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Category 1 is the baseline that every grid connected photovoltaic system receives: continuity of protective earthing and bonding, polarity, combiner box checks, string open circuit voltage, string current, functional checks of switchgear and inverter, and insulation resistance of the direct current circuits. Category 2 is the additional tier, led by string curve tracing and infrared thermography, applied where the baseline results leave a doubt or where the specification demands a performance record. The second tier does not replace the first; it is layered on top of a complete first tier.
Category 1 is the baseline every grid connected array gets
Before any instrument comes out, the standard expects an inspection and a documentation pack: system data, a wiring diagram showing string configuration and protection, datasheets for modules and inverters, and the operation and maintenance information the owner will need. Testing a system that does not match its own drawings produces evidence about something other than the installed array, which is why the paperwork step is genuinely first rather than ceremonially first.
The baseline electrical tests then follow a logical order. Continuity of protective earthing conductors and any equipotential bonding is proven first, because the protective arrangement has to be intact before anything else is energised deliberately. Polarity on every string is confirmed next, since a reversed string will otherwise be discovered by the combiner. The combiner itself is checked for correct string connection and correct fusing. Open circuit voltage is then read per string and compared against the design, and string current is measured either as a short circuit value or as an operational one.
Functional tests close the baseline: isolators operate, interlocks behave, the inverter starts and disconnects as designed. Insulation resistance of the direct current circuits is measured last among the electrical tests, because it is the one that requires the array to be isolated and configured deliberately. That sequence is the skeleton of every commissioning sheet in a commissioning kit, whichever manufacturer supplied the instruments.
Where the insulation resistance thresholds come from
Insulation resistance is the one baseline test with a numeric pass mark that people routinely misquote, because the test voltage and the minimum resistance both depend on how much voltage the array can present. The array figure used is not the nameplate open circuit voltage but an uprated version of it, because a cold clear morning pushes open circuit voltage well above its standard conditions value, and the insulation has to be proven against that worst case.
| System voltage, open circuit voltage uprated for cold conditions | Test voltage applied | Minimum insulation resistance |
|---|---|---|
| Below 120 V | 250 V | 0.5 megohm |
| 120 V up to 500 V | 500 V | 1 megohm |
| Above 500 V | 1000 V | 1 megohm |
Two practical points follow. The first is that a reading can pass in dry conditions and fail after rain, which is the reason a wet repeat is sometimes specified for arrays with a history of ground faults, and why ground fault testing tools are a separate purchase from a general insulation tester. The second is that the exact bands and the procedure around them belong to the current edition of the published standard, so confirm the figures in the document before signing anything that carries a pass mark.
Category 2 adds the tests that need a controlled source or a sweep
The second tier exists because the baseline proves that a system is correctly built and safe, not that it performs. String curve tracing is the principal addition. Sweeping each string and comparing the measured trace against the manufacturer curve, after translating both to a common set of conditions, is the only way to evidence that a string is delivering the shape it should rather than merely the voltage it should.
Infrared thermography of modules and connections is the other main addition. A warm cell, a warm bypass diode or a hot connector shows a fault that is thermal before it is electrical, and often before it is large enough to change a string reading. Thermography under the standard is a procedure, not a photograph: irradiance has to be high enough for faults to develop a temperature difference, wind has to be low enough not to wash it away, and the viewing angle and emissivity assumptions have to be recorded with the image.
The two additions share a characteristic. Both produce a record that can be compared against a later record, which is what turns a commissioning exercise into a baseline for the operating life of the array. That is the practical argument for doing the second tier even where it is not demanded, and it is why a basic testing toolkit grows in that direction rather than in any other.
How instruments declare conformity to an IEC standard
Instrument listings in this field cite standard numbers constantly, and the citations vary enormously in how much they actually commit to. A useful declaration names the standard, the part, the class or category within it where the standard defines one, and ideally the fact that an independent calibration or certification accompanies the instrument. A weak declaration names a number and stops.
