Voltage between array negative and ground

I recently wired up a combiner box, where several strings of panels come together, and are combined into a single set of +, -, G wires that run to the inverter. While poking around in the combiner box with a multi-meter, checking for any weirdness, i'm finding that there is some voltage between the array negative output and the ground wire. When i test there, i see 35v or so very briefly, and the voltage rapidly (2-3 seconds) drops to 15v or so, and then continues to drop more slowly, approaching zero. The rate of voltage drop seems to change, perhaps on some kind of exponential curve such that it drops a lot at first and then more and more slowly, and may take a long time to get all the way to zero. Note that this is with the array circuits open, and no current flowing. I'm not sure what to make of this. The way that the voltage seems to dissipate when measured makes me think that there's not really much energy there, but i really don't understand it, so i don't want to dismiss it without further investigation.
Any ideas on what could be causing this, or how to determine if it constitutes a problem? Should I not turn on the inverter before resolving this?
More information if helpful: The array is 40 panels, 65W each. They are arranged in 10 series strings of 4 panels each. The 10 strings are then combined into two series groups of 5 strings each, and the two groups are paralleled and fed to the inverter in a single wire pair. Voc of the final output is 410V. The array ground is connected to a grounding rod and the house distribution panel ground.
Any ideas on what could be causing this, or how to determine if it constitutes a problem? Should I not turn on the inverter before resolving this?
More information if helpful: The array is 40 panels, 65W each. They are arranged in 10 series strings of 4 panels each. The 10 strings are then combined into two series groups of 5 strings each, and the two groups are paralleled and fed to the inverter in a single wire pair. Voc of the final output is 410V. The array ground is connected to a grounding rod and the house distribution panel ground.
Comments
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A
solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,
Are the panels rated for that string voltage? If I'm reading right, it's essentially 20 panels in a 410v series string, in parallel with another 20 panel string.
The array ground goes to both the AC panel and the ground rod? Is AC neutral bonded to ground in the panel?
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
Yes, basically 2 strings of 20 panels each.
There is a grounding rod, and both the AC panel and the solar array grounds are tied to it. Yes, neutral and ground are bonded in the AC panel. As far as i can determine, the array DC(-) is not connected to anything.
The panels are rated for a max system volltage of 600v.
The reason the module strings are wired somewhat oddly is that it was originally an off-grid system with a 48v battery bank. There were 10 strings, each made of 4 modules in series. The system voltage was 82Voc. I'm switching it over to grid-tied and the new grid tie inverter wants much higher DC input voltage, so i took the 10 original strings of 4 panels and made it into 2 strings of 20 panels to bump the voltage from 82v to 410v.
I don't think that the configuration is relevant to the issue, but i could be wrong.
When i put a meter between ground and the final DC- I expect to see zero, but instead i see the voltage described in the original post.
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A
solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,
It could be a phantom reading, as McGivor suggests, or a real voltage which diminishes over time as the meter in essence creates the missing DC negative to ground bond.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
-Bill