Fire at combiner box

[thisissean]

Today I was testing a system while hosting a city inspection. 3 strings of 14 modules combined at a combiner box on the roof. Voltage reading at the inverter was only 18V positive to ground also 18V and negative to ground 0V. I went up to test the shoals fused combiner box on the roof. To test the string voltage independently I normally pop up the fuses at the combiner box rather than search around for the home runs. When I opened up the first fuse the block arced but did not catch fire. I could at least test that string now since it was disconnected from the others and got normal readings, around 400V. Now I was getting 14V from the 2 strings still combined. I opened up a 2nd fuse, presumably of a string where the polarity was reversed and the block caught fire. Once the fire was out I found that 1 of the strings was indeed reversed. It goes without saying that the inverter was off during all of this testing. The home runs also arced when I eventually disconnected them. I couldn't get a good explanation on why reverse polarity on 1 string would start a fire.

Am I wrong in saying there is no current traveling though the home runs when the inverter is off?
Can lines arc when there is no current?
Why does 1 string being reversed result in my voltage reading being 18V?

Thanks for your input

[Sunking]

Why does 1 string being reversed result in my voltage reading being 18V?

Thanks for your input

Because it shorted out the rest of the other strings which means little voltage and full current.

Try this experiment under the hood of your car. Take a volt meter and connect it to the battery and monitor the voltage. While watching the voltage take a jumper cable. You only need one of them red is best. Connect 1 clamp to the positive battery post, and the other clamp to the negative post. Note the voltage when you do that and see what happens.

The voltage will drop down toward Zero and a big ball of fire turning your red jumper cable black and crispy.

[Mike90250]

... Try this experiment under the hood of your car. Take a volt meter and connect it to the battery and monitor the voltage. While watching the voltage take a jumper cable. You only need one of them red is best. Connect 1 clamp to the positive battery post, and the other clamp to the negative post. Note the voltage when you do that and see what happens.

The voltage will drop down toward Zero and a big ball of fire turning your red jumper cable black and crispy.

NO. NEVER DO THIS. use the black cable The black insulation resists heat for 3 more degrees than the red plastic. This will allow it to contain the plasma ball for 0.005 seconds longer, and give you time to blink and fuse your eyelids to your eyeball, protecting them from the battery acid.

[russ]

This fellow should consider becoming a Walmart greeter!

Until you understand electricity better than this it is totally unsafe to have him in the neighborhood!

[thisissean]

This fellow should consider becoming a Walmart greeter!

Until you understand electricity better than this it is totally unsafe to have him in the neighborhood!

Really helpful, thanks dude.

[Mike90250]

....

Am I wrong in saying there is no current traveling though the home runs when the inverter is off?

In a properly wired system, when the inverter is off, the PV DC wires should have full voltage, but no current flow.

Can lines arc when there is no current?

From hot line to chassis, or some other conductive part, yes they can arc.
If you are asking if a " Touch - Safe " fuse holder can arc when you pull a fuse, yes it can, even at it's full rated voltage, but it should not catch fire. Unless you are using AC components in a DC circuit.

Why does 1 string being reversed result in my voltage reading being 18V?

because one string has a few MA more power than the other string, and the winning string shows some voltage.

My question is, the combiner box, was it a commercially made unit for DC, or a DIY box of DIN rails and AC fuse holders ?
It should not have burst into flame, and that worries me, because there is not supposed to be any flammable materials in it.

What did the inspector think of it ?

[FloridaSun]

Try this experiment under the hood of your car. Take a volt meter and connect it to the battery and monitor the voltage. While watching the voltage take a jumper cable. You only need one of them red is best. Connect 1 clamp to the positive battery post, and the other clamp to the negative post. Note the voltage when you do that and see what happens.

HAHAHAHA! you sadist...

[bcroe]

Today I was testing a system while hosting a city inspection. 3 strings of 14 modules combined at a combiner box on the roof. Voltage reading at the inverter was only 18V positive to ground also 18V and negative to ground 0V. I went up to test the shoals fused combiner box on the roof. To test the string voltage independently I normally pop up the fuses at the combiner box rather than search around for the home runs. When I opened up the first fuse the block arced but did not catch fire. I could at least test that string now since it was disconnected from the others and got normal readings, around 400V. Now I was getting 14V from the 2 strings still combined. I opened up a 2nd fuse, presumably of a string where the polarity was reversed and the block caught fire. Once the fire was out I found that 1 of the strings was indeed reversed. It goes without saying that the inverter was off during all of this testing. The home runs also arced when I eventually disconnected them. I couldn't get a good explanation on why reverse polarity on 1 string would start a fire.

Am I wrong in saying there is no current traveling though the home runs when the inverter is off?
Can lines arc when there is no current?
Why does 1 string being reversed result in my voltage reading being 18V?

This may now be obvious, but when the strings were wired, but open circuit at the combiner,
each string should be tested for proper voltage. Then the polarity of all strings to be connected
in parallel should be noted as THE SAME. Just measuring the voltage between one hot (fused)
lead and the others should show nearly no voltage.

If a reversed string IS connected to a normal string(s), you will have a lot of current flowing
around the double loop, and turning off the inverter won't help. A voltage measurement here
won't mean much, its just a matter of which circuit is over powering which. A blown fuse could
save the day, but it will need to interrupt double the normal voltage; same for a combiner
disconnect. Do proper testing, good luck. Bruce Roe