Burned out fuses

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Zee

Senior Member
Location
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Interesting problem.

Why did the fuses burn out on the following system?

SYSTEM SPECS:
Built in 2014.

4 strings of Solar World 270W modules. (8.81A Imp and 9.44A Isc)
All 4 strings pass through a Soladeck J-box mounted to roof surface.
8 fuses inside Soladeck: 4 + and 4 - fuses.
20A fuses.
Strings just pass through fuses to inverters - no combining in Soladeck. (All PV source circuit conductors continue individually to inverters.)

2 strings feed one inverter and 2 feed the other.
2 @ SMA SB5000TL-US-22, 5,000 W inverters.
Dual input/MPPT inverter.
Ungrounded system.

ISSUE:
Found 2 fuses burned out. Probably over the last year, one fuse in string #2 and one in string #3 burned out.
These feed separate inverters.

Insides of most white plastic fuse-holders browned and several fuseholders fused/melted together.
Wire ends near terminals of fuse-holders blackened.
Fuseholders browned even where fuse was intact, but to slightly lesser degree than the fuse-holders that had failed fuses inside.

Both inverters working fine on one remaining string each when I visited. Green lite solid.
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I have 4 questions and I will try not lead in any direction.

1. Why would 20A fuses burn out on a roughly 9 Amp circuit?

2. Was fusing even required per NEC on this system? (No combining of strings and separate inverters also. But 4 pv source circuits do share same conduit to inverter area.)

3. Regardless of NEC, does fusing serve any safety purpose in such a case?

4. Would you replace fuses with 20A, 25A, 30A, "slugs" (dummy "fuse", solid metal, not a fuse), or....?


 
Interesting situation. With only two strings per inverter, I don't believe fusing would be required on any of the strings, even if the strings were all in the same conduit. What size homerun wire did you use? Why did you put the fuses in the Soladeck? If you are going to install fuses, they should be as close as possible to the point where the circuits are combined, which is generally in the inverter, although with only 2 strings per inverter, this shouldn't matter. Seems like the system should have been fine with 15A fuses? Is the Soladeck exposed to sunlight? Could high temperatures have something to do with it? I will be curious to see what others suggest.
 
1) I would suspect from what you described that the fuse holders and/or fuses were not rated for the rooftop temperatures, or that something about the install made the temps in the Soladeck extra-ordinarily high. Hard to see how an actual overcurrent could occur, I would need some other type of evidence of unusual events (damage to inverter? Lightning strike?) to take that seriously.
2) No, fusing was never required.
3) No safety benefit.
4) I would probably try to remove the fuse holders entirely and splice the wiring with whatever you normally use in a j-box. Double check that whatever you splice with is appropriately rated for rooftop temps upwards of 120F.
 
I've been installing solar for almost 3 years and over a year of it was strictly O&M. I've seen situations similar to this several times. A few things I have seen are people using din rail terminals to splice/change the PV source circuits from USE-2 or PV wire to some other wire like 10AWG THHN stranded. Condensation will buildup in the box and drop from the lid causing the strings to shortcircuit against each other. Sometimes as many as 10 strings going to 2 different inverters in 1 box. Makes a big charred mess and they are usually still welded together when I show up to diag the issue. Sometimes the people land the wire in the terminal incorrectly causing minor arcs and overheating. I've seen fuses that were not rated for the correct DC voltage. I've seen a lot. I'd bet it's one of the issues I mentioned. As far as code or fuses required, fuses aren't technically required until you have MORE THAN 2 STRINGS on a single MPPT. On those particular inverters you could have 4 strings and require no fusing on the PV source circuits. However there is code in 2014 in 690 in wiring methods permitted or what ever it's called, that says the circuits of separate systems are not to be installed in the same raceway or enclosure UNLESS they are neatly grouped together and separated by a partition. The fuses do add benefit as longs as the fuse blocks dont melt and allow the lugs to short to each other. The fuses will open if the strings short together and you have excessive current traveling through the 12AWG PV leads on the panels. However, I've managed to avoid all of the aforementioned issues by just splicing my wires in junction boxes with MC4's. They are rated for everywhere you would use them, just a little bulky. Do you have pictures?

