Solar PV Commissioning: The Tests Before Handover, and What IEC 62446 Requires

Commissioning is the last thing the contractor does and the first thing the owner inherits. On too many sites it is a formality: the inverters come on, the display shows power, somebody signs a sheet, and the crew moves on. That is a start-up, and calling it commissioning does not make it one.

Commissioning has two jobs. The first is to prove that the system is safe, that it was built as designed, and that it produces what the design says it should. The second outlasts the first by twenty years: it records the baseline. Every future warranty claim, performance dispute and fault diagnosis is a comparison against numbers taken in the first weeks of operation. A system commissioned without a baseline can work perfectly well. It simply cannot prove anything about itself later.

What IEC 62446-1 Covers

The reference document is IEC 62446-1, whose full title describes its scope: requirements for testing, documentation and maintenance of grid-connected PV systems, covering documentation, commissioning tests and inspection. The current text is the 2016 edition with its 2018 amendment, sold as a consolidated version. Older specifications still cite plain IEC 62446, the number used before the standard was split into parts.

It defines the documentation to be handed to the customer, an inspection that precedes testing and is normally done before the installation is energised, and two categories of test regime.

Category 1 is the set every system should receive, whatever its size.

Category 2 adds IV curve tracing and infrared inspection, and is aimed at larger or more complex systems. All Category 1 tests are completed and passed before Category 2 begins.

It also lists additional tests for particular situations: voltage to ground on resistively earthed systems, blocking diode tests, wet insulation resistance testing and shade evaluation.

Two limits matter. The standard is written for grid-connected systems and does not address battery storage or hybrid systems, which is why those installations need the additional checks set out later. And the AC side is inspected and tested under the low voltage wiring rules that apply to any electrical installation, IEC 60364-6 or its national equivalent.

Inspection Comes First

Before a test lead touches a conductor, the installation gets walked. Visual inspection catches faults no electrical test will find: connectors from one manufacturer mated with another's, cable hanging unsupported across a sharp rail edge, a roof penetration without flashing, unlabelled DC isolators, a backsheet scuffed through during handling.

The inspection checks the installation against the design and the wiring rules: DC equipment rated for the maximum voltage and current, protective devices matching the drawings, isolation where the design puts it, signage in place, and the mounting structure complete and correctly earthed. Where equipment was substituted during construction, this is where the substitution comes to light.

The output is a written checklist, signed item by item, with photographs. A verbal assurance that the inspection happened is worth nothing in a dispute two years later.

The Category 1 Test Sequence

The standard lists the Category 1 tests in a particular order, and the logic is practical: some tests are only safe once an earlier one has passed, and some can only be done before strings are connected together.

Continuity of protective earthing and equipotential bonding. Where the array frame and mounting structure are earthed or bonded, continuity is proved first. A broken earth path makes every later test on energised equipment more dangerous.

Polarity. Every string is checked for correct polarity before it is connected to a combiner, fuse or inverter. A reversed string in parallel with healthy ones can drive current backwards through modules and protection, and damage caused that way is unlikely to be accepted as a product defect.

Combiner box test. Where combiner boxes are used, a check that every string is connected where the drawings say, with none missing and none doubled.

String open-circuit voltage. Each string is measured and compared with the value expected from the datasheet, the number of modules in series and the module temperature. Identical strings under the same conditions should agree closely. A string low by roughly one module's voltage usually has a module short in the count, a reversed module, or a bypass diode failed short.

String current. Either short-circuit current, measured with a switching device rated for the job, or operating current with the system running. Current tracks irradiance, so the reading means little without an irradiance measurement taken at the same moment.

Functional tests. Switchgear, isolators, protective devices and the inverter are operated to confirm they behave as intended: the inverter starts, synchronises and shuts down correctly, and isolators actually isolate.

Insulation resistance of DC circuits. A test between live conductors and earth, at a test voltage chosen for the system voltage, to find damaged insulation, conductors crushed in trunking, water in junction boxes and connectors never fully seated. The standard tabulates minimum acceptable values. Left in place, this fault becomes an earth fault trip on the first wet morning, and occasionally a fire.

Record every reading. A test sheet showing "pass" against each string, with no measured values, has recorded the verdict and thrown away the evidence.

The Category 1 commissioning tests in the order IEC 62446-1 lists them. Every reading should be recorded as a value, because the values are the baseline. Source: IEC 62446-1 and MIMAH engineering practice.

Category 2: IV Curves and Thermography

Category 1 proves the system is safe and wired correctly. It says relatively little about whether every module is performing. Category 2 adds the two tests that do.

IV curve tracing. A curve tracer sweeps each string from short circuit to open circuit and records its full current and voltage characteristic. The shape of the curve carries information two spot readings cannot. Steps point to shading, mismatch or a bypass diode conducting. A soft knee suggests added series resistance at connections or cell interconnects. A steeper than normal slope near short circuit suggests leakage paths. Translated to standard test conditions using simultaneous irradiance and module temperature readings, each string's peak power can be compared with the datasheet and its neighbours. In our view this is the most useful baseline a PV system can have: a curve taken on the same string by the same method five years later shows exactly what has changed.

Infrared thermography. With the system operating in strong, steady sunlight, a thermal camera finds modules and cells running hotter than their neighbours: cracked cells, failed solder bonds, bypass diodes carrying current, and hot terminations in combiner boxes and at the inverter. IEC TS 62446-3 sets out the equipment, ambient conditions, procedure, reporting and personnel qualification for outdoor thermography of PV plants. A survey done in weak or patchy irradiance produces photographs and very few findings.