An example from a completely different discipline shows the strong form clearly. The CRYSOUND CRY2831 sound level meter states conformity to IEC 61672-1 at Class 2, names the measurement span of 29 dB to 138 dB and the frequency response to 12.5 kHz, states an IP54 rating, and states that a calibration certificate is supplied. That is the complete shape of a conformity claim: standard, part, class, the quantities the class governs, and evidence. Nothing about a sound meter helps on a roof, but the pattern is exactly what a photovoltaic tester listing should match when it claims Category 1 and Category 2 coverage.
Compare that with an instrument whose function overlaps part of the baseline without claiming the standard. The Testo 750-3 voltage tester states ground fault circuit interrupter testing on alternating current outlets, a three digit display of actual voltage, an all around viewable LED voltage bar and polarity detection for direct current measurements. Polarity detection overlaps one baseline requirement, but a measurement category and voltage rating are not stated in the listing bullets, and nothing in them addresses insulation resistance, string current or curve tracing. An instrument that performs one test on the list is not an instrument for the list.
Working through a commissioning sequence in order
Start with documents and inspection, so that every later reading belongs to a system you can describe. Run the baseline tier in its intended order, with protective continuity before energisation and insulation resistance after the array has been configured for it. Record conditions beside every reading, because a string current without an irradiance figure cannot be defended and cannot be repeated. Use a low resistance meter for the continuity work rather than the continuity buzzer on a general purpose meter, since the figure that matters is a value in milliohms, not a beep.
Escalate to the second tier wherever the baseline leaves a question, and keep the sweeps and thermal images whether or not anybody asked for them. Years later the only thing that distinguishes a degrading string from a string that was always like that is a trace taken on the day it was handed over.
Frequently Asked Questions
Is Category 2 optional?
It is additional rather than optional in the ordinary sense. The baseline tier applies to every grid connected system, and the second tier is applied where the first leaves a question unanswered, where the system size or contract specifies it, or where a performance claim has to be evidenced rather than asserted. Many commercial specifications simply require the second tier on every string as a condition of handover, which removes the discretion entirely.
What is the difference between inspection and testing here?
Inspection is the visual and documentary part: checking that the installed system matches the design, that components carry the right ratings, that cable management and earthing arrangements are as specified, and that the documentation pack exists. Testing is the instrument part that follows. Inspection comes first deliberately, because a test result from a system that was installed differently from its drawings is evidence about the wrong system.
Why is system voltage defined as open circuit voltage multiplied by a factor?
Because the voltage a photovoltaic circuit can actually present is higher than its nameplate open circuit voltage on a cold day, and the insulation has to be proven against the worst case rather than the catalogue case. Applying a factor above unity to the standard conditions figure approximates that cold weather maximum without requiring a site specific temperature study for every array, which is why the test voltage bands are set against the uprated number.
Does the alternating current side fall under the same standard?
No. The inverter output and everything downstream of it is verified as an electrical installation under the general installation verification rules, which cover continuity, insulation resistance, earth fault loop impedance, residual current device operation and polarity in the usual way. The photovoltaic standard concerns itself with the direct current array and the interface, and expects the alternating current verification to have been done to its own rules.
Does a thermal camera count as a Category 2 instrument on its own?
A camera can perform the thermography part, but thermography under the standard is a defined procedure rather than a snapshot, with requirements on irradiance, wind, viewing angle, emissivity and the reporting of anomalies. An image that shows a warm cell is a finding only when the conditions under which it was taken are recorded with it. The camera is necessary and the method is what makes the result admissible.
What should a test certificate contain beyond the numbers?
The identity of each string tested, the instrument used with its calibration status, the conditions at the time including irradiance and ambient temperature, the limits applied and the pass or fail judgement against them, and the signature of the person responsible. A page of readings without conditions and instrument identity cannot be re created later, which defeats the purpose of keeping it.