How to test and troubleshoot solar PV panels
Testing a PV system correctly requires understanding which electrical parameters matter and why. This article covers the four key measurements used in professional PV diagnostics: open circuit voltage (Voc), short circuit current (Isc), isolation resistance (Riso), series resistance (Rs) and system impedance.
Safety note: Troubleshooting a PV system may be a complex and dangerous task. It is best to leave this kind of work to an experienced solar PV professional. Even small PV systems can generate substantial direct currents that can be dangerous.
Quick answers: PV testing at a glance
What is a Riso test?
A Riso test measures insulation resistance between a PV system’s live conductors and earth, verifying that cables, connectors and modules are electrically isolated. IEC 62446-1 requires at least 1 MΩ for systems above 120 V. Low values indicate insulation faults that can cause ground faults and fire risk.
What is a Voc test?
An open circuit voltage (Voc) test measures the voltage of a PV string with no load connected. Comparing measured Voc against the calculated value (module datasheet × modules in string, temperature-corrected) verifies string configuration and reveals faulty bypass diodes, shading or wiring errors.
What is series resistance (Rs) in a PV string?
Series resistance is the accumulated resistance of cables, connectors, cell interconnects and solder joints in a PV string. Rising Rs from corrosion, loose MC4 connectors or damaged joints reduces fill factor and power output, and shows up before total failure. Trending Rs over time catches degradation early.
What is PV impedance testing?
Impedance testing is emazys’ patented diagnostic method: a small AC signal (1–100 kHz) is injected into the PV string and the frequency response analysed. It goes beyond the mandatory IEC 62446 tests: instead of only detecting that a fault exists, it locates ground faults, open circuits and weak connections along the string, from a single measurement point.
The mandatory IEC 62446-1 tests
IEC 62446-1 defines the verification tests every grid-connected PV system must pass at commissioning and periodic inspection: continuity of protective conductors, polarity, string open circuit voltage (Voc), string current (Isc) and insulation resistance (Riso). The Z300 PVT and Z300 HE perform these mandatory tests robustly, including on high-efficiency and bifacial modules, and log every result automatically to the emazys cloud, so your IEC 62446 documentation is complete and audit-ready.
Open circuit voltage (Voc) and short circuit current (Isc)
The maximum voltage generated by a PV string is measured at the open terminals and referred to as Voc. Similarly, the maximum current available is called Isc. Together, Voc and Isc serve as the primary indicators of PV system condition and are typically measured in the field close to Standard Test Conditions (STC) and compared to the module manufacturer specifications.
Photovoltaic cells generate electricity through the photovoltaic effect. Made from semiconductors, the cells conduct electricity upon exposure to light. When photons strike the surface of a PV cell, the semiconductor material absorbs them, energising the electrons and causing them to break free from their atomic bonds. This creates electron-hole pairs essential for generating voltage.
PV cells are manufactured with a built-in electric field from two different semiconductor layers: one with an excess of electrons (n-type) and another with an excess of holes (p-type). The interaction between these layers generates a static electric field that drives an electric current. The main structure of the PV cell is that of a PN junction diode, which explains the shape of the IV curve used to characterise PV performance.
Testing against Standard Test Conditions (STC)
Module datasheets specify Voc, Isc and power at Standard Test Conditions: an irradiance of 1,000 W/m², a cell temperature of 25 °C and an air mass of 1.5. Field measurements are almost never taken under those conditions. Voc falls as cell temperature rises, and Isc scales with irradiance, so a raw field reading cannot be compared directly to the datasheet or to a measurement taken on another day.
To make measurements comparable, they must be translated to STC using the measured irradiance, the module temperature and the temperature coefficients from the module datasheet. Done by hand, this is a common source of calculation errors in commissioning reports.
On the emazys cloud this STC reference calculation happens automatically. When a Z300 string test is synced together with irradiance and module temperature data, for example from a pyranometer and the Tempirra logger, the platform converts every Voc and Isc reading to its STC equivalent and compares it against the module datasheet and your stored baseline. Measurements taken in March fog and July sun become directly comparable, and deviations reflect the condition of the string, not the weather.
Isolation resistance (Riso) and ground fault location in PV systems
Isolation resistance is one of the most important safety parameters in a PV system. PV systems can produce DC voltages well above 1000 volts. Without adequate isolation resistance, these voltages create severe electric shock hazards and risk of fire through unintended current paths to ground.
A PV ground fault occurs when an unintended electrical path is created between the system and earth. When this happens, current that should flow to the inverter flows directly to ground instead. Above a certain threshold, the inverter shuts down and energy production stops until the fault is resolved.
High isolation resistance also protects the long-term integrity of insulation materials, preventing moisture ingress, corrosion and degradation of electrical components. Regulatory standards such as the National Electrical Code (NEC) in the US mandate specific isolation resistance requirements for PV systems.
Read our detailed guide to PV ground fault troubleshooting for practical field procedures.
