---|---|
| Several machines trip at once, no fault found | Voltage sag | Contactors, VFDs, PLCs | Analyzer event log vs. trip times |
| VFD "DC bus undervoltage" fault | Voltage sag | Variable frequency drives | Drive fault history timestamps |
| Motor runs hot at normal load | Harmonics or voltage imbalance | Induction motors | Current THD and thermography |
| Repeat bearing failures on VFD motors | Bearing currents | VFD-fed motors | Shaft voltage test; check races for fluting |
| Transformer hot below rated load | Harmonics | Distribution transformers | K-factor and current THD |
| Neutral hotter than the phases | Triplen harmonics | Neutrals and panels | Neutral vs. phase current |
| Capacitor fuses keep blowing | Harmonic resonance | Power factor correction banks | Voltage THD with bank on and off |
| Breakers trip with no overload | Harmonics, high crest factor | Breakers and fuses | True-RMS current reading |
A match in the matrix does not prove the cause, but it tells you exactly where to measure first.
Diagnosing power quality problems takes four steps: spot the pattern, pin the timestamps, observe with instruments, then treat and track the fix.
The SPOT Power Quality Framework:
Maintenance teams that skip the first two steps often buy filters they don't need. Most facilities can find the likely cause in their own failure records before they spend on instruments.
The cheapest fix for most power quality problems is to harden the controls that trip, not to clean up the whole supply. Match each fix to the problem you confirmed.
Always measure again after any change. Filters and capacitors that are sized wrong can create a new resonance problem.
Cryotos doesn't filter harmonics or stop sags. It fixes the reason power quality problems stay hidden, which is scattered failure records. As a Computerized Maintenance Management System, it ties every trip, repair and reading to the asset and the time it happened.
Maintenance teams using Cryotos have reported up to 30% reduction in unplanned downtime and 25% faster repair turnaround. For power quality, the bigger gain is proof. Before-and-after MTBF shows whether a filter or ride-through upgrade paid off.
Look for repeat failures on the same supply, several machines tripping at once, or equipment running hot below its rated load. Then confirm with a one- to two-week analyzer survey and a thermography scan before you change any hardware.
IEEE 519 recommends voltage THD of 8% or less on systems at 1 kV and below, and 5% or less from 1 kV to 69 kV. Current limits depend on how strong your supply is compared with your load. Check the latest edition before you use these figures in a contract.
Most sags cause trips rather than direct damage. The secondary effects still hurt: restart inrush stresses motors, uncontrolled stops shock gearboxes, contactors wear from chatter, and product in the line gets scrapped.
They do both. Standard six-pulse drives are the biggest harmonic source in most plants, and the same drives trip easily during sags. Line reactors, filters and ride-through settings address both sides.
Many facilities keep a permanent meter on the main incomer and run a portable survey after adding large drives, UPS units or capacitor banks. Pair that with scheduled thermography of panels and transformers, so slow damage from power quality problems shows up early.
Power quality problems stay hidden until your failure data connects the dots. Schedule a free demo to see how Cryotos links every trip, repair and reading to the asset so you can prove the cause and stop repeat failures.
Cryotos AI predicts failures, automates work orders, and simplifies maintenance—before problems slow you down.
