Insulation Resistance Testing (IRT) in solar installations

Insulation Resistance Testing (IRT) is a fundamental part of verifying electrical insulation integrity in solar installations. It involves applying a high-voltage, low-current charge to a conductor and measuring the leakage current. This ensures no insulation defects or degradation are present that may cause system failure or create safety hazards.

The IEC 62446-1 standard outlines specific requirements for IRT in grid-connected PV systems, specifying test voltages and minimum insulation resistance values. According to this standard:

• For PV arrays with a system voltage within the range of 120 V to 500 V, a test voltage of 500 V DC is required.
• For PV arrays with a system voltage within the range of 500 V to 1000 V, a test voltage of 1000 V DC is required.
• For PV arrays with a system voltage exceeding 1000 V, a test voltage of 1500 V DC is required.
• The measured insulation resistance should be no less than 1 MΩ.

Similarly, the ANSI/NETA Maintenance Testing Specifications (MTS) provide guidelines for conducting maintenance tests on electrical power equipment, including solar components, and specify the following:

• Equipment rated from 6000 V to 1000 V requires a test voltage of 1000 V DC.
• Equipment rated up to 5000 V requires a test voltage of 2500 V DC.
• The measured insulation resistance should be no less than 100 MΩ for systems up to 1000 V, and 1000 MΩ for systems up to 5000 V

Utilizing Fluke insulation resistance testers
The Fluke SMFT-1000, 1587, 1507, and 1537 are designed to meet the standards set by IEC 62446-1 and ANSI/NETA MTS for solar installations. These testers offer:

• Adjustable Test Voltage: Users can select the appropriate test voltage, up to 1000 V for the SMFT-1000, 1587, and 1507, and up to 2500 V for the 1537, depending upon installation specifications.
• Any of these Fluke resistance tester models can test from negative to ground or positive to ground. Additionally, the SMFT-1000 can short-circuit the positive and negative of a string of PV modules. Then, it tests the short-circuited string to ground for circuits up to 20 A, as recommended in IEC 62446-1. This procedure has numerous benefits:
  • Improved Safety: By short-circuiting the string, you eliminate the risk of any voltage potential difference between the positive and negative terminals. This reduces the risk of electric shock during testing, ensuring a safer working environment for the electrician.
  • Higher-accuracy results: When testing between the short-circuited string and ground, effectively, you are testing the entire string's insulation resistance. This more accurately represents the insulation resistance of the entire string, enabling better issue identification.
  • Better issue detection: Testing between the short-circuited string and ground helps you identify potential issues with the insulation resistance of the entire string, including any weak points or damaged modules. This is crucial to maintain the safety and efficiency of the solar PV system.
  • Industry standard compliance: Testing between the short-circuited string and ground is compliant with industry standards, like IEC 62446-1, and best practices for insulation resistance testing of solar PV systems. By following these guidelines, you can trust that your testing procedures align with expert recommendations.

Applications of Fluke testers in solar

• Installation: Verifying cable integrity after wire installation, but before the ends have been terminated. The wire must be isolated from other conductors and any electronic components before IRT, so conducting an IRT at installation is the optimal time to verify the wire wasn’t damaged during the test.
• Commissioning New Installations: Ensuring all components meet safety standards. IRT can be done on any conductor in the system, including PV source circuits, combiner output circuits, and inverter output circuits. IRT can also be done between current-carrying conductors like L1, L2, or L3 on the AC output side to verify the absence of parallel arc faults.
• Maintenance and Troubleshooting: Monitoring insulation condition to preemptively address issues. Transformerless non-isolated inverters conduct IRT each morning before start-up. This test often identifies insulation fault concerns like ground or arc faults, but won’t pinpoint their location. IRT conducted with a handheld Fluke meter can identify which circuit has an insulation fault issue, and then the Fluke GFL-1500 can help you pinpoint the fault's location.
• Post-Repair Verification: Confirming the quality and safety of repairs and ensuring no wires were damaged during the repair.

What about CAT ratings and insulation resistance testing?

No voltage should be present when conducting an insulation resistance test. Any Fluke IRT meter that senses anything over 30 volts between the test point and ground will stop the test. In most applications, IRT is performed on isolated, de-energized conductors. For example, the AC inverter output circuits can be isolated by opening the inverter AC disconnect and the breaker or disconnect at the point of interconnection. This isolates the AC conductors from both the inverter and the power grid.

PV conductors are always energized when the array is exposed to sunlight. Voltage is measured as the potential between two points. Assuming there are no issues, measuring between the positive and negative of a string of modules will produce the full open-circuit voltage of the string. However, when conducting an insulation resistance test, you’re measuring between the positive and ground or the negative and ground, not between the positive and negative. In a properly operating string with no ground faults, voltage shouldn’t be present between the positive and ground or the negative and ground.

Conclusion

Solar professionals can certify that their installations are compliant, safe, and reliable by adhering to IEC 62446-1 and ANSI/NETA MTS standards for IRT and using Fluke insulation resistance testers. These practices are essential for the durability and efficiency of solar installations.