What an inspection report under IEC TS 62446-3 contains

Almost anyone can take thermal images of a photovoltaic system today. The difference lies in the report. A usable inspection report records the conditions under which the measurement was taken, how a finding comes about and where the statement ends. Without those details a collection of colourful images cannot be checked and is worth little in a warranty case.

What the standard actually covers

IEC TS 62446-3 is a technical specification from 2017 for the thermographic inspection of photovoltaic modules and plants in operation. It covers five areas: the minimum requirements for the equipment, the inspection procedure including the permitted conditions, the software used for the evaluation, the assessment of thermal anomalies and the contents of the inspection report. It also knows two levels of inspection, a simplified and a detailed one, which differ in the depth of the evaluation and in the qualification required.

The conditions belong in the report

Thermography is a measurement, and a measurement without documented conditions cannot be followed. These details therefore belong in the report as a minimum:

• Irradiance in the module plane at the time of the recording, at least 600 W/m² is required
• Air temperature, wind and cloud cover, because wind cools the modules and reduces temperature differences
• Operating state of the system, it has to be on the grid and in a thermally steady state
• The emissivity and the reflected temperature that were set, both influence the absolute values
• The viewing angle onto the module surface and the resolution reached at the object

Resolution, the point almost nobody states

For the inspection of modules the standard asks for a geometric resolution of no more than 3 cm per pixel edge. If the flight is higher, a conspicuous cell disappears into the average of its neighbouring pixels and never appears in the report at all. That is why the resolution actually reached belongs in the report, not only the flight altitude. For comparison, the camera used by QopterVision reaches 1.011 mm per metre of flight altitude, so at 10 m it is 1.01 cm per pixel edge, about one third of the permitted maximum.

How a finding is made

A warm spot alone is not a finding. It can come from the viewing angle, from the mounting rail underneath, from the roof membrane at the edge or from a reflection. At QopterVision a category above the level normal is only given when two criteria are met at the same time, a sufficient temperature difference and a matching fault pattern, that is the shape a failed cell, a switched off module string or a whole dead module leaves in the thermal image.

The five categories

• Normal, below 2 K temperature difference, no action needed
• Observe, 2 to below 5 K, look at it again at the next inspection
• Warning, 5 to below 10 K, an inspection by a qualified electrician is recommended
• Defect, 10 to below 20 K, inspection and repair needed
• Critical, from 20 K or above 80 degrees C on the module surface, act promptly

Why the temperature difference is normalised

The same anomaly shows a larger temperature difference in strong sunlight than in weak sunlight. So that values from different dates stay comparable, the measured difference is normalised to 1000 W/m². The report shows both, the measured and the normalised value, and the category is deliberately based on the measured one. Anyone who shows only the normalised value makes a system look worse or better on paper than it was on the day of the measurement.

What is in the report in the end

• Property and order data, and the documented coverage, meaning which part of the system was actually recorded
• The conditions of the measurement and the equipment used, with the resolution reached
• A site sketch with continuous module numbering, so that a finding can be found again at the next inspection
• One image pair per finding, visible spectrum and thermal image of the same section, with the measured values
• The category per finding, the recommended action and its deadline
• The limits of the statement and, on request, the radiometric raw data

What is deliberately not in it

A thermography report is not a certification and not an acceptance in the legal sense. It gives no figure for a loss of yield in kWh or euro, because that would need electrical measurements which are not part of thermography. It does not replace an IV curve measurement or an insulation measurement. A finding from the image is a well founded indication of a cause and not a proof of it, the confirmation belongs in the hands of a qualified electrician. A report that promises more promises too much.

How to recognise a usable report

Before you place an order, ask three things: what the irradiance was on the day of the measurement, what resolution was reached at the object, and by which rule the categories are given. Anyone who has no answer to that delivers images and not an inspection. A complete sample report with example data is sent on request, so that you can judge the result in advance.

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