Why I ignore panels Isc rating.

[Mattb4]

To kick off the controversy.

Of the various number found on the panels specifications I find the Isc (short circuit current) to have the least practical use. I say it is no sense to even worry about it. It really only happens when you direct connect the positive of a panel (or panels in Series) to the negative. As a test of the diodes perhaps it has some significance but it is not gong to happen under a load situation to a SCC. If you size your circuit protection for Imp than it will react before you can reach Isc.

While using Isc for setting the SCC input capability is often mentioned in real use the Imp is what the SCC is tasked to handle. Amperage limits and time are important to heat dissipation capability of the SCC. I can not see a circumstance where you would have Isc at your SCC for any length of time and especially not if your PV circuit is fused for Imp.

So in my thinking you should plan your PV and SCC requirements using Voc, Vmp and Imp. Isc is simply a curiosity.

 

[efficientPV]

Isc is used for PWM and direct connect water heating and those people can ignore Vmp and Imp. The cultural divide will never be healed.

 

[Bop]

You are welcome to your 'belief', but the fact remains that it is a useful parameter, especially for 'quick tests' of their output, regularly used by installers worldwide on a daily basis...

 

[Mattb4]

You are welcome to your 'belief', but the fact remains that it is a useful parameter, especially for 'quick tests' of their output, regularly used by installers worldwide on a daily basis...

Grid tie installers. Them folks have different things and equipment they worry about.

I have no beliefs or sacred cows just simple observations. Some wrong or based on not seeing correctly.

 

[Hedges]

Use Isc to size wires and fuses.
Isc x 1.56 is in NEC. I think Isc x 1.25 should be sufficient for hydraulic-magnetic (not thermal) breakers or unfused.

 

[Bop]

Grid tie installers. Them folks have different things and equipment they worry about.

I have no beliefs or sacred cows just simple observations. Some wrong or based on not seeing correctly.

Yes I did start installing gridties- in 2004...
I started installing offgrid solar- in the early 1980's....
I no longer install gridtie- but live and install offgrid still (occasionally- blew past 50 a while back- too old to be climbing on roofs and crawling under houses...)

Like I said- you can believe whatever you want...

 

[Mattb4]

Yes I did start installing gridties- in 2004...
I started installing offgrid solar- in the early 1980's....
I no longer install gridtie- but live and install offgrid still (occasionally- blew past 50 a while back- too old to be climbing on roofs and crawling under houses...)

Like I said- you can believe whatever you want...

And, as I said, I have no beliefs. Prove to me using Solar cell construction and circuit analysis of how testing them in multiple configurations can show how voltage and short circuit current alters with damage. Than you will have more than simple appeal to Authority type argument.

BTW this discussion is meant to be fun and promote thinking. Do not take it personally.

 

[Hedges]

I won't explain how, but I will document that it does.

Measuring PV panel performance

I suppose I could buy a solar panel tester like Will reviewed, but that doesn't feel DIY enough to me. Besides, I'm a cheapskate, and I try to do things in the most difficult (and expensive if the equipment wasn't on hand) way. I'm presently working on an underperforming string of Sharp 165W...

diysolarforum.com

 

[robbob2112]

Why is Isc important? What if you have a string of panels and you sized the wire for Imp. Now through some fluke the wires get shorted together inside conduit because you did a bad pull and damaged the insulation at a corner. Now they're is running at Isc for an unknown period of time generating heat at the short. You will eventually have melted wires and maybe a fire.
Now have three strings in parallel and no fuses, or fuses at one string and not using a combiner. If the shorts is in the wires of one string just before they come together. Each string dumps their Isc into that short and you have 3x the current of that the wire is sized for.

Now, those are a stretch but use Imp to size the wire and don't do the 1.56 capacity, or in some cases the 1.25 capacity and melting wires abound.

When I get to my computer I'll run some numbers with various configurations and shorts in different places...or you @MattB can do it since you opened the topic.

 

[Mattb4]

I won't explain how, but I will document that it does.

Measuring PV panel performance

I suppose I could buy a solar panel tester like Will reviewed, but that doesn't feel DIY enough to me. Besides, I'm a cheapskate, and I try to do things in the most difficult (and expensive if the equipment wasn't on hand) way. I'm presently working on an underperforming string of Sharp 165W...

diysolarforum.com

Perhaps I am misunderstanding but this is a panel that is undamaged.

 

[Mattb4]

Why is Isc important? What if you have a string of panels and you sized the wire for Imp. Now through some fluke the wires get shorted together inside conduit because you did a bad pull and damaged the insulation at a corner. Now they're is running at Isc for an unknown period of time generating heat at the short. You will eventually have melted wires and maybe a fire.
Now have three strings in parallel and no fuses, or fuses at one string and not using a combiner. If the shorts is in the wires of one string just before they come together. Each string dumps their Isc into that short and you have 3x the current of that the wire is sized for.

Now, those are a stretch but use Imp to size the wire and don't do the 1.56 capacity, or in some cases the 1.25 capacity and melting wires abound.

When I get to my computer I'll run some numbers with various configurations and shorts in different places...or you @MattB can do it since you opened the topic.

