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Well i am no longer sure that such thing doesn’t exist : Look at this one https://www.specialized.net/test-equipment/power-quality-analyzers/non-contact-volt-detectors/extech-381676a-pen-digital-multimeter-w-voltage-detector.html

@jimf909 @hwy17 I just found out that my digital clamp Meter measures only AC current !! I am looking online for a new clamp meter that can measure DC current as well, but before I buy one I want to ask you if you know a clamp meter that can also measure DC voltage in a contactless way : I...

@All Thank you for the insightful tips. However there is still a missing piece of puzzle and nobody mentioned it : A current leakage somewhere in the string ( a damaged MC4 or Panel itself ) could reduce the whole current transmitted to the inverter Isn’t this a mechanistically plausible cause ?

Yes I have Clamp meter, please correct me if I am wrong : Clamp meters offer a contactless way of measuring current but it can’t measure voltage in a contactless way.

MPPT tracker range 350-800

So longer wires means automatically lower ohms resistance ? If we measure insulation resistance we expect higher resistance at the smaller string than the bigger one, right?

Ok, I have an additional question for you : Is it safe to measure string’s current while the inverter is running with clamp meter ( measuring current at the inverter’s DC input with clamp meter ) ?

but why should the MPPT reduce the current ? Isn’t that counter-productive ? The string that has 20 panels has a max VOC 620 volt while the smaller one has 320 volt, so in total 940 volt

No! The smaller string lays approx 3 meters below the bigger one !

Well your previous explanation isn’t quite wrong : Both strings are connected in series, and the bigger one has quite longer wires than the smaller one !

Please look at my reply to &seneysolar I hope you could give some useful tips on this Thx

Thx but that doesn’t answer my question : I have two strings, one string has 10 panels and the other has 20 panels. The panels are the same and have the same specs. By looking at the output of each string, I notice that the bigger string ( that has more panels ) has a higher voltage but less...

Hi Does more voltage mean automatically more generated current ?

No combiner boxes, strings are totally separate

Only string 1 is plugged in : voltage 606.1

SGI 9K And what’s the MPPT operating range ? As i previously said the inverter works fine even if one string has a VMP < 300 volt. The Panel‘s rated voltage ( U mppt ) = 27.2 volt Yes error 19 ( isolation fault ) pops up Yes that’s exactly what I tried to measure, however I measured in the...

No it didn’t matter Each string worked fine separately. After reboot I plugged both strings into the inverter but showed an isolation error, before the reboot there was no issue even with both strings plugged in. If I let both strings plugged in overnight, the inverter starts in the next...

Well, the inverter displays the voltage under load but I don’t think it’s 100% accurate. here is the manual uploaded as pdf as i didn’t find it online in english version ( only in German! ) As I said it’s a German made inverter ( production’s year 2011 or 2012 )

@Zwy one additional info I forgot to add : Behind the inverter I have an AC breaker ( 3 poles !) Every time I shut down the inverter, i turn this breaker off ! I have a 3 phase inverter, do you think this is causing the problem ? Perhaps a 4 pole AC breaker ( for the neutral line also ) is...

okay, just wanted to say a big thank you for all the help ! I just measured the VOC voltage after shutting down the inverter and here are the results : String 1 had 310 Volt String 2 had 618 volt String 1 is still below the MPP threshold as it has only 10 panels. Then I turned on the...

@Zwy Thank you very much for this. A faulty isolation relay is a potential cause. if there is a sustained earth fault on the DC side, why the inverter would pass the morning check and starts producing power ?

The error is a typical isolation fault

- No ,i didn’t install it - String 1 ( 20 panels ) is on the house‘s roof While string 2 (10 panels ) is over the garage ( so only 10 panels could fit there ) - yes if strings are connected separately, there is no issue and inverter starts working smoothly after reboot

AC side or DC side ? For the DC side I am using 4 mm2 DC cable up to 1000 volt

May i ask you what is 15s? If I connect only one string ( no matter which one ) the inverter connects to grid and starts working without any issue

Well according to the manufacturer if the MPP voltage is not reached, current is generated but not too much

Exactly, string one has 10 panels ( voc 31 volt per string ) while string 2 has 20 panels ( voc 31 volt per string ) The panels are the same! My understanding is that the inverter could still work even below the minimal MPP track voltage

Ok,good point! your explanation does make sense. However the documentation contradicts what you saying as the MPP track range is 350-800 volt ! At least that’s what‘s written in the manual !!

The ground wire is the yellow one. I have a 2012 kaco inverter (made in Germany ) it has two MPP tracker, according to the documentation the range is 350-800 volt. I have two strings : string one is connected to MPP1 ( voc 310 volt ) and string 2 is connected to the MPP2 ( voc 620 volt )

@wpns Great find ! I am happy for you My inverter is a 3 phase inverter but not hybrid. I didn’t have yet issues with tripping the AC breaker and the AC wires ( on the right side ) look fine, my issue is that my string inverter shows an isolation error immediately after reboot. let’s...

Thank you for your comment and information. I am aware of the safety precautions, but let’s forget the insulation resistance for now and focus on measuring the voltage to earth.when a ground fault is present, measurement will show Voc between positive and negative conductors ( dc+ to earth...

@wpns You mentioned that sometimes there may be a short at high applied voltages. Could you please explain a little ? Thx

Please don’t get me wrong : I understand that a megger is needed if you want to measure the insulation resistance by shortconnecting the positive and negative electrodes of the PV string, and measure the insulation resistance between the shorting point and earth ( the megger applies testing...

well this is actually a very important tip. My inverter shows an isolation fault after I reboot it! My inverter is a transformer-less ( No Galvanic isolation ) so every morning before it connects to the grid it measures the voltage to earth and if it’s fine it starts producing power. as I...

Yes I know however they should still be able to find if there is an earth fault, according to the inverter‘s manufacturer one should measure voltage between + and - to earth ( must be 0 volt ) And measure insulation resistance between + and - to earth ( without shortcircuit ) and result should...