[RE-wrenches] String fuse blown

[Amos Post]

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/>
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