What is an Isolation Fault?

It is when DC electricity accidentally leaks to the ground. This usually happens due to:

• Damaged wires (unshielded or cut).

• Defective solar modules or power optimizers.

• Internal inverter faults.

When to Troubleshoot

Do it immediately. These faults often happen in the morning due to moisture and can "disappear" once the sun dries things out. You want to catch it while the error is active.

Locating the Leaking String

• Step 1
  Locate the Faulty String
  If multiple strings are connected to the inverter, find the faulty string by connecting one string at a time to the inverter and checking if the error is still displayed

• Step 2
  Verify
  Check each of the string separately to determine if there is more than one faulty string

Locating the Leakage within a String Using SetApp

• Step 1
  Access the Isolation Status Screen
  In SetApp, from the Commissioning menu select Maintenance -> Diagnostics -> Isolation Status

• Step 2
  Location of the Isolation Fault
  The value of the isolation resistance (in kΩ) indicates whether the fault is at DC+ or DC-:

  • 0% indicates the fault is at the DC+ side

  • 100% indicates the fault is at the DC- side

• To identify the location of the faulty Power Optimizer:
  Multiply the number of Power Optimizers in the string by the percentage value. The result is the module near which the fault occurred.
  
  
  For example: In a string with 15 modules and Power Optimizers and a percentage value of 68% :
  15*68% = 10.2
  
  This means that the fault is near module #10, counted from the DC+ side.

Replacing the faulty component

Replace the faulty component(s) as determined by the troubleshooting process.

Troubleshooting an Isolation Fault with SetApp using an Insulation Tester

In this procedure, you can check the resistance of system components, drilling-down from string level, through Power Optimizer/module pairs, and finally a single component – a Power Optimizer or a module. You can also check the wiring between the components.

Equipment Needed

• Insulation tester

• Connector branch cable

• Step 1
  Turn OFF the Inverter
  Turn OFF the inverter ON/OFF switch, and wait until SetApp indicates that the DC voltage is safe (<50V), or wait five minutes before continuing to the next step. 

• Step 2
  Disconnect DC
  Disconnect all the DC cables connecting the strings to the inverter or the Safety Switch. 

• Step 3
  Test the Resistance
  Test the resistance of the extension DC cables between the strings and the inverter (home-run cables):

  1. Disconnect the home-runs at both ends and measure each wire by connecting it to the positive probe, while the negative probe remains connected to the ground.

  2.  If the wiring resistance is less than 200MΩ, the wire is faulty; replace any faulty wires. 

  3. If the wiring resistance is 200MΩ or more, the fault is in the string that was connected to this extension cable;
     check the string (without the extension cable) as described in the following step.

Testing the String Itself:

• Step 4
  Test the String Resistance

  1. Verify safe string voltage and current: max. 50V and 500mA.

  2. Connect the positive and negative connectors of the first or last Power Optimizer in the string to the branch cable

  3. Connect the positive probe of the insulation tester to the branch cable. 

  4. Connect the negative probe of the insulation tester to a ground point.

  5. Select 500V testing on the insulation tester.

  6. Test the insulation.

• Step 5
  String Insulation Results
  If the resistance is less than 600kΩ in a single phase inverter or less than 1MΩ in a three phase inverter, a leakage is occurring. Continue checking this string’s components to isolate the source of leakage, as detailed in the following steps.

Test the Power Optimizer's and Module's Resistance:

• Step 6
  Test the Power Optimizers and Modules

  1. Disconnect the Power Optimizer from the string; it should remain connected to the PV module

  2. Verify safe Power Optimizer voltage and current: max. 1V and 500mA

  3. Connect the Power Optimizer’s positive and negative output connectors to the branch cable

  4.  Connect the positive probe of the insulation tester to the branch cable.

  5.  Connect the negative probe of the insulation tester to a ground point.

  6. Select 500V testing on the insulation tester and measure the resistance.

• Step 7
  Power Optimizer and Module Results
  If the measured resistance is 200MΩ or more, the Power Optimizer and module are not faulty.
  If the measured value is less than 200MΩ, the Power Optimizer, the module or both are faulty. Continue checking as detailed in the following steps in order to know which one is faulty:

Test the Power Optimizers Resistance:

Find whether the leakage is from the module or from the Power Optimizer. Test each component individually by measuring the resistance between negative wires and ground:

• Step 8
  Testing the Power Optimizer

  1. Disconnect the module from the Power Optimizer

  2. Connect the power optimizer’s positive and negative output connectors to the branch cable

  3. Connect the positive probe of the insulation tester to the branch cable

  4. Connect the negative probe of the insulation tester to the power optimizer frame (ground point)

  5. Select 500V testing on the insulation tester

  6. Measure the resistance.

• Step 9
  Power Optimizer Test Results
  If the value is 200MΩ or more – the Power Optimizer is fine, but the module is faulty.
  
  
  If the value is less than 200MΩ, replace the Power Optimizer and test the module in case it is also faulty, as described in the next step.

Test the Module's Resistance:

• Step 10
  Connect the Module
  Disconnect the branch cable from the power optimizer and connect it to the module:

• Step 11
  Test results
  Measure the resistance:

  • If the value is with 40MΩ*m2 or more – the module is OK.

  • If the value is less than 40MΩ*m2, the module is faulty.

  • The typical value is 80MΩ for each module.

Replacing the faulty component

Replace the faulty component(s) as determined by the troubleshooting process.