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)

Figure 1. Klein ET600 Tester in 1000 V Mode for Insulation Resistance Test

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.
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.
Disconnect both (positive and negative) conductors to test a string. The conductors can be disconnected from the inverter (on the ground) or on the roof.
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.
Test the string's positive conductor:
1. Connect the positive tester lead (red) to the positive conductor.
2. Connect the negative tester lead (black) to ground / equipment grounding conductor (EGC).
  Figure 2. Testing the Insulation Resistance of the Positive Conductor
3. Tape the ends of the wire on the opposite end of the run to help isolate.
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. If the insulation resistance is less than 1 Mega-Ohm, this string is causing the isolation failure.
  Note
  String voltage to ground (V DC) is expected to be a high value because it indicates an open circuit.
Test the string's negative conductor:
1. Connect the positive tester lead (red) to the negative conductor.
2. Connect the negative tester lead (black) to ground / EGC.
  Figure 3. Testing the Insulation Resistance of the Negative Conductor
3. Tape the ends of the wire on the opposite end of the run to help isolate.
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. If the insulation resistance is less than 1 Mega-Ohm, this string is causing the isolation failure.
  Note
  String voltage to ground (V DC) is expected to be a high value because it indicates an open circuit.

Identify the Failure in the Faulty String

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

Note

Watch the video above for visual examples of these failures.

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.