DC Voltage Test (Completed String and Polarity)

The purpose of this test is to:

Verify that the string polarity is correct
Detect any open circuits
Identify whether a string has been combined after a parallel connection

This test is performed before connecting the solar conductors to the Powerwall 3 MPPTs and turning the system on, and before performing an insulation resistance test on the string. Because MCIs have a very high resistance when open, normal DC voltage testing will not provide accurate V_OC readings. The test is especially useful when performed near the Powerwall 3.

Warning

This test must be performed when Powerwall 3 is OFF and the system is OFF (stopped) in the device setup interface in Tesla One. Failure to do so could cause irreparable out-of-warranty damage to the Powerwall 3 when swapping strings between MPPTs.

Note

The test is performed open air, meaning the conductors are not connected to the MPPTs during the test.

Note

Different testers have varying impedance levels, which can lead to different readings depending on the brand used.

Required Tools

Insulation resistance tester or multimeter in DC test mode

Tesla recommends one of the following:

Klein ET600
Klein CL800
Fluke 374 FC

Perform DC Voltage Test

Set the tester to DC test.
Klein ET600 Klein CL800 Fluke 374 FC
Note
If using the Klein CL800 multimeter, push the orange SEL button after setting the tester to DC test (indicated on the tester screen). Without pushing the orange button, the tester will perform the AC voltage test.
DC Test (Correct) AC Test (Incorrect)
Plug the positive and negative meter side leads into the meter.
Connect the positive lead to the positive lead input.
Connect the negative lead to the negative lead input.
Klein ET600 Klein CL800 Fluke 374 FC

Confirm the voltage reading:


Tester	Individual String	Combined String
Klein ET600	Voltage reading between 10 V and 29 V	Voltage reading between 25 V and 48 V
Klein CL800	Voltage reading between 10 V and 29 V	Voltage reading between 25 V and 48 V
Fluke 374 FC	Voltage reading of approximately 2 V	Voltage reading of approximately 4 V

Note

This voltage is not an accurate reflection of the module configuration. The voltage measurement can vary based on several factors, including time of day, the number of MCIs in the series and their placement, and the number of modules in the series.

Because of these variables, the voltage reading likely will not be consistent even across strings with the same number of modules, especially if they are on a different mounting plane or tested at a different time.

Note

It is still technically possible for a combined string to show a voltage below 25 V (if using the Klein ET600 or Klein CL800 tester), depending on factors like time of day, how many modules are in the string, or MCI placement. However, it is highly unlikely that a single, isolated string will produce a voltage as high as 29 V unless MCIs were misplaced or miswired. Use the elevated voltage as a clue when trying to identify or relabel strings during troubleshooting. The voltage of a combined string is typically equal to the sum of the two individual string voltages after they’ve been paralleled.

For example, if using the Klein ET600 or Klein CL800 tester, string 1A reads 18 V and string 1B reads 17 V. The combined string will show an additive value of ~35 V when tested after paralleling the connection.

If using the Fluke 347 FC tester, string 1A reads 2 V and string 1B reads 2 V. The combined string will show an additive value of 4 V when tested after paralleling the connection.

If the voltage reading is 0 V, it may be due to one of the following causes:
- There is an open circuit (no string closing the loop between the positive and negative terminals of the MPPT). This is most likely due to a missed connection while wiring the solar modules. Please double check that all module connections have been made and that all home run connections are fully secured.
  Note
  If using the Klein CL800 multimeter, the tester will display a very small amount of voltage (measured in mV); this is the equivalent of a 0 V result on other testers and generally indicates an open circuit.
- The positive and negative conductors of the string have been mixed up with other conductors, likely due to incorrect labeling.
  - In this example, the conductors for strings 1 and 3 have been swapped (which often occurs because of incorrect labeling), as indicated by the DC test result of 0 V:
  - After correcting the wiring by swapping the positive conductors for strings 1 and 3, both strings are now producing voltage readings between 10 and 29 V (indicating that wiring is correct):
If the voltage reading is negative:
- The string likely has reverse polarity, meaning the positive and negative home runs were mixed up during wire management. This can be corrected either on the roof or by re-labeling the wires correctly at the inverter, and landing them in the appropriate MPPT input.
- Alternatively, check the meter:
  - Make sure the test leads are plugged into the correct ports, and that the correct lead is being used on the correct wire.
  - A negative reading can also occur if using the negative lead on the positive wire, and vice versa.