When two engineers measure the same supply with two different analyzers and get different answers, the argument is usually not about the network. It is about how each instrument measures. IEC 61000-4-30 exists to remove that argument: it defines exactly how power quality parameters are measured and aggregated. Its Class A and Class S say how strictly an instrument follows those methods.
What IEC 61000-4-30 defines
The standard fixes the measurement method for each parameter — supply frequency, voltage magnitude, flicker, dips and swells, interruptions, unbalance, harmonics, interharmonics and rapid voltage changes. RMS values are computed over a 10-cycle window at 50 Hz (12 cycles at 60 Hz) and aggregated into 150/180-cycle, 10-minute and 2-hour values. During a dip, swell or interruption the aggregated values are flagged, so one event is not counted twice in other statistics.
Class A and Class S side by side
| Class A | Class S | |
|---|---|---|
| Purpose | Contractual applications, verifying compliance with standards, settling disputes | Statistical surveys and power quality assessment |
| What it guarantees | Two Class A instruments measuring the same signal give matching results within the specified uncertainty | Results suitable for surveys; tighter agreement between instruments is not required |
| Voltage magnitude uncertainty | ±0.1 % of declared input voltage | ±0.5 % of declared input voltage |
| Frequency uncertainty | ±10 mHz | ±50 mHz |
| Time stamps | Synchronised clock, so events can be lined up between instruments and sites | Looser requirements |
| How it is verified | Tests defined in IEC 62586-2 (power quality instruments, “PQI-A”) | Tests defined in IEC 62586-2 (“PQI-S”) |
Edition 3 of the standard (2015, amended in 2021) keeps these two classes. The older Class B was moved out of the normative part and survives only for legacy instruments.
What “certified Class A” should mean
Class A is declared per parameter: an instrument can be Class A for voltage and frequency but not for flicker or harmonics. When a datasheet says “Class A”, ask:
- Which parameters are Class A, and which edition of IEC 61000-4-30 does the claim refer to?
- Was it tested against IEC 62586-2, and by which laboratory? Ask for the test report, not a logo.
- What accuracy do the current sensors add? Clamps and Rogowski coils often dominate the total current uncertainty, whatever the instrument class.
When you need Class A
- A contractual or regulatory report, such as proving that the supply does not meet EN 50160 or a grid code.
- A dispute in which the other party will also measure, so both results must agree.
- Compliance verification against IEEE 519 limits written into a connection agreement.
When Class S, or a well-built monitor using the same methods, is enough
Most power quality work is troubleshooting, not litigation. The question is what happened, when, and what caused it — and a 0.4 % difference in voltage uncertainty does not change the answer to a 40 % dip. Class S or a monitoring instrument that follows the IEC 61000-4-30 methods is the practical choice for:
- Finding the cause of trips and stops (symptoms and causes).
- Continuous monitoring of your own plant, hospital or data centre.
- Before-and-after comparisons made with the same instrument, such as checking a harmonic filter.
- Ride-through analysis against the ITIC and SEMI F47 curves.
What matters more than the class for troubleshooting
- Event capture with waveforms, so you can see how an event started and ended.
- How long you measure. An intermittent fault missed by a two-week survey is not caught by better accuracy.
- Sampling rate high enough for the disturbances you are chasing.
- Timestamps you can line up with the equipment’s own alarm log.
Where Inspector fits
Inspector is built for continuous monitoring and troubleshooting: it classifies events with IEC 61000-4-30 methods and stores five cycles of waveform before and after every event, 24/7. If you need a certified Class A report for a contractual dispute, tell us what the report must show and we will say honestly whether Inspector or a dedicated Class A recorder is the right tool for that job. Terms used here are defined in the power quality glossary.
