High vs low? DC string fuse sizing

[Zee]

While troubleshooting a commercial system i found 2 out of 14 PV string fuses blown.
The fuses are 12 A/600VDC.
1. Funny thing is 156% times PV Isc is covered by above 12 Amp fuse rating. ( PV Isc = 7.38 A. Multiply by 156% = 11.51 A. < 12 A fuse)
Shouldn't the 12A fuse be just adequate?
2. Funny thing #2, I seem to recall Wiles mentioning that fuses only need the 125% times Isc rating........
That could - in THEORY - mean even LOWER Amp fuses, no??
3. Funny thing #3, this is happening on both systems 7 years after install.
a. Do fuses get tired?:?
b. Did we just have some freak solar storm?
c. Should i be happy and think these panels reached their peak burst of power (momentarily) 7 years in???

OK, I realize original designer was a bit over-zealous in choosing the minimum 12 A size that was calculated as possible. The panel could use 15A fuses based on their series fuse rating.
Here, the problem is solved ( I BELIEVE).
I am getting all 15 Amp fuses for all strings.
I hate the thought of one fuse causing 2KW to shut down for who knows how long.....
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
However, it makes me think: i don't want these problems on my systems in the coming years! :rant::rant::rant:
I kinda like the type of work....that pays.:thumbsup:
For example, i am installing SUNIVA 260's. Isc = 9.01 A. at 156% = 14.06 A.
The wholesaler says series fuse rating is 15A. YET, 15 A fuse relative to the 156% of Isc is really just as much as the problems above.
I'm gonna do 20A fuses.
(Especially as my DC CB is on roof, as always, in the HEAT.)

Your thoughts, as always, appreciated!

 

[SolarPro]

Heat. Fuses are thermal devices. What is the temperature inside the fuse box? What temperature is assumed in the nominal rating of the fuse? If a fuse is exposed to elevated ambient temperatures, then it will trip at lower currents.

If you study the trip curve for the fuse versus the expected environmental conditions, you may find the cause of your nuisance tripping. There may be something else going on, but if you can't find something obviously wrong, I suspect the thermal environment is the problem. Typically you can avoid nuisance tripping by sizing the fuses according to the max allowable series fuse rating as listed on the product label.

(If you size the fuse higher than specified on the label, then you will have violated both the NEC and the UL listing. So be sure to use the product as it is intended to be used.)

 

[Zee]

Right. I always knew heat affected the fuses, just never considered it in this case (DUH!), as it seemed the CB was somewhat shaded. I did paint the other system's Combiner Box white (very exposed)! Big difference in Temp. (I have become a fan of cans of white spray paint for AC load centers, CB's and other equipment on) roofs!


I agree, heat is a huge problem in sizing fuses to properly protect PV in ALL seasons.
My point is, On a rooftop, I don't see how nuisance tripping can be avoided given the requirement to follow the max series fuse rating of the mfctr. (as you pointed out)

A very typical panel today has about 8 A to over 9 A - Isc, and the mfctr very often prints "15A fuse rating" on its spec sheet. If i determine 20A is needed to protect in the ambient environment it is placed in: i still CANNOT do so per UL and NEC.......
It seems only properly locating CB's in sheltered environs can solve that issue.

 

[ggunn]

FWIW, the string fuses are not to protect the wiring from overproduction from PV, but to protect the properly functioning strings from backfeeding a ground fault in another string. If you only have one or two strings, fuses are not required.

 

[Zee]

Exactly. Yes, PV is current limited.
95% of the time, I just install these Combiner Boxes at the array to have a way of isolating circuits from array to equipment at ground level. Usually 1 or 2 strings. So i'll just use oversized fuses as they aren't needed in teh first place.

 

[tallgirl]

Transient high output events happen -- cold, wet panels, with a stiff breeze and some cloud edges will get you to 50% above STC. I have a client with a 9,400 watt array. His peak output is over 14kW. My 2,800 watt array has made 4,400 watts.

 

[Zee]

Transient high output events happen -- cold, wet panels, with a stiff breeze and some cloud edges will get you to 50% above STC. I have a client with a 9,400 watt array. His peak output is over 14kW. My 2,800 watt array has made 4,400 watts.

Cool! Curious, in mountains?
That truly is exceptional as 125% is the factor that covers that in ampacity calcs .
(You must have a Fronius or really good timing to be in front of inverter when it happens.)

You just made fuse sizing even tougher for me now
or rather, that power surge is really due to V (Cold and wind)
and Amps (edge of cloud, strong Sun), right?
So it's not like the amps jumped 50%.

 

[tallgirl]

Cool! Curious, in mountains?
That truly is exceptional as 125% is the factor that covers that in ampacity calcs .
(You must have a Fronius or really good timing to be in front of inverter when it happens.)

You just made fuse sizing even tougher for me now
or rather, that power surge is really due to V (Cold and wind)
and Amps (edge of cloud, strong Sun), right?
So it's not like the amps jumped 50%.

Nope, all OutBack gear.

My recollection is that it was a combination of volts and amps, not just one or the other. It really was the "perfect storm" of events -- we'd had overnight freezing rain that was rapidly cleared out by cold and dry airflow from the due south. I didn't have a back-of-panel sensor on the large array, but I had one on mine and the module temperatures were well below STC.

As for the measuring kit, it's my company's products. My company makes really good measuring gear that way