Most power quality problems are reported as something else: a drive that trips, a transformer that runs hot, a scanner that aborts, a capacitor bank that keeps blowing fuses. The symptom is real; the cause is often invisible because it lasts a few cycles or hides in the waveform. This guide maps the common symptoms to their usual power quality causes and to the measurement that proves or rules out each one.
Use it as a starting point, not a diagnosis. Several causes can produce the same symptom, which is exactly why you measure before you buy the fix.
Symptom, likely cause, what to measure
| Symptom | Likely power quality causes | What to measure |
|---|---|---|
| Breakers, contactors or drives trip with no fault found | Voltage dips and short interruptions, transients, inrush current | RMS events with waveform capture at the equipment and at the incomer, time-stamped against the trip log. Dips explained |
| Production line stops for a few seconds, then restarts | Voltage dips from network faults or large motor starts | Dip depth and duration, plotted against the ITIC and SEMI F47 curves |
| Variable speed drives trip on DC-bus overvoltage | Oscillatory transients from capacitor bank switching, swells | Transient capture with the time of each capacitor step. Transients explained |
| Transformer or neutral conductor runs hot; high neutral current | Triplen harmonics, unbalance, overload | Harmonic current spectrum per phase and in the neutral, K-factor, phase currents |
| Capacitor bank fuses blow or capacitors fail early | Harmonic resonance between the bank and the supply | Voltage and current harmonics with the bank in and out of service. Resonance check |
| Motors run hot or fail before their rated life | Voltage unbalance, harmonics, sustained undervoltage | Negative-sequence unbalance, voltage THD and RMS at the motor terminals. Motors guide |
| Lights flicker | Voltage fluctuations from welders, arc furnaces, compressors; interharmonics | Flicker severity Pst and Plt, rapid voltage changes, interharmonics |
| PCs, PLCs or controllers lock up or reboot | Dips and short interruptions, transients, neutral-to-earth voltage | RMS events with waveforms, neutral-to-earth voltage at the equipment |
| UPS keeps transferring to battery | Input dips, frequency deviation on generator supply, high input distortion | Input voltage events, frequency and THD at the UPS input. UPS guide |
| Power supplies, LED drivers and electronics fail early | Transients, sustained overvoltage, high crest factor | Transients, long-duration RMS levels, crest factor |
| MRI, CT or X-ray errors and aborted scans | Dips, transients, earthing problems | Events with waveforms on the imaging feeder, earth current and earth resistance. Hospital imaging guide |
| Residual current devices trip without an obvious fault | Earth leakage from filters and degrading insulation, harmonics | Earth leakage current trend over days, harmonic content |
| Power factor penalties on the electricity bill | Low displacement power factor, harmonics lowering true power factor | True and displacement power factor, kVAR, current THD. Harmonics and power factor |
| Cables or busbars overheat at normal load | Harmonic currents, unbalance | RMS and harmonic current per phase and neutral, unbalance |
Where to measure
Measure at two points if you can: at the equipment that fails and at the main incomer. If an event appears at both, it came from the supply. If it appears only at the equipment, it was created inside the site, by a neighbouring load, a loose connection or the equipment itself. That single comparison settles most arguments between a site and its utility.
How long to measure
At least one full week, because loads follow a weekly cycle and EN 50160 judges compliance on a week of 10-minute values. Intermittent problems — the trip that happens twice a month — need longer, which is where a permanent monitor beats a portable survey. The event that matters is often the one that happens after the analyzer has been removed.
Mistakes that hide the cause
- Averaged data only. A one-minute average cannot show a 100 ms dip. You need event capture with the waveform.
- Measuring after the event. A healthy reading today says nothing about last Tuesday.
- No time link to the equipment. Without matching timestamps you cannot prove that a dip caused a trip.
- One measurement point. You can see that the voltage dipped, but not where it came from.
- Ignoring the earth. Earthing faults show up as nuisance trips and equipment errors, not as a voltage problem.
How Inspector helps
Inspector classifies dips, swells, interruptions, transients and rapid voltage changes with IEC 61000-4-30 methods, stores five cycles of waveform before and after every event, and records harmonics, flicker, unbalance and power factor continuously. Inspector V4 also measures earth current and earth resistance, so an earthing fault appears on the same timeline as the voltage event. Alerts reach the team by SMS, e-mail or app.
Not sure which instrument fits your problem? Try the model selector, look up a term in the power quality glossary, or describe the symptom to an engineer.
