Why did this combiner box catch on fire?

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SolarPro said:
If you can convince the installers to leave the fuse holders open until the voltage check happen, I think you can avoid situations like this in the future. Once the fuse holder is closed, you are relying on someone taking a current measurement before opening the fuse holder. While that is a best practice on paper, it's an easy step to skip in the field. It not only requires a more specialized meter but also assumes everyone is always paying attention to what they are doing, which isn't the case on any job I've ever been on. You always have noobs who only know how to do the tasks you have trained them to do. Plus even the best and brightest get fried after a day in the sun. Seems better to design the workflow around that step, which I think you can do by keeping the fuse holders open, then closing them one at a time after the voltage tests.

Maybe there's an even better way. What are other people doing?
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This.

Leaving fuseholders open (don't even leave fuses there) is absolutely crucial for safety to installers.
This is key for many reasons: including the fact that you may be at inverter, far away from Combiner Box, wiring it up or checking things and installer on roof closes the fuseholder.....

I don't know if larger commercial inverters or any residential ones still have GFI fuses, but removing them while installing will add a lot to installation safety also.
 
Check the wires from chB going up into the inverter are wired correctly inside the inverter. I've ran into that before on those inverters. That will drop out the voltage and cause it to arc like that.


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Just a question to the OP has the root cause been determined for this? Curious to know if it lines up with what was covered in the thread.
 
MechEdetour said:
Just a question to the OP has the root cause been determined for this? Curious to know if it lines up with what was covered in the thread.
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Yes. Connecting the short string in parallel with the 1,000 volt strings caused the failure:

He basically theorized that two modules in series hooked up to strings of 800V were not able to resist that higher voltage and shorted out, backwards. You would have current flowing backwards across those two modules because the potential difference from their positive to negative terminals is in essence about 700V the wrong way, which their cells and diodes might not stand up to. If he guessed right, those two modules were the load. Inspecting those modules might possibly tell you something (or the damage might not be visible).
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This meant that current was flowing in the circuit when the OP opened the fuseholder. that's all it takes to start a fire in a combiner.
 
So I know that strings in parallel should match each other in length or keep close but is there a cushion? Say if the fuses were removed and disconnect open would it have blown up too once the disconnect was closed after commissioning? If it made it thru and for some reason one was to troubleshoot a string if one opened the disconnect if would have blown up as well. I don't have experience with combiner boxes.
 
Ozymandias said:
So I know that strings in parallel should match each other in length or keep close but is there a cushion? Say if the fuses were removed and disconnect open would it have blown up too once the disconnect was closed after commissioning? If it made it thru and for some reason one was to troubleshoot a string if one opened the disconnect if would have blown up as well. I don't have experience with combiner boxes.
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IMHO the combiner box blew up because there was current in an unexpected place (from long strings back through short strings) even when the disconnect was open. And trying to remove a fuse which was carrying current resulted in an arc, since the fuse hold was not meant to interrupt load by being mechanically opened rather than by the fuse blowing.
It is also possible that the fuse holders were not rated for that DC voltage in the first place.

In any case, closing or opening the disconnect only (output side of combiner) should not cause any direct problems.
 
As the OP said, what the technician did - hook up a string of two modules, not per plan - was inexcusable.

If the fuses had not been installed, there wouldn't have been a blowup. The failure in process here was the closing of the fuse holders on all the strings before commissioning. As other comments in this thread have alluded to, it's not a bad idea to simply withhold the fuses from the installation crew, and only give them to the senior tech who does the commissioning. (In this case, the OP.) That way, the incorrect voltage on the string of two modules could have been caught and fixed before damage was done. At commissioning, you test each string for correct voltage, polarity, etc, and then after that put in the fuses and close the fuse holders.
 
Seen fuse holders burnt up before what happens is when you connect two strings together instead of just one example.
You have a 20 string and connect another 20 strings by accident now you have 40 panels in series and fuses were rolled in at the time.
Open it bang arc flash.
 
Also appears no hubs used on tops of enclosures if on exterior? Water entry?312.2.
 
Greentagger said:
Also appears no hubs used on tops of enclosures if on exterior? Water entry?312.2.
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Probably raintight connectors with gaskets that meet the letter of the code, if not best practice.

However both your comment and the previous one are not the reason for the problem here, and the original thread is over four years old.
 
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