Why is this fuse blowing?

[adobejoe]

I have a bergey 10 KW unit here in Wyoming. I generate about 8,000 kWh per year and it has operated over 15 yrs with little maintenance.  There is a 35 amp fuse at disconnect at base of tower then about 600 ft buried run and a 50 amp fuse on same conductor at entry to inverter. I just blew the 50 amp fuse which is the second time in two years. I guess my question is why the 35 amp fuse is not blowing first. The 35 amp fuse has never blown.

Going to get a backup fuse (busman Fwh-50) butt maybe I should get a 60 amp....?

Andy

[XeonPony]

No if it is blowing there is a reason for it. So some reason over 50 amps are getting pushed through that fuse to some where.

Do you have a camp on amp meter ?

Start tracing current flows. See if there was a defective batch of fuses, ambient temps, and such.

[SparWeb]

Wild a** guess #1:
Different fuses blow at different speeds. 
I can't really explain all the details but it is possible that a 50Amp fuse is so much faster than a different type of slow 35A fuse.
Wild a** guess #2:
There is enough capacitance in the buried 600ft run that when the inverter flips On, it accepts a momentary surge as the capacitor is discharged.

[adobejoe]

Spar. I like your answer...capacitance in wire. The 35 amp is closer to the turbine so you would think that would go first.  I will keep a 60 amp spare. Thanks!,

[DamonHD]

I find it difficult to believe that the energy stored in wire capacitance could even tickle a substantial physical fuse wire never mind get it hot enough to melt.

I'd be more inclined to believe a fast-blow vs slow-blow thing.

Or maybe there is some other curious intermittent wiring fault.

Rgds

Damon

[SparWeb]

Try 0.3 uF per kilometer for dry wire.  The run is 600 feet so we have 2 conductors * (600*0.3048/1000*0.3) = 0.055 uF
That will store a micro Coulomb of charge.  It will discharge about 10 microAmps at 60 Hz. 

After a day to think about it, that's not it at all.

Can your wires handle the 60 Amps?