Insulation Resistance Testing Explained
Megohm or insulation resistance tests validate the insulating properties of electrical conductors. A conductor with “good” insulation will have high insulation resistance – often measured in gigaohms, GΩ. A conductor with “bad” insulation will have lower insulation resistance, which can create a pathway for leakage current.
No insulation is perfect, but the goal of the test is to quantify the insulation’s resistance value to better understand the conductor’s health. In this article, we will use a PV string example to explain how the test process works and what steps to follow when performing the test in the field.
What is an insulation resistance test?
Insulation resistance testing is a non-destructive test procedure that measures the insulation resistance between a conductor and ground. The PV industry commonly uses the test before energizing the cables during project commissioning, during regularly scheduled maintenance, and as a tool for diagnosing system performance issues, especially earth/ground faults.
Performing an insulation resistance test can put higher than usual voltage on a conductor. The test instrument will also inject a small amount of current into the conductor during the test. If the insulation is compromised, the current will ‘leak’ out, and the instrument will use Ohm’s law to determine the insulation’s resistance.
When running an insulation resistance test on a compromised conductor, more current will leak out, and the tool will record a low resistance value. To pass International Electrotechnical Commission (IEC) standards for insulation resistance testing, PV systems with an open circuit voltage rating greater than 120 V DC must have an insulation resistance greater than 1 MΩ.
How are insulation resistance tests performed?
Insulation resistance tests place a high voltage on the components under test. Technicians must maintain a safe working environment for themselves and others working on the electrical system, starting by isolating all power electronics and surge protection devices from the circuit under test. Failure to isolate these components can result in damage to the components. Technicians who wish to perform the insulation resistance test through the PV modules must obtain approval from the module manufacturer. To avoid damage, the test voltages applied should remain less than or equal to the PV module’s maximum voltage rating.
Before performing tests, the working area must be appropriately identified and isolated. All technicians on-site must be notified of the tests so they do not inadvertently come in contact with any components under test. Technicians must also wear proper personal protective equipment (PPE). At a minimum, high-voltage gloves and safety glasses must be worn when performing tests.
Once the circuit under test is fully isolated—with disconnect(s) in the OFF position—and after verifying that expected voltage and current values are present within the circuit, it’s time to pull out the insulation resistance tester. The test setup will vary depending on which circuit is being tested.
To test through the PV string conductor, attach the tester’s red lead to the negative homerun termination point for that string and the black lead to the ground bar inside the inverter or combiner box.
When the test runs, DC voltage will be applied across the entire string. Note that the result displayed on the tool is 50 kΩ, which is below the 1 MΩ threshold set by the IEC and indicates that one or more of the conductors in the string could be damaged. Let’s attach the guard wire and retest to see if surface leakage is the cause of the low resistance reading.
To test with a guard wire (when available), the third terminal of the tester is connected to a conductive material (e.g., bare copper wire) wrapped around the conductor’s jacket. The guard wire will detect any surface leakage caused by contamination and adjust the insulation resistance reading accordingly.
Since we’re still testing the PV string conductor, the placement of the red and black test leads will remain the same. A third, typically blue-colored, lead is attached to a bare copper wire wrapped around the positive and negative homerun conductors. Note that the result displayed on the tool is now above 1 MΩ, indicating the PV string conductor is in good condition but likely covered in moisture or contaminants that increase conduction along the outer surface.
Re-testing with the guard wire won’t always lead to a passing result, however. What happens if the test still measures a low resistance of around 50 kΩ? We can isolate the string even further and retest one of the homerun conductors to pinpoint the problem.
To test a homerun conductor, completely isolate the series string homerun by going into the array and disconnecting the quick connects at the beginning and end of the string. Retest with the black lead attached to the ground bar and the red lead attached to either the positive or negative homerun termination point – depending on which homerun you intend to test.
Here, we’re testing the insulation resistance of the negative homerun conductor. Note that the tool displays a non-passing result. Since we have isolated the string circuit down to a single conductor, we can now assume there is damage somewhere along the negative homerun. Replace this homerun conductor, retest, and verify that the insulation resistance has risen above 1 MΩ.
Repeat the processes above for all strings in the array, retesting with the guard wire and isolating down to the homerun conductors when necessary. Document all results to track conductor health over time.
