A plant's drives trip twice a month and nobody can say why. The meters on the feeders show normal kW and kWh before and after each trip, because they average over seconds or minutes and keep no record of a 120 ms dip. An analyser on the incomer would have recorded the dip, its depth and its duration, and the trips would have lined up with it. The opposite mistake is as common: a site fits an analyser to every board and pays for event capture nobody reads, when it needed kWh per circuit. This guide explains what each instrument measures, which questions need which, and how both feed one monitoring system.
The most quoted power quality figure, THD, is explained with its limits in the total harmonic distortion guide.
What each instrument measures
| Energy meter | Power quality analyser | |
|---|---|---|
| Question it answers | How much energy, when, and what it cost | How good the supply is, and what disturbed it |
| Main quantities | kWh, kvarh, kW demand, voltage, current, power factor | Voltage dips, swells and interruptions, flicker, unbalance, harmonics and interharmonics, frequency |
| Measurement standard | Accuracy classes in IEC 62053-21 or IEC 62053-22; MID approval for billing in the EU | Measurement methods in IEC 61000-4-30, Class A or Class S |
| Time resolution | Energy registers, plus averages over seconds or a demand interval | Values over 10 cycles at 50 Hz (12 at 60 Hz), about 200 ms, aggregated to 3 s, 10 min and 2 h, plus event records |
| Typical installation | Permanent, one per circuit | Portable for a survey, or fixed at an incomer |
An energy meter's class says how closely it counts energy. It says nothing about events, because a meter is not required to detect them. An analyser's class says how it measures each power quality parameter, so that two instruments on the same supply report the same dip.
Network analyser, power analyser or multifunction meter
The names overlap. In Spanish (analizador de redes) and Italian (analizzatore di rete), the same word covers a portable power quality analyser and a panel-mounted multifunction meter that also shows harmonics. French specifications call the panel instrument a centrale de mesure. In English, "network analyser" usually means a radio-frequency instrument, so name the function instead.
IEC 61557-12 calls panel instruments that measure energy and electrical quantities in an installation power metering and monitoring devices (PMDs). A PMD may display THD or individual harmonics without measuring them to the IEC 61000-4-30 method. Check the datasheet for the standard and class of each function you intend to rely on.
Measurement classes
IEC 61000-4-30 defines two measurement classes. With Class A, two instruments connected to the same signal produce matching results within the stated uncertainty. Use it where a result may be compared or disputed: checking a supply against EN 50160 or a supply contract, or a grid-code compliance test. Class S is for surveys and statistical assessment, when you need to find a problem rather than prove it to someone else. The class applies to each parameter an instrument declares, so read which parameters a datasheet covers.
How long to measure
EN 50160 describes the voltage at the supply terminals of a public low-voltage network over one week of 10-minute mean values. During each week, 95% of the 10-minute RMS voltages must be within ±10% of nominal, and all of them within +10% and −15%. Voltage THD, including harmonics up to the 40th order, must not exceed 8%.
A survey shorter than a week, or one that misses the busy shift, cannot show this. Plan the survey around the plant's working pattern, not around the engineer's visit.
Which instrument for which question
| Question | Instrument |
|---|---|
| Is the supply within EN 50160 or the supply contract? | A Class A analyser at the supply point for at least a week |
| Why do drives or contactors trip? | A portable analyser with event capture at the affected board; Class S is often enough |
| Are harmonics overheating a transformer or capacitors? | An analyser that measures individual harmonics on the feeder, then the THD and TDD calculator to express current distortion against the demand current |
| Are the phase voltages or currents unbalanced? | Voltage and current from either instrument, checked with the voltage unbalance method |
| Where does the energy go, and what does it cost? | Energy meters on each circuit, read continuously |
| Did the night load fall after a change? | Energy meters with interval data |
Many sites need both: one fixed analyser at the incomer for events, and energy meters on the circuits for consumption. The three-phase power and power factor correction calculators work from the same voltage, current and power factor either instrument reports.
Modbus meters and analysers in one system
Panel analysers and energy meters usually share an interface: RS-485 with Modbus RTU, or Ethernet with Modbus TCP. A gateway can poll both and keep their readings on one time axis. The Device Directory has guides for three analysers: the Janitza UMG 512-PRO, the Janitza UMG 96-PQ-L and the Camille Bauer LINAX PQ5000. The three-phase Modbus meter guide covers the meters.
Polling copies the values at the poll interval, for example voltage, THD and unbalance once a minute. It does not copy the analyser's event records or waveforms, and the copy does not carry the analyser's Class A status. Keep events and reports in the analyser's own software, and use the polled values for trends, alerts and comparison with the energy data.
Where EpiSensor fits
A ZEM electricity monitor is a three-phase energy monitor, not a power quality analyser. It reports kWh, kVAh, RMS voltage and current, power factor and frequency, at Class 0.5S to IEC 62053-22 for the meter and its current sensors together. It does not measure harmonics or flicker or record voltage events. The accuracy guide explains what that class covers.
The Gateway runs Edge, which reads ZEMs and Modbus meters and analysers into the same dashboards, alerts and exports. To try it with a meter or analyser you already have, run Edge on a Raspberry Pi imaged with System Manager: one third-party Modbus device of up to 32 registers is licensed free, with all dashboards, analysis and alerting, and the data stays on your network. The Pi runs from a microSD card, so use it for evaluation, not for a business-critical site. See evaluating Edge on a Raspberry Pi.
Common questions
What is the difference between a power quality analyser and an energy meter?
An energy meter measures energy, power, voltage and current to an accuracy class for consumption and cost. A power quality analyser measures how the supply departs from a clean, steady sine wave, using the methods of IEC 61000-4-30: dips, swells, interruptions, flicker, unbalance, harmonics and interharmonics, each with a time stamp and duration.
What does a network analyser measure?
The name is used for two instruments. A portable power quality analyser records voltage events, flicker, unbalance and harmonics during a survey. A panel network analyser, a multifunction meter in a switchboard, measures energy, power, voltage, current, power factor and usually harmonics continuously. Check the datasheet for the standard each function meets.
Do I need a Class A power quality analyser?
Use Class A when the result may be compared with another instrument or disputed: verifying a supply against EN 50160 or a contract, or a grid-code compliance test. A Class S instrument is usually enough to find the cause of a problem on your own site.
Can an energy meter measure harmonics?
Many meters display voltage and current THD, and some list individual harmonics. Unless the datasheet states the IEC 61000-4-30 method and class for that parameter, treat the value as a trend indicator, not as evidence of conformity.
How long should a power quality survey run?
At least one week, so that it covers every shift and the weekend. EN 50160 assesses a low-voltage supply over one week of 10-minute values, and a shorter survey can miss the load pattern that causes the problem.