Power quality describes how clean and stable the electrical supply is, and poor power quality causes nuisance tripping, overheating, premature equipment failure, flickering lights and wasted energy. A standard multimeter shows you a voltage at one instant; a power quality analyser or logger records voltage, current, power, harmonics and events over time, so you can find the cause of intermittent and load-dependent problems that a spot reading will always miss.
The common power-quality problems
| Problem | What it is | Typical symptom |
|---|---|---|
| Harmonics | Distortion from non-linear loads (drives, electronics, LED). | Overheating neutrals, transformers and motors; tripping. |
| Sags / dips | Short drops in voltage. | Resets, dropouts, contactor chatter. |
| Swells / over-voltage | Short rises in voltage. | Stressed and failing equipment. |
| Unbalance | Unequal voltage/current across three phases. | Motor overheating and loss of efficiency. |
| Transients | Very fast voltage spikes. | Damaged electronics, unexplained failures. |
| Poor power factor | Reactive load drawing current that does no work. | Higher demand charges, wasted capacity. |
Why you need a logger, not a snapshot
Most power-quality problems are intermittent or load-dependent. They appear when a big machine starts, at shift change, or only at certain times. A spot measurement almost never catches them. A power logger records continuously over hours, days or weeks, time-stamps every event, and lets you correlate a fault with what the load was doing at that moment. That correlation is what turns "it trips sometimes" into a diagnosed root cause. As a rule of thumb, a logging campaign should run long enough to capture a full production cycle, typically at least one full week, so it includes every shift pattern, every startup and shutdown sequence, and ideally a weekend or low-load period for comparison.
Common causes, and what to check first
- Harmonics: almost always traced to non-linear loads. Variable-speed drives, UPS systems, LED and fluorescent lighting with electronic ballasts, and switch-mode power supplies. Check which of these shares a distribution board or neutral with the affected circuit.
- Sags and dips: often caused by large motors starting elsewhere on the same supply (their inrush current pulls the local voltage down momentarily), or by faults and switching events upstream on the utility network. Correlate the sag timing against known start-up events on site before assuming the fault is external.
- Swells: commonly follow a large load switching off suddenly, or a loose neutral connection allowing voltage to rise on a lightly loaded phase. A loose neutral is a genuine safety issue and should be treated as urgent, not just a power-quality nuisance.
- Unbalance: usually comes from uneven distribution of single-phase loads across the three phases, or a partially failed component in a three-phase load. A logger showing steady, rather than intermittent, unbalance points toward a load-distribution problem rather than a transient event.
What an analyser measures
- Voltage and current on each phase, plus neutral.
- Real, reactive and apparent power, and power factor.
- Harmonics (THD and individual orders).
- Sags, swells, transients and interruptions, with time stamps.
- Energy consumption over the logging period.
How harmonics are actually quantified
A harmonic is a component of the waveform at a whole-number multiple of the fundamental frequency (50 Hz in Singapore), so the 5th harmonic sits at 250 Hz, the 7th at 350 Hz, and so on. A power quality analyser samples the waveform fast enough to resolve these individual orders, then reports two things: the magnitude of each individual harmonic order, and Total Harmonic Distortion (THD), a single figure summarising how distorted the overall waveform is. THD on current (THDi) is usually the more revealing figure on a mixed industrial load, since non-linear equipment (variable-speed drives, switch-mode power supplies, LED drivers) draws current in short pulses rather than a smooth sine wave, even when the supply voltage itself looks clean. IEEE 519 and IEC 61000-3 are the reference standards most facilities are assessed against for harmonic limits, and a competent power quality analyser reports against these directly rather than leaving you to interpret raw numbers.
Instrument class matters here. IEC 61000-4-30 defines Class A and Class S measurement classes for power quality instruments. Class A instruments meet a tightly defined accuracy and timing specification suitable for contractual, standards-compliance and dispute-resolution measurements; Class S instruments are adequate for general troubleshooting and statistical surveys but not for a measurement that needs to stand up as formal evidence. If a reading might end up supporting a warranty claim, a dispute with a utility, or a compliance submission, confirm the analyser is Class A rated for that measurement.
Worked example: tracing a nuisance-tripping complaint
A facility reports that an RCD on a lighting circuit trips intermittently, always in the afternoon, with no obvious cause. A spot check with a multimeter during a site visit shows nothing unusual, because the fault is not present at that moment. Installing a three-phase power logger for a week, set to capture events continuously rather than just log averages, changes the picture: the log shows the tripping events correlate precisely with a large variable-speed drive on a nearby production line starting up each afternoon shift. The logger's harmonic capture shows THDi spiking well above normal at exactly those times, consistent with the drive's non-linear switching injecting distortion onto a shared neutral. This is the value proposition in one example: a spot reading would never have caught it, but a week of continuous, time-stamped logging turned "it trips sometimes" into "it trips when the line 3 drive starts," a specific, actionable finding.
Where it pays off
- Troubleshooting: find the cause of nuisance tripping and equipment failures.
- Energy: identify poor power factor and wasted energy to target savings.
- Commissioning / disputes: document supply quality, verify after a fix, or support a claim with the utility.
- Compliance: demonstrate that harmonic emissions and supply quality stay within the limits your facility, equipment supplier or utility connection agreement requires.
Unitest Instruments supplies Fluke power quality analysers and three-phase loggers, available to buy or rent for a logging campaign, and our engineers can carry out the study and report for you.