Riso fault Power-One Aurora

[goodwell]

I have a small Pv farm 5KW. Some months ago inverter started to measure low Riso<1MΩhm (0.8). I tried to test each string alone and with one the inverter read good Riso and started normally. The other had very low Riso. I tried to check the two pv strings but both have same Voc 340Volts. So I think the ground fault may be in DC cable of the second string until inverter. How can I check it with the multimeter? In which way can I check the DC cable (~10m length)? In Ωhm position in multimeter which value should I see and how should I connect with the probes? With ground to one side or not?

 

[Solar_Phil]

You'll need to do an IR test on the string. More than likely a failing MC4.

 

[goodwell]

You'll need to do an IR test on the string. More than likely a failing MC4.

Please could you explain more the procedure?
I tested the Voc of two strings in the roof and they have the same Voc.
So afterwards should I check V between ground and + to see the remaining voltage to find the faulty panel/MC4? And the same procedure down to inverter to see if the cable has any issue?
Thank you very much for your help.