maximum series fuse rating?

[TomW]

Sorry Tom,  I was just using the term back-feeding to mean excess current (fries the wires and busses) and that's sloppy.  Otherwise, did my logic sound Okay to you?

No worries.

As someone mentioned in this thread (I think) you need to beware of "nuisance tripping" if you shave it too close on breaker ratings, especially if the breaker is used as a shutoff and gets cycled.

I find it hard to give good advice when the target audience has such a varied level of knowledge in the subject.

Things I often automatically "figure in" without conscious thought can be a bad thing to ignore in use for the unaware.  Giving too much info can confuse folks, as well.

I learned electronics back when the components were huge and the hot ones were called "valves" also known as vacuum tubes.  Today, they stuff a couple billion transistors (valves) on a CPU in your laptop.

Its all good. Just want to be sure I didn't miss something or develop Alsheimers.

Tom

[Vic]

Also,  older Thermal and Thermal Magnetic breakers have developed a reputation as getting more-and-more sensitive and tripping at a lower current with more trips.   Some/many were NOT "Switch_Rated"  --  the ability of these breakers to be used as switches,  under load.

The breakers that MN sells are all Magnetic-Hydraulic (or sometimes,  Hydraulic-Magnetic).

Here is a statement from Halfcrazy Ryan,  about the CBI breakers that MidNite sells,  under the MNEPV part number,  copied from Reply #7 in this thread:
http://kb1uas.com/mnsforum/index.php?topic=351.msg2024#msg2024

"Straight from our CBI distributor when I talked to him in Dallas at SPI. The CBI breakers are good for 10,000 or more switches under partial to no load and 5,000 or more under full load rating. We have sold millions of CBI breakers and I can count on one hand with fingers left over the failures we have had. Rony the distributor for CBI is in Pennsylvania and sells to a lot more than us and he says the same thing they just do not break.

The Over center thing you mention is just part of the spring or snap action it has to open fast to help extinguish the arc. I have a 150vdc Din rail breaker here that is on my lister and I always open it under load it runs a VOC of 195vdc and an operating voltage of 135vdc and 60 amps. I have been using this for about 6 months and have opened it a few dozen times and it works just fine.
Ryan"

Hope that stealing words of others is OK,  FWIW,  Vic

[dgd]

... The theory here is that if you have 3 strings of modules or more and one module shorts the other strings can backfeed that one string. Thus giving it to much current. If you use no series fusing and had say 6 strings of pv paralleled and one string shorts YES the house will burn down but if you had the proper fuses or breakers life is good. See picture for how NOT to parallel modules 

I'm not disputing this but am having difficulty figuring out how this can happen with  several parallel strings of PVs if there is a blocking diode for each string and bypass diodes in the PVs of each string. How can any form of backfeeding work to burn down a house?
I understand, I think, that the MNPV6 breakers, one per string, protect the string if for some reason ( that I can't imaging)  produces abnormally high current that exceeds the breaker switch rating. That scenario never concerned me as I use the MNPV6 breakers  simply to isolate a string when I want to work on its wiring.
dgd

[Powerplay]

I'm not disputing this but am having difficulty figuring out how this can happen with  several parallel strings of PVs if there is a blocking diode for each string and bypass diodes in the PVs of each string. How can any form of backfeeding work to burn down a house?

I just went with the premise for the sake of argument, and, like most of us electrical types, I'm a belt AND suspenders type of guy.    Maybe some math can tell whether there would or could ever be a serious issue despite one's super bad luck scenario of rocks - meteors - hail, etc.

Mine goes like this - -imagine one of the strings to a given combiner is shorted and open to back feeding all the current available to a given combiner box.  My biggest combiner at the moment has 2000W of PV total.  But one of the strings is shorted out and not producing anything - fair?  Okay  2000 - 400 = 1600W.  Now the PV is only about 80% efficient on a good day.  (Assuming I wasn't home during the hail storm and didn't open the breakers before the sun came out.      Okay 1280W.  The voltage at the combiner 76V so 1280 / 76 = 16.8A  - so that's borderline possible since the max fuse is 15A.  Have to weigh the likely hood of occurrence for this I think.  My other combiner only has 1500W available.  Btw, it looks like a 20A breaker wouldn't even get involved in this.

[Powerplay]

I just noticed the VoC on those strings would be 90V so the the 15A breaker likely also wouldn't come into play at 1280 / 90 = 14.2A.  So...

[CDN-VT]

I just noticed the VoC on those strings would be 90V so the the 15A breaker likely also wouldn't come into play at 1280 / 90 = 14.2A.  So...

Also on that NOTE !!
The folks who use The Classic  200 & 250 units Your Breakers are NOT the same as what the Classic 150 use due to voltages . They also have & sell the same 10 amp breaker in 250Vdc and it the same size & money ..

SO do the MATH & order the correct size & voltages .
Inspectors with real know-how will look & judge .

VT