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PWDickerson said:
Interesting situation. With only two strings per inverter, I don't believe fusing would be required on any of the strings, even if the strings were all in the same conduit. What size homerun wire did you use? Why did you put the fuses in the Soladeck? If you are going to install fuses, they should be as close as possible to the point where the circuits are combined, which is generally in the inverter, although with only 2 strings per inverter, this shouldn't matter. Seems like the system should have been fine with 15A fuses? Is the Soladeck exposed to sunlight? Could high temperatures have something to do with it? I will be curious to see what others suggest.
Click to expand...

I put fuses in the system to have a future means to troubleshoot individual circuits....should I ever need to. As a terminal point to use a multimeter and to open circuits.:cool:

Also, frankly, I may have been unsure whether 4 circuits really are safe together w/o fuses. It just made me wary enough that i threw in fuses, why not?!/?
(been doing this so long, that when I see 3 or more circuits...... i think fuses :roll: )

Yes, Soladeck is exposed in sunlight on roof...I wonder about heat myself...but do not know if it is a significant factor.
 
jaggedben said:
1) I would suspect from what you described that the fuse holders and/or fuses were not rated for the rooftop temperatures, or that something about the install made the temps in the Soladeck extra-ordinarily high. Hard to see how an actual overcurrent could occur, I would need some other type of evidence of unusual events (damage to inverter? Lightning strike?) to take that seriously.
2) No, fusing was never required.
3) No safety benefit.
4) I would probably try to remove the fuse holders entirely and splice the wiring with whatever you normally use in a j-box. Double check that whatever you splice with is appropriately rated for rooftop temps upwards of 120F.
Click to expand...

Appreciate it, JBen!

1. They are 600 vdc fuseholders for pv...and "PV" fuses!
No damage to inv.s.
No lightning. I mean I see no damage anywhere on roof or at inverters.
Homeowner would probably know if his house was hit???
We effectively do not have lightning in this area....although he is up on hilltop.
2. and 3. Good to know.
4. OK. Wirenut OK?
 
Wirenuts are not ok. Pv course circuits should be connected using devices that latch/lock together or be landed in a terminal that is rated for the wire voltage and current.

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With regard to temperature, you might try these. The indicators are irreversible. Once the temperature is exceeded it's permanently marked.

8M_LABELS_l.jpg

They come in different temperature ranges. Two labels would cover you from 110°F to 250°F in 10 degree intervals. It's about $30 for a package of 10, less shipping.
 
Because wire nuts in a humid location exposed to the heat and cold expanding and contracting is asking for future failures. Remember the PV source circuits can not be turned off and when those wire nuts fail and the strings start parallel arcing both inverters will likely shut down do to the AFCI protection. Also correctly installing the wire nuts on PV wire is going to be difficult if not impossible. The amount of insulation stripped off the PV wire to allow the wire nuts installed securely would be potential for arcing. And when water gets in the soladeck(which it will) the wire nuts are not going to prevent the water from causing series and/or parallel arcing amongst the circuits. Would you put wire nuts on SER supplying the MSP? I think anytime there is no OCPD between the power source and the conductors termination wire nuts are a bad idea, especially outside. I've never had any issues using MC4's to splice my wires. There may not be code against using wire nuts but if your in the solar business you wont be doing anyone a favor installing wire nuts.

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solar.stevepvip said:
I've been installing solar for almost 3 years and over a year of it was strictly O&M. I've seen situations..........Do you have pictures?
Click to expand...

Interesting experiences. Appreciated!

On a side note: My understanding of Code is two "separate systems" are two systems feeding two meters, i.e. with two points of connection.
However, two inverters on one meter (this case) is not 2 "systems" per Code, IMHO.

I should be clear it wasn't a big charred mess, the insulation had browned on the wire ends and fuse-holders were intact (albeit browned inside around fuse).

Interestingly, fuses # 2 and # 3 failed....but they each go to separate inverters.

