The insulation resistance test of a photovoltaic array uses direct voltage to measure the resistance between the active conductors of the array and ground. It detects cables with damaged sheaths, connectors with water, damp string boxes and leaking modules before they turn into a ground fault. IEC 62446-1 includes it in the category 1 tests for every system, and the inverter repeats it in its own way every time it starts.
What it measures
A photovoltaic array has hundreds or thousands of meters of direct current conductor, connectors at every module, string boxes, and insulation exposed to sun, rain and rodents. Between those active conductors and ground (the structure, the module frames, the protective conductor) there must be a very high resistance. The insulation resistance test applies a direct voltage between the active conductors and ground, measures the leakage current and calculates the resistance.
A low value indicates a leakage path: a point where the insulation gave way and the conductor ended up close to a grounded part, or where water closes the circuit.
What it detects
| Defect | Why insulation drops | Where it is usually found |
|---|---|---|
| Damaged cable sheath | Rubbing against the structure, cuts during installation, rodents | Crossings over metal edges, trays, trenches |
| Connector with water or poorly assembled | Connector not crimped properly or from different manufacturers, no seal | Joints between modules and at box entries |
| Damp string box | Open cable glands, condensation | Boxes outdoors or without drainage |
| Leaking module | Damaged frame or backsheet, delamination | Modules knocked during mounting or aged |
| Degraded insulation | Aging from ultraviolet radiation and temperature | Cables exposed to the sun without protection |
The test does not say where the defect is, only that it exists. It is located by measuring in parts: string by string and, within the failing string, section by section.
What IEC 62446-1 says
IEC 62446-1:2016, with its 2018 amendment, includes the insulation resistance of the PV array in the category 1 tests, which apply to all grid-connected systems. The standard describes the measurement methods between the array poles and ground, the test voltages and the minimum values according to the system voltage. Among its additional tests is the wet insulation resistance test, used when there are doubts about the behavior of the wet array or when a defect only appears with humidity.
Minimum values and test voltages are taken from the table of the current edition of the standard or from the project specification. Figures that circulate online without the table in view are not valid as an acceptance criterion.
The general guide to the test, with its principle and the influence of temperature, is in insulation resistance and polarization index. The full framework of array tests is in IEC 62446-1: testing and documentation of PV systems, and the tests done beforehand are in polarity, Voc and Isc of strings.
What makes it different
Three particulars separate this test from that of a transformer or a medium voltage cable:
- The object generates voltage. As long as there is light, the string has voltage between poles and each pole has voltage with respect to ground. The test is prepared with the string isolated from the inverter and with a method that accounts for that voltage and the string’s short-circuit current.
- Capacitance to ground. A large array has appreciable capacitance between conductors and ground, which charges with the test voltage. The instrument must discharge it at the end, and the operator must wait for that discharge before touching anything.
- Sensitivity to humidity. The same string can give a high value at midday and a low one at dawn. Recording the time, the humidity and whether the array was wet is as important as the value.
Inverter monitoring
Many inverters measure the array insulation resistance before connecting to the grid and monitor the residual current while operating. When the value falls below the manufacturer’s threshold, the inverter does not start or disconnects and raises an alarm. Inverters for PV systems are certified to IEC 62109-1 and IEC 62109-2, the safety standard for power converters used in photovoltaic systems and its particular requirements for inverters; the specific thresholds are declared by each manufacturer in its manual. On ungrounded networks and in DC systems, dedicated insulation monitors are also installed; read Bender ISOMETER: insulation monitoring in IT networks.
This monitoring and the insulation tester measurement complement each other:
- The inverter sees its whole array at once and under the real conditions of each morning, but does not say which string is failing.
- The insulation tester measurement is done string by string and allows the defect to be located, but at a single moment. Some PV analyzers include it together with polarity and the curve; read Metrel MI 3105 and MI 3115.
When the inverter reports low insulation intermittently, the usual route is in typical PV array faults: record the alarm with time and humidity, then measure by string to find the one that lowers the value. If the defect is in a buried cable, the route continues in faults in buried low voltage and DC cables. Substation DC systems have the same problem with another solution, described in ground fault detection in direct current.
- Does the alarm appear only with humidity or at dawn?
- Yes: A defect that shows up with water: measure string by string with the array damp, or use the wet test
- Permanent: Does the low value repeat when each string is measured separately?
- In one: Locate the section inside that string: connectors, cable, modules
- In none: Check the common wiring: string box, main DC cable, inverter input
Where it applies
| System | Application |
|---|---|
| Residential | At commissioning, as part of category 1 |
| Commercial and industrial | At acceptance and on inverter alarms |
| Solar plant | At acceptance by strings or by boxes, and in periodic maintenance |
The array is always a low voltage direct current circuit, even if the plant delivers at medium or high voltage. The medium voltage cables of the collector network are tested with other techniques, described in the cable diagnostic testing guide.
How to keep it from coming back
- Connectors of the same manufacturer and model on both sides of each joint, crimped with their own tool. The standard and the selection of cables and connectors are covered in solar cable and MC4 connectors.
- Cables protected against the edges of the structure and against radiation where the specification requires it.
- String boxes with suitable cable glands and drainage.
- A record of the acceptance insulation resistance as a baseline to compare against in maintenance.
We leave your plant ready to operate
Our team carries out commissioning of solar plants at medium, high and extra high voltage, from the transformer stations to the interconnection protections, in Colombia or elsewhere on request. We also run the DC array tests (insulation resistance, I-V curve, polarity, Voc and Isc, and module thermography, also by drone), and their records are integrated into the energization documentation. Write to us through contact.
Related guides
- IEC 62446-1: testing and documentation of PV systems
- Polarity, Voc and Isc of strings: initial tests
- Bender ISOMETER: insulation monitoring in IT networks
- Faults in buried low voltage and DC cables
- Insulation resistance and polarization index
- Ground fault detection in direct current
References
- IEC 62446-1:2016+AMD1:2018, Photovoltaic (PV) systems – Requirements for testing, documentation and maintenance – Part 1: Grid connected systems – Documentation, commissioning tests and inspection.
- IEC 62109-2:2011, Safety of power converters for use in photovoltaic power systems – Part 2: Particular requirements for inverters, used together with IEC 62109-1.
- IEC 62548-1:2023, Photovoltaic (PV) arrays – Part 1: Design requirements.
Frequently asked questions
Why does the inverter report low insulation in the morning and not in the afternoon?
Because early-morning dew and humidity lower the insulation resistance of an array that has a weak point, and the inverter measures it before connecting. When the sun dries the array the value rises and the inverter starts. The defect is still there: testing string by string with an insulation tester, ideally in similar conditions, is how to locate it.
What minimum insulation value does IEC 62446-1 require?
The standard sets the minimum values and test voltages according to the system voltage, in its own table. We do not reproduce them here without the table in view: they must be taken from the current edition of IEC 62446-1 or from the project specification, and compared with those of the inverter manufacturer.
Is the array insulation test the same as that of a transformer?
The principle is the same: direct voltage and a resistance measurement. The object changes: the array is a generator that keeps producing voltage in daylight, has a lot of capacitance to ground because of its extent, and its insulation varies widely with humidity. For that reason the method and the safety of the test are specific to photovoltaic systems.