Perform Insulation Resistance Test to Identify Faulty String

Insulation Resistance Test Overview

Insulation resistance testing is performed by “injecting” potential onto the conductor in the form of DC voltage, with a second conductor or ground conductor as a reference point. Any “leakage" of the potential indicates a fault in the insulation.

Note

If an Isolation Fault error is present in the device setup interface, look for poor wire termination at the Powerwall 3 (wires not fully seated in terminals, wire strands are frayed). If poor wire terminations is not the issue, disconnect all strings from the terminals before proceeding, as the insulation resistance test is performed open air (meaning the conductors are not connected to the MPPTs during the test).

Required Tools:

Insulation Resistance Tester in 1000 V test mode (Tesla recommends Klein ET600)
(2) extra MCIs (see Explanation for Testing with Temporary MCIs)

Identify the Faulty String

Note

If testing a long string, perform isolation testing on shorter sections within the string to isolate the problematic segment.

Note

Perform this testing after wires have been pulled through but before they have been landed at the Powerwall 3.

Safely shut the system down:
1. Push the System Shutdown Switch if one is present, then turn the Enable switch OFF.
2. Open the Powerwall 3 circuit breaker (turn OFF).
3. Wait 30 seconds before proceeding with any work.
  Warning
  Confirm Powerwall 3 is de-energized before proceeding. Confirm the breaker is open (OFF) and solar production is disabled in Tesla One.
Temporarily connect the (2) extra MCIs to the string to test for ground faults:
Note
If possible, connect the extra MCIs at Powerwall 3 to test the entire string. The extra MCIs can be connected on the roof, but they will not test the wiring between the roof and Powerwall 3 in this configuration.
Note
The extra MCIs are not required if the string already has an MCI at either end of the string.
1. Create (4) jumpers, two for each MCI, by crimping MC4 connectors to short lengths of PV wire. Connect the jumpers to the extra MCIs.
2. Connect the (2) extra MCIs to each end of the PV string.
Plug the positive and negative meter tester leads into the meter. For combination insulation resistance and multimeter testers, insert the test leads into the designated insulation testing ports.
Set the tester to 1000 V mode.
Test positive to Ground :
1. Connect the insulation resistance tester positive lead (red) to the string's positive conductor.
2. Connect the insulation resistance tester negative lead (black) to the string's equipment grounding conductor (EGC).
3. Insulate and protect the string's negative conductor, as it is live and is not being tested during this step.
4. Press and hold the TEST button to begin the test. Allow the reading to stabilize before recording the measurement (take the resistance value while STILL holding the TEST button).
  Note
  Keep the test leads connected to the circuit until it is completely discharged, and the display shows near zero volts.
5. Confirm the resistance value is greater than 1 MΩ. If the value is less than 1 MΩ, there is likely a damaged wire and/or ground fault in the system. Look for nicked or pinched wires, improper field-made connectors or improper crimps, or evidence of damaged modules that need to be replaced.
  Note
  String voltage to ground (V DC) is expected to be a high value because it indicates an open circuit.
Test negative to Ground :
1. Connect the insulation resistance tester negative lead (black) to the string's negative conductor.
2. Connect the insulation resistance tester positive lead (red) to the string's equipment grounding conductor (EGC).
3. Insulate and protect the string's positive conductor, as it is live and is not being tested during this step.
4. Press and hold the TEST button to begin the test. Allow the reading to stabilize before recording the measurement (take the resistance value while STILL holding the TEST button).
  Note
  Keep the test leads connected to the circuit until it is completely discharged, and the display shows near zero volts.
5. Confirm the resistance value is greater than 1 MΩ. If the value is less than 1 MΩ, there is likely a damaged wire and/or ground fault in the system. Look for nicked or pinched wires, improper field-made connectors or improper crimps, or evidence of damaged modules that need to be replaced.
  Note
  String voltage to ground (V DC) is expected to be a high value because it indicates an open circuit.

Note

The diagrams above illustrate example PV stringing configurations; the number of MCIs and where they are installed in the string varies by installation configuration.

Note

Paralleled strings must be tested independently. This must be done on the roof, and can be done by undoing Y-connectors or removing jumpers inside the combiner box.

Explanation for Testing with Temporary MCIs

The insulation resistance test outlined above uses a temporary MCI connected at either end of the PV string to mitigate the following scenario:

Panel(s) are present at the end of the string (between Powerwall 3 and the first MCI in the string)
A ground fault is present in the string upstream of the first MCI
Because of the ground fault, voltage is present on the string, meaning the insulation resistance test cannot be completed (the tester will display a failure due to presence of voltage)
Figure 1. Failed Test Due to Missing Temporary MCIs

The addition of a temporary MCI at either end of the string ensures the insulation resistance test can be completed in this scenario. While it is possible to perform an insulation resistance test without connecting the temporary MCIs, and to simply add the MCIs if the test fails due to the presence of voltage, Tesla recommends starting with the temporary MCIs so that the test need only be completed once.

Note

This scenario is not the only one which will result in an error on the insulation resistance tester, and the error(s) displayed depend on the tester being used. See MCI and PV String Issues in the Powerwall 3 Device Setup Guide for additional troubleshooting steps depending on the issue(s) observed.

Identify the Failure in the Faulty String

To find the failure, perform a close inspection of all the components in the faulty string:

Inspect where the wires exit conduit, and where wires pass through conduit bodies, as nicked wires may occur here
Check for wires pinched by mounting hardware or damaged by fasteners
Note
For Solar Roof installations, hardware may be accidentally run through wires when installing the tiles.
Check for improper field-made connectors or improper crimps
Look for evidence of damaged modules:
- Microcracks
- Signs of water intrusion
- Module leads are bent at sharp angles
For the faulty string, try testing the string with the DC grounding conductor (EGC) removed from the inverter's ground terminal; if the string passes self-test with the grounding conductor removed, it is likely the string is faulting to ground somewhere

If any of the above issues are found, fully replace the damaged wire and/or perform the necessary repairs to ensure the system's lifespan is not compromised.

If none of the issues above are observed, see MCI Diode Test and Resistance Test (MCI Health Tests) to determine if a faulty MCI is causing the isolation failure.