Also FWIW...a direct short is hard as I place all + together and also all - next to each other. See photo: all + in a row, and then all - in a row.
(older CB's had alternating + and - terminals......poor design, IMHO )
 
solar.stevepvip said:
Because wire nuts in a humid location exposed to the heat and cold expanding and contracting is asking for future failures. Remember the PV source circuits can not be turned off and when those wire nuts fail and the strings start parallel arcing both inverters will likely shut down do to the AFCI protection. Also correctly installing the wire nuts on PV wire is going to be difficult if not impossible. The amount of insulation stripped off the PV wire to allow the wire nuts installed securely would be potential for arcing. And when water gets in the soladeck(which it will) the wire nuts are not going to prevent the water from causing series and/or parallel arcing amongst the circuits. Would you put wire nuts on SER supplying the MSP? I think anytime there is no OCPD between the power source and the conductors termination wire nuts are a bad idea, especially outside. I've never had any issues using MC4's to splice my wires. There may not be code against using wire nuts but if your in the solar business you wont be doing anyone a favor installing wire nuts.

Sent from my SM-G955U using Tapatalk
Click to expand...

Well I've yet to see any failures so far. Taped up wire nuts are probably less likely to short than most other options. The question about the MSP is a bit disingenuous because you don't see wire nuts for 100A conductors. I'd use a listed connector like polaris for that; wire nuts on #10 is also listed. Fortunately I'm not splicing too many straight up source circuits these days, almost all optimizers and microinverters. From experience I don't agree about PV wire insulation. I think using MC4s is a fine idea but many jboxes are too small for that. I've used Buchanan splices a lot, but they actually have worrisome temperature limitations. I just kind of think that with what's available it comes down to workmanship, mainly facing things up so they drain, adding tape to prevent untwisting.
 
Zee said:
Interesting problem.

Why did the fuses burn out on the following system?

--------------------------------------------------------------------
I have 4 questions and I will try not lead in any direction.

1. Why would 20A fuses burn out on a roughly 9 Amp circuit?
Click to expand...

High ambient temperature reducing the fuse rating to the point that the fuse blows at a lower current. Most fuses are rated up to 40°C.
High temperature in the terminals due to poor contact heats up the whole fuse holder and fuse. Eventually melts the fuse holders and causes fuses to blow at lower current. (I would assume this myself)

2. Was fusing even required per NEC on this system? (No combining of strings and separate inverters also. But 4 pv source circuits do share same conduit to inverter area.)
Click to expand...

No, it's not required.

3. Regardless of NEC, does fusing serve any safety purpose in such a case?
Click to expand...

No

4. Would you replace fuses with 20A, 25A, 30A, "slugs" (dummy "fuse", solid metal, not a fuse), or....?
Click to expand...

Assuming you are just using this to transition conductor types use a simple box. Check with the manufacturer to see if using a slug is approved. That would be an easy out. It would not solve the problem if the terminations are high resistance though, you will still come back to melted parts.
 
pv_n00b said:
High ambient temperature reducing the fuse rating to the point that the fuse blows at a lower current. Most fuses are rated up to 40°C.
High temperature in the terminals due to poor contact heats up the whole fuse holder and fuse. Eventually melts the fuse holders and causes fuses to blow at lower current. (I would assume this myself)
Click to expand...

Yes.
I thought it may be Ambient T that pushed the fuses over the edge here...but didn't want to lead in that direction...... Would ambient T really add so much that a 20A fuse blows on a 9A circuit?

In any case, I have made a sheet metal shade cover to slip over the Soladeck Junction Box....and spray painted it white.

I will replace fuses with wire nuts.
 

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RE Wire nuts:
- I used to split bolt, rubber tape and electrical tape all PV DC splices for years. Then...somewhere along the way I became convinced wire nuts are acceptable - as long as they are done well.

- MC connectors are made for PV or USE-2 wire. I do not think they work for THHN wire.
ALthough I like that idea.
 
I am doubtful that ambient temperature would derate the fuse that far. Based on your description of the damage I am going with one or more hot terminals due to a high resistance connection to the conductor. That would cause both the thermal damage to the fuse housing and conductor insulation and make the fuse hot enough to blow at the operating current. If there is a high ambient temperature it would just add to that.
 
Yes. It may be the combined effect of all those factors.

I could not touch the black panel up there with my bare hands, it was so hot.
 
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