[RE-wrenches] String fuse blown

[Mac Lewis]

Hello Amos,

Have you checked for voltage imbalance for each string?  If you have a bad
diode, that string may run low on voltage and be trying to send current
toward the weak string (this is the reason you have these breakers).  I'd
run each string independently and see what Vmp is.  If you are missing some
voltage that may be a clue.  You can also independently clamp each string
during operation.

I like IR cameras for this type of thing.  It may just be a bad connection,
heat build up, fuse holder issue.

Let us know what you find out

On Fri, Apr 11, 2025 at 8:07 AM Amos Post via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> Hi Wrenches,
>
> I have a bit of a puzzler that I’m trying to get sorted out.  I’ve been
> servicing a Grid-Tied Battery Backup system that’s about 17-18 years old.
> The system comprises of 2-GVFX3648s in series, with 2-MX60(at least I think
> they are 60s) CC’s.  Each CC has 3 strings of 3 tied to it, and each string
> has a 15A in-line 600V fuse.  The modules are I believe are like
> 230/240Watt 60 cell modules (sorry I don’t have pictures/specifics).   We
> replaced the flooded batteries with AGMs about a year and a half ago.
>
> The situation is one of the 15A string fuses keeps blowing.  It’s now
> happened 3 times and its the same fuse every time.  First we just replaced
> the fuse and under the surge/lightning assumption.  Fuse blew again.  Then
> I thought I found the culprit.  The #6 PV output from the combiner to the
> PV input breaker for the CC was a bit darkened and very warm to the touch
> when I replaced the fuse the first time.   So I though maybe it was a loose
> connection on that lug and that caused everything to heat up and eventually
> blow the fuse.  I cut and stripped the wire back and cleaned the lug and
> re-seated the wire in the lug.  Fuse blew again within a few months time.
> I then checked the Junction box on the roof where the USE-2 wire connects
> to THHN, and did a visual inspection of all the wiring on the roof.  All of
> the Jbox connections and everything on the roof “looked” ok.  I have also
> check all of the connections inside the Outback DC load center, and they
> look ok.  Fuse replaced 3rd time and then it blew again.
>
> Before I take all of the panels off the rack, check panels, re-check
> string wiring on the roof, is there anything else I should be checking?
> Seems like that warm wire in the Combiner box is telling me something but I
> can’t figure out what...I believe this is a new problem in the last 2-3
> years but we are not 100% sure when the problem first came up…
>
> As always, thanks for your help/ suggestions!
> Amos
>
>
>
>
>        Amos Post
>    Integrity Energy
>   W 802.763.7023
>    C 802.291.2188
>     ienergyVT.com <http://www.ienergyvt.com>
>         Facebook <https://www.facebook.com/integrityenergyllp?ref=hl>
>
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-- 



Mac Lewis

*"Yo solo sé que no sé nada." -Sócrates*
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