Code Corner: Insulation Resistance Testing, 2023 NFPA 70B
In 2023, for the first time, the National Fire Protection Agency (NFPA) explicitly included photovoltaic (PV) systems in their Standard for Electrical Maintenance Handbook. In our most recent episode of Code Corner, Ryan Mayfield opens up 2023 NFPA 70B to analyze one of the crucial tests prescribed in Chapter 30, the insulation resistance test. Even with proper commissioning, and gentle aging, wear, and tear, the insulation for a given circuit conductor cannot be taken for granted.
What is an insulation resistance test?
A helpful analogy to understand electricity flow is water flowing in a pipe. Say you wanted to verify the water was flowing at a desired rate, free of leaks. One way to check would be to isolate a section of the pipe, increase water pressure, and observe any leakage.
It’s a similar principle when testing for insulation resistance, but rather than upping the water pressure, a high voltage is applied across the conductor. Take, for example, a homerun conductor of a given PV system under test:
After disconnecting the conductor under test to isolate it from the rest of the circuit, and setting the leads of your trusty high-range resistance meter (ohmmeter) properly, the meter can both:
- apply a known voltage across the isolated conductor, and;
- inject a very small direct current.
The meter measures any return current, and the rest of the story is Ohm’s Law: using the known voltage across the conductor and the measured current through the conductor, the ohmmeter calculates an insulation resistance value. If the insulation is sufficiently damaged, the meter may not be able to complete the test and display an error message.
Watch the 10-minute Code Corner episode below for a closer look at the test process.
For an even deeper dive into this topic, check out the Electrical Testing Standards Guide we’ve been developing with Megger. There, you can find full-length chapters and how-to videos on insulation resistance testing, ground-fault location, electrical continuity verification, and much more.
If you have further questions or considerations regarding any PV maintenance tests or other code topics, our team is happy to help. Contact us to start a project or learn more. More of our Code Corner videos can be found on the Mayfield Renewables YouTube channel.
