Insulation and dielectric
Wet insulation resistance testing wets a solar photovoltaic array and measures the resistance between its live conductors and ground. It shows whether rain, dew or irrigation could find a leakage path through damaged modules, connectors, junction boxes or cable insulation.
Wet insulation resistance testing is an acceptance measurement for solar photovoltaic arrays. The array is deliberately wetted, and the resistance between the live parts of the DC circuit and ground is measured with an insulation tester. In a dry insulation resistance test the surface of the modules, connectors and cables is dry and any small defect may be hidden, because air and dry plastic insulate well. Water changes that. It runs into cracks in the backsheet, through damaged connector seals, into junction boxes with failed gaskets and along scraped cable jackets, and if there is a path to the frame or the ground, water completes it. The measurement is made on the string, the sub-array or the whole array with the positive and negative conductors tied together so that every live part is tested at once against ground. The test belongs to the larger set of acceptance checks on a new or modified solar installation and is listed in the standard as an optional measurement. It complements the dry insulation reading, the string open-circuit voltage and short-circuit current checks and the I-V curve measurement, and it covers a failure mode that none of them can show.
A photovoltaic array lives outdoors for decades in rain, dew, snow melt, dust, heat cycles and ultraviolet light. Connectors loosen, cable jackets are cut against racking edges, backsheets crack, and junction box seals age. Many of those faults pass a dry test and only reveal themselves when the array is damp, which is exactly when the system is exposed to leakage and ground faults. A leakage path in a live DC circuit is a safety issue as well as a performance one. It can cause nuisance ground-fault trips in the inverter, corrosion in connectors, heat at the defect and, in the worst case, arcing or a fire. It can also let a metal frame become energized, which creates a shock hazard for people who work on or near the array. Finding these faults at commissioning, when the installer still has the crew and the access to correct them, is far cheaper than chasing intermittent ground-fault alarms after the first rainfall. The test also documents the condition of the array at handover, which supports warranty claims and later comparison.
Insulation resistance is the voltage applied divided by the small leakage current that flows through and over the insulation. When the insulation is dry and intact, that current is tiny, so the resistance is very high. When water bridges a defect, it forms a conductive film or fills a crack, and the leakage current rises, so the resistance falls. The wetting therefore acts as a probe, revealing flaws that would stay hidden in dry conditions. Because the array is a large network of modules and cables in parallel to ground, the measured resistance is the combination of every leakage path, and a larger array naturally has more surface and more paths. For that reason published criteria take the size of the array into account, and the result is interpreted accordingly. Another consequence of the physics is a safety one. Modules generate voltage whenever they are illuminated, so the array must be treated as live during the test, and the method relies on shorting the DC conductors together to keep the test safe and repeatable.
Safety comes first, because a photovoltaic array produces voltage whenever light falls on it and cannot be de-energized by opening a switch alone. The inverter and any other connected equipment are isolated from the array, and the array is covered, or the test is carried out in low light, as the installer's procedure and the owner's permit require, with insulated tools and appropriate protective equipment. The positive and negative conductors of the circuit under test are connected together with a suitable shorting device so that all live parts are at one potential, and the exposed conductive parts and the frame are connected to the ground reference. An insulation tester rated for the system voltage is connected between the shorted conductors and ground. The array is then wetted evenly with water, typically from a hose with a gentle spray, avoiding high-pressure jets that could drive water into seals and create a defect, and the water is allowed to run over modules, connectors and junction boxes, with the wetting extended to the cabling and combiner locations where practical. The test voltage is chosen according to the system rating and the governing procedure, and it is applied for the required time while the reading stabilizes. The reading is noted, the circuit is discharged and grounded, and the shorting device is removed in the order the procedure states. Weather, ambient temperature, wetting method and the circuit tested are recorded, and any reading that looks unusual is traced by testing smaller sections.
The record describes what was tested, how the array was wetted and under what weather, and what was measured, together with the instrument used and its calibration status. It also notes the condition of the connectors, junction boxes and cable runs that were inspected. Where a low reading was traced to a component, the location, the cause and the corrective action are kept with the retest. These details allow a later reader to understand how the wet test was performed and to repeat it under comparable conditions, and the results sit beside the dry insulation readings for the same array.
No single number decides whether an array is acceptable, and this page does not give one. Published criteria for wet insulation resistance take several shapes that are used together. The first is a minimum resistance that depends on the system voltage and, in many methods, on the size of the array, so a larger array is expected to show a lower raw reading, and the result is judged against a figure scaled to that size. The second is comparison between similar strings or sub-arrays built the same way, where one that stands apart from the others points to a local defect. The third is comparison with the dry reading, since a large drop between dry and wet conditions indicates a water-dependent leak. The fourth is correction for the conditions at the time, including temperature and the completeness of the wetting. The governing text and its tables are in ANSI/NETA ATS-2025, where the wet test is described among the optional measurements for photovoltaic systems, together with the general table that applies to insulation resistance. The project specification and the manufacturer's data for modules and cable may set stricter requirements. For the actual values, refer to your own licensed copy of the standard named above, which remains the authority for acceptance.
The wet test is chiefly an acceptance measurement, made on new or modified installations before they are handed over, so that defects in workmanship and in components are found while the installer is still responsible for them. Where it is repeated later, as part of an investigation of a ground-fault alarm or after storm damage, it is used to isolate which part of the array is leaking, and the result is compared with the acceptance record. The scale of the installation changes the approach. A small array can be tested as a whole, while a large one is divided into sections so that a fault can be localized, and the wetting and the safety arrangements are planned accordingly. Ground-mounted and roof-mounted arrays differ in access, water supply and risk of falls. Arrays with modules from different suppliers, or with long cable runs, call for particular attention to the connectors and the combiner enclosures. The measurement is not a substitute for the dry insulation test, the continuity and polarity checks or the performance tests, and it is made in addition to them.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
Water finds defects that a dry test cannot. It enters cracked backsheets, damaged connector seals and failed junction box gaskets and completes a leakage path to ground. Wetting the array shows what would happen in rain or dew, which is when ground faults and corrosion problems actually appear.
Only with a suitable procedure. The array is live whenever there is light, so the circuit is isolated from the inverter, the conductors are shorted together, and the work is done under the owner's permit with insulated tools and protective equipment. The array may be covered or tested in low light to reduce risk.
Yes. A larger array has more modules, more cable and more surface, so more leakage paths in parallel to ground, and the raw resistance falls as it grows. For that reason published criteria are scaled to the size of the array, and results are judged against the scaled figure rather than a single fixed number.
The array is divided into smaller sections and retested to locate the leaking part. The suspect components, such as connectors, junction boxes and cable runs, are inspected and repaired or replaced, and the circuit is dried and retested. The cause and the correction are recorded with the final result.
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