Minimum insulation resistance values (IEC 62446-1)
Insulation resistance readings are strongly affected by weather. See our article on the impact of humid conditions on Riso testing.
| System voltage (Voc stc) | Test voltage | Minimum Riso |
|---|---|---|
| < 120 V | 250 V | 0.5 MΩ |
| 120 – 500 V | 500 V | 1.0 MΩ |
| > 500 V | 1,000 V | 1.0 MΩ |
Beyond the standard: patented impedance-based fault location
The following methods, series resistance diagnostics and system impedance measurement, are not part of IEC 62446. They are emazys’ patented diagnostic technology, and they answer the question the standard tests cannot: not just whether a fault exists, but where it is.
Series resistance (Rs) in solar PV systems
Series resistance in a PV system comes from multiple sources: DC cables, connectors, junction box connections, bypass diodes and the solar cells themselves. The general principle is straightforward: low series resistance is desirable, and elevated series resistance reduces system performance and creates safety risks.
Increased series resistance reduces the fill factor (FF) of the PV system. More critically, elevated resistance causes localised power dissipation in the high-resistance area, which can lead to burn marks, connector failure and disconnections in PV strings.
In practice, worn or corroded connectors and damaged cabling are the most common root cause of elevated series resistance in the field. Mechanical damage to PV cables can cause both loss of isolation and increased series resistance. These faults are often intermittent and appear only in moist conditions, making them difficult to locate with conventional test equipment.
PV system impedance measurement
PV system impedance is measured at the open circuit voltage of the string across a broad frequency range of approximately 1 to 100 kHz. The test signal amplitude is kept below a few volts, making this measurement method very gentle on the solar cells.
At low frequencies below 5 to 10 kHz, a healthy fully illuminated PV string shows minimal impedance, and the spectrum is dominated by the series resistance of the string. Deviations from the expected series resistance value indicate abnormalities in the conductor path.
Interpreting impedance results: Voc measurement vs load measurement
Comparing the impedance measured at open circuit voltage with the impedance measured under load provides additional diagnostic information. If both values are elevated, a broken cable or open circuit is likely. If the values differ significantly from each other, the DC current flowing under load is influencing the result, which points to a fault internal to the PV cells or modules rather than in the external circuit.
Frequently asked questions about solar PV testing
What is a Riso test on a solar system?
A Riso test (insulation resistance test) measures the electrical resistance between the live PV conductors and earth. A healthy PV string typically shows values well above 1 MΩ. Low Riso values indicate insulation faults such as damaged cables, moisture ingress or faulty modules, which can cause ground faults, fire risk and inverter shutdowns.
What is a good Riso value for a PV system?
IEC 62446-1 requires a minimum of 1 MΩ for systems above 120 V (tested at 500–1,000 V DC). In practice, healthy dry systems measure 10–200+ MΩ. Values that drop sharply in humid conditions point to developing insulation faults, which is why readings should be trended over time rather than judged on a single pass/fail test.
How do you test a solar panel string?
These are the core measurements required by IEC 62446: open circuit voltage (Voc), string current (Isc) and insulation resistance (Riso). With a dedicated PV tester such as the emazys Z300, a complete string test takes about 20 seconds, every result is logged to the cloud for documentation, and patented impedance-based fault location, not just fault detection, is included for ground faults and open circuits.
How often should a PV system be tested?
IEC 62446 requires full commissioning tests at handover, and periodic verification is typically performed annually or bi-annually for commercial systems. High-value or safety-critical sites benefit from baseline measurements stored in the cloud, so every later test can be compared against the original healthy state.
Can you test solar panels without disconnecting them?
Yes. emazys’ patented impedance-based diagnostics injects a small AC signal into the string and can locate ground faults and weak connections from one end of the string, without removing modules or opening every connector. This method is unique to the emazys Z300 and complements the mandatory IEC 62446 tests.
Why does insulation resistance drop in wet weather?
Moisture penetrates damaged cable insulation, corroded connectors and module defects, creating conductive paths to earth. A system that measures 100 MΩ on a dry day can drop below 1 MΩ in rain. That pattern is diagnostic: intermittent low Riso in humid conditions almost always means a physical insulation defect that should be located and repaired.
What is the difference between an IV curve test and impedance testing?
An IV curve characterises string performance under irradiance and shows that a problem exists, but not where. Impedance testing measures the electrical health of the circuit itself and can locate the position of faults along the string. The two methods complement each other: IV curves for performance benchmarking, patented impedance diagnostics (unique to emazys) for troubleshooting and fault location.
What are Standard Test Conditions (STC) in PV testing?
Standard Test Conditions are the reference conditions used on module datasheets: 1,000 W/m² irradiance, 25 °C cell temperature and air mass 1.5. Field measurements are translated to STC using measured irradiance and module temperature, so results can be compared with datasheet values and earlier tests. The emazys cloud performs this STC correction automatically.
Store your baseline measurements free: create an emazys cloud account, get audit-ready IEC 62446 documentation, and compare every future test against day one.