14.2 Minimum and Additional DC Tests

Quick Answer: Minimum DC tests are continuity of earthing and/or equipotential bonding, polarity, insulation resistance, combiner polarity and voltage, string open-circuit voltage (Voc), string circuit current, and functional tests. Additional tests include I-V curve tracing, infrared thermography, voltage to ground, wet insulation resistance, and shade evaluation. Polarity is confirmed before Voc so the meter and inverter are not reverse-fed.

Minimum tests versus additional tests

BS EN 62446-1 groups commissioning tests so that every grid-connected array gets a defined minimum set, with extra tests used where the contract, the IET CoP, a fault investigation, or a performance dispute requires more evidence. The 2922 handbook expects you to know which tests are the minimum set and which are additional — and to know that order matters.

Order: polarity before Voc

After visual inspection, continuity of bonding is a dead test: you are proving conductors, not measuring generation. The next electrical check on a completed string is polarity. Only when polarity is proven do you measure open-circuit voltage.

If you measure Voc first on a reversed string, the meter sees reverse voltage. Many instruments are not designed to be reverse-fed at array voltage. If the DC isolator is closed onto the inverter, reverse polarity can damage the inverter input. Polarity testers, a DC voltmeter with clearly identified leads, and a physical check of connector gender at the combiner all come before you treat a Voc figure as a commissioning result.

Then work the rest of the minimum set:

1. Isolate the inverter and any SPD that the insulation-resistance test voltage could damage.
2. Insulation resistance at the test voltage specified in BS EN 62446-1 and the IET CoP for that system voltage — do not pick a megger range from habit. Those documents tabulate the test voltage and the minimum resistance against system Voc. Using 5 kV because higher feels safer can destroy SPDs and inverter input circuits. Leaving the inverter connected can make the reading meaningless and can damage the unit.
3. Combiner polarity and voltage: each string landed correctly, voltages consistent with one another.
4. String Voc compared with design.
5. String circuit current compared with design (short-circuit current with a correctly rated procedure, or operational current under generation, as the CoP method in use requires).
6. Functional tests once the AC side is complete: inverter starts and generates, G98 or G99 loss-of-mains and related protection behaves as required for that connection type, DC and AC isolators operate and isolate.

Functional tests are not a substitute for the dead DC set. An inverter that starts on a sunny day can still be fed by a reversed spare string sitting in a combiner, or by insulation that was never tested because someone skipped the isolation step.

Comparing Voc and Isc with design

Nameplate Voc and Isc are standard test conditions (STC): 1000 W/m², 25 °C cell temperature, air mass 1.5. On a real roof the irradiance is rarely 1000 W/m² and the cells are rarely 25 °C.

• Voc falls as cell temperature rises (negative temperature coefficient). A hot summer roof can show Voc several percent below the STC string total even when every module is present. Correct using the module temperature coefficient and a measured or estimated cell temperature, and note irradiance because very low light also depresses Voc.
• Isc (or operational current) scales mainly with irradiance. Measure irradiance in the plane of the array at the same time as the current. A string at 600 W/m² should not be failed for delivering about 60% of STC Isc if the correction has been applied.

Record the design string Voc and Isc, the correction method, the meter readings, and the pass/fail decision. Looks-about-right is not a result. Do not compare an uncorrected cold-morning Voc on one string with an uncorrected hot-afternoon Voc on another and call the difference a fault without doing the arithmetic.

Additional tests in brief

I-V curve tracing plots current against voltage from Isc to Voc. A notch or shelf suggests shading or a failed bypass diode. A slope change suggests series resistance (connectors, fuses, underrated cable). It is additional evidence, not a replacement for polarity and Voc.

Infrared thermography under load finds hot connectors, hot bypass diodes, and hot cells. It needs sufficient irradiance and generation as specified in the thermography method you are using; a first-fix array with the inverter not live will not show load-related hot spots.

Voltage to ground looks at each polarity relative to earth. A large imbalance can indicate an insulation fault on one pole that a combined IR test might still need follow-up to locate.

Wet insulation resistance finds faults that only appear when moisture bridges a damaged backsheet, a poorly glanded combiner, or a nicked cable in a wet roof void. It is not a substitute for the dry minimum IR test.

Shade evaluation checks that stringing does not put a chimney shadow across every module in one string at the time of day the design claimed was clear. It explains a low Isc that irradiance correction cannot justify.

Scenario: Voc 10% below the adjacent string

Two nominally identical strings. String A Voc matches the temperature-corrected design. String B is about 10% low at the same time, same irradiance.

Do not energise the inverter to see if MPPT corrects it — MPPT cannot invent missing voltage. Do not pass it as meter error; 10% is far outside typical DC meter uncertainty on a string voltage measurement.

Work the sequence:

1. Reconfirm polarity on string B. A reversed measurement or a reversed jumper will not be cured by a better irradiance reading.
2. Count modules and check the string diagram. On a ten-module string, 10% low is about one module missing or not in circuit. A single reversed module subtracts rather than adds, so the drop is about two module voltages (larger than 10% on a long string, but the same diagnostic family).
3. Check combiner fuses, landed ferrules, and unterminated tails.
4. Look for shading, mixed orientations stuffed into one string, or a damaged module.
5. Only then consider additional tests: I-V curve, thermography under load, voltage to ground.

Record string B as a fail against design until the cause is found and corrected. Commissioning is not an average of the good strings.