Category 2 was written with larger plants in mind, but it earns its cost on commercial systems too. A defect found at handover is the contractor's to fix. Found a year later, it becomes an argument about who caused it.

Inverter Settings, Protection and Grid Witness Tests

The inverter's settings decide how the system behaves when the grid misbehaves. Commissioning should record the settings actually loaded, not the datasheet defaults: voltage and frequency trip limits and delays, reconnection delay, active and reactive power settings, export limitation where the connection agreement requires it, and the grid code profile selected.

Anti-islanding, the inverter's ability to stop exporting when the grid is lost, protects anyone working on the network. On larger connections the utility will usually want to witness protection tests in person, and export waits for their signature. These requirements come from the local grid code and connection agreement, not from IEC 62446-1, and they vary between utilities. List them in the commissioning plan with a booked date, because a finished plant idle for weeks awaiting an unscheduled witness test is a familiar sight.

The Performance Test Period

Tests at a single moment prove the wiring, not the energy. For that, larger systems and any contract with a performance guarantee include a test period, typically days to weeks, in which measured output is compared with what the design should have produced in the weather that actually occurred.

The usual metric is the performance ratio: measured energy divided by what the array's nameplate rating would deliver, with no losses, under the plane-of-array irradiation measured over the same period. With irradiation in the denominator, the weather drops out. IEC publishes capacity and energy evaluation methods as technical specifications in the IEC 61724 series (IEC TS 61724-2 and IEC TS 61724-3), and a contract that defines its test by reference to them avoids most arguments about method.

Three things decide whether the test means anything. The irradiance sensor must be in the plane of the array, clean and within calibration. The temperature treatment must be agreed in advance, because a ratio calculated without temperature correction looks worse in a hot month than a cool one for the same healthy plant. And the exclusions, such as grid outages and curtailment, must be defined before the numbers arrive rather than negotiated afterwards.

The stages between a finished build and a handed-over system. Each one produces records the owner keeps for the life of the plant. Source: MIMAH engineering practice, after IEC 62446-1.

Hybrid and Off-Grid Systems

IEC 62446-1 stops at the grid-connected inverter. Systems with batteries and generators, which is what most sites with an unreliable grid need, require further checks.

Battery commissioning. Charge parameters must match the battery manufacturer's figures, not a generic preset: absorption and float voltages and the initial charge procedure for lead-acid, and for lithium, a working communication link through which the battery management system actually controls charge current. Usable capacity should be checked against the design; the arithmetic behind that figure is set out in our guide to tubular battery kWh calculation. Battery room ventilation and temperature belong on the inspection checklist, because high temperature is one of the main reasons batteries fall short of their rated life.

Generator changeover. Start and stop signals are tested at the conditions that should trigger them, usually a state of charge or load threshold. Changeover is tested under real load in both directions, with the interlocks that prevent unwanted paralleling, and with the generator deliberately failing to start. A hybrid whose generator start has only been tried with the load switched off has not been tested.

A full cycle. A full day and night on real load, with the generator called and released at least once, finds logic errors that individual tests miss.

The Handover Pack

IEC 62446-1 is specific about documentation, and it is what the owner will use for the next twenty years.

System data. Rated power, equipment models and quantities, key dates, and the designer's and installer's contacts.

As-built drawings. A wiring diagram showing string configuration, cable sizes, protective devices and earthing, as built rather than as designed.

Datasheets. For every major component, matching the models actually installed.

Test results. Every reading from every test, per string, with the instrument used, its calibration date, and the irradiance and temperature at the time. For Category 2, the IV curves and thermal images.

Operation and maintenance information. Isolation and shutdown procedures, emergency procedures, the maintenance schedule, and the warranty documents for each component.

Settings record. Inverter and protection settings as loaded, and the utility's witness certificate where one applies.

Serial numbers deserve their own mention. A register tying each module and inverter serial to its position is tedious to compile and invaluable at the first warranty claim, for reasons covered in our article on solar PV warranties.

What to Watch For as a Buyer

The builder commissioning its own work, unwitnessed. The EPC contractor normally runs the tests. The owner should not accept the results with nobody from its side present. Attend, or send an independent engineer, at least for Category 2 and the performance test. The wider question of contractor selection is covered in our guide to choosing a solar EPC contractor.

Verdicts without values. A report that records "OK" against each string tells you a test happened and nothing about its result.

No baseline. Without string-level readings, irradiance and temperature at commissioning, there is nothing to compare against when output falls. Several of the failures in why solar systems fail early, undersized cable and a hot battery room among them, leave measurable evidence at commissioning for anyone who records it.

Documents promised for later. Drawings and manuals that will follow after handover frequently do not. Tie the final payment or the release of retention to the complete pack.

Commissioning squeezed by the programme. When construction runs late, commissioning is what gets compressed, because the completion date rarely moves. Tests rushed on a cloudy afternoon are the ones that miss things.

Five commissioning warning signs, all of them avoidable if they are written into the contract before construction starts. Source: MIMAH engineering practice.

After Handover

The handover pack is the first document an O&M provider should read. Periodic testing repeats the commissioning tests by the same methods, so each set of readings can be compared with the last, and that only works if the first set exists. What a maintenance agreement should contain is covered in our solar O&M contract guide.

MIMAH's renewable energy team commissions the solar systems it builds and carries out performance audits on systems built by others. If you have a system approaching handover, or one already handed over with a thin commissioning record, get in touch.