What is Insulation Resistance Testing (IRT)?

When it comes to solar PV system maintenance and troubleshooting, few skills are as critical as understanding insulation resistance testing. This fundamental diagnostic technique serves as both a preventive measure and a powerful troubleshooting tool, helping technicians identify potential issues before they become costly problems and track down elusive ground faults that can plague solar installations. With numerous IRT meters available on the market – from traditional manufacturers like Megger and Fluke to specialized PV-specific instruments – choosing the right equipment and understanding how to use it effectively is essential for reliable field work.

The video excerpt below is taken from Solar Tech Collective’s comprehensive course on “Solar PV Ground Fault Troubleshooting: Theory, Tools, and Field Application.” Led by industry experts with decades of combined experience, this course dives deep into the practical skills needed to diagnose and resolve ground faults in both central and string inverter systems. If you’re looking to enhance your troubleshooting capabilities and gain the expertise that separates seasoned professionals from the rest, this course offers the hands-on knowledge and field-tested techniques you need to excel in solar system maintenance and repair.

Transcript below.

Testing insulation resistance is a key part of commissioning and troubleshooting any type of electrical system. Let’s go ahead and start off with the fundamentals. Now you notice on the screen here, IRT insulation resistance testing, you’re going to hear me say that over and over again. Sometimes it’ll mean IRT insulation resistance testing. Other times I’m going to use it in regards to the meter we use to perform the test. That’s right – an insulation resistance tester. 

So what is this all about anyway? We use an IRT meter to measure resistance – that’d be measured in ohms – by applying a constant voltage to a circuit, a conductor, or a piece of equipment that’s under test.

The analogy here is it’s like pressure testing water pipes. We apply a high pressure, in this case, a high voltage, and we make sure none of these electrons are popping out and leaking out of the circuit. If we have a low resistance value, that indicates we have damaged insulation or some path for electrons to escape the circuit.

Now you also hear this commonly called “meggering” because Megger (one of the several manufacturers of these meters) developed the first one back in 1889. You’ll also hear the term megaohm meter used pretty much interchangeably for IRT or insulation resistance tester.

 What is IRT Insulation resistance testing used for?

Well, one big thing is commissioning. We want to make sure that our conductors and equipment have proper insulation before we turn that system on for the first time. If we look at various standards out there that are applicable to PV systems – IEC 62446- testing insulation resistance is one of the things we’re supposed to do when we commission a system.

This allows us to find faults that may exist before we turn it on and use the power source to find the fault. We can also establish baseline values for that installation so that we can see that if it degrades over time, we know that something’s happening. 

This also might be performed as part of preventive maintenance, a regular testing regime that we do as part of maintaining the system. This would be based on standards, contracts, and equipment manuals. It’s a little invasive of a procedure, and there’s other ways to verify that things are working okay, so we don’t see this quite as often in PV systems, but it can allow us to identify conductor insulation degradation, and verify the integrity of equipment such as bussing insulation in switchgear and switchboards, transformer insulation breakers, and other gear associated with our system.