Having short of your wires to create an ISC condition is possible. But would the wires get hot enough to melt or would the panel cells get damaged first?

ETA: BTW what happens to a solar panel when you connect the pos and negative together and leave it like that?

 

[efficientPV]

All I know is that your Chinese breakers will never open.

 

[Mattb4]

Did some searching on the issue of shorting out a solar panel and found this at the Solar panel Forum.

I shorted my Panels ... Are they Dead? - Solar Panels - Solar Panels Forum

OK so I know it was a dumb thing to do. I was hot and had been on the roof most of the day and was not thinking clearly. I bought 9 285w REC TwinPeak 2

www.solarpaneltalk.com

 

[sunshine_eggo]

Isc can be easily exceeded with cloud edge effects. My system has produced 20% over rated power due to high elevation and cloud edging.

Many charge controllers deliberately short the PV input to protect against damage from reverse polarity or if they fail via another means. I encountered a Midnite classic that had failed, and it shorted the PV input.

Isc and beyond happen in the real world, and as @Hedges pointed out, there is an NEC requirement to size your PV wire on a multiple of it.

 

[Mattb4]

Isc can be easily exceeded with cloud edge effects. My system has produced 20% over rated power due to high elevation and cloud edging.

...

Well to be pedantic ISC is not exceeded it just goes higher due to more solar irradiance than test conditions.

Isc can happen but what does that mean if a shorted solar panel will not even warm the wires?

 

[Bop]

Did some searching on the issue of shorting out a solar panel and found this at the Solar panel Forum.

I shorted my Panels ... Are they Dead? - Solar Panels - Solar Panels Forum

OK so I know it was a dumb thing to do. I was hot and had been on the roof most of the day and was not thinking clearly. I bought 9 285w REC TwinPeak 2

www.solarpaneltalk.com

View attachment 237379

So you have declared that a well known test and specification is safely 'ignored'- with apparently little experience with solar and apparently knowing very little about them (yes panels are a current source, this has been well known for half a century now!!!) and the Isc is not that much higher than the Imp- certainly not enough to 'melt wires', indeed it will not even be noticeable unless you have already screwed up badly in the design and install phases....
This is stuff that is well known already...

 

[sunshine_eggo]

Well to be pedantic ISC is not exceeded it just goes higher due to more solar irradiance than test conditions.

Yes, but the reason it exceed STC rated Isc exist in the real world.

Isc can happen but what does that mean if a shorted solar panel will not even warm the wires?

The reason you respect Isc on a Victron controller is due to the rating of the contactor protecting against reverse polarity. Of course if you choose to base on Imp, you'll be under that, so it doesn't matter, but if you're sizing wires based on Imp, you may under-spec your wires.

 

[Mattb4]

So you have declared that a well known test and specification is safely 'ignored'- with apparently little experience with solar and apparently knowing very little about them (yes panels are a current source, this has been well known for half a century now!!!) and the Isc is not that much higher than the Imp- certainly not enough to 'melt wires', indeed it will not even be noticeable unless you have already screwed up badly in the design and install phases....
This is stuff that is well known already...

Uh you misunderstand me. I never declared a test is safely ignored. Sorry that I upset you but do not read into things that are not there.

You also impugn me on my solar experience. You have zero idea of how much I have. I asked you to not take it personally that I was exploring something and wanting people to think about it. But you did take it personal as well as out of context. Now you insult me. That I do not take.

 

[Bop]

Uh you misunderstand me. I never declared a test is safely ignored. Sorry that I upset you but do not read into things that are not there.

You also impugn me on my solar experience. You have zero idea of how much I have. I asked you to not take it personally that I was exploring something and wanting people to think about it. But you did take it personal as well as out of context. Now you insult me. That I do not take.

It is LITERALLY the title you assigned this thread!!!!

https://diysolarforum.com/threads/why-i-ignore-panels-isc-rating.

You were the first one to start slinging insults as well...
"Grid tie installers. Them folks have different things and equipment they worry about.
I have no beliefs or sacred cows just simple observations. Some wrong or based on not seeing correctly."

Your 'knowledge' level is readily apparent by your questions you asked (especially for those with a LOT of training in the electrical and electronics fields, of which I am one- trade qualified electrical fitter since the 1980's, auto electrical (boats, cars, and motorhomes), solar designs and installations (in the mining game for over a decade doing remote solar monitoring equipment- at the design level...) and level 3 electronics tech- design and repairs as a bench tech...

Not 'bignoting' myself here- but your questions you asked in this thread alone, show a certain level of knowledge in the electrical field....

 

[Hedges]

Perhaps I am misunderstanding but this is a panel that is undamaged.

The panels have no mechanical damage, but some have degraded severely.
Apparently a known issue for this model.

Don't know if it is cracks in cells or chemical (dopant) issues, but a few were 70% the Isc of others, even lower Imp.
Some only producing 40% the power of others.

I will say that measuring Isc finds some but not all bad panels.
So I recommend measuring W(mp) or W(load)

 

[Mattb4]

..

I will say that measuring Isc finds some but not all bad panels.
So I recommend measuring W(mp) or W(load)

Yeah that was a point about measuring Isc I tried to make but received a hostile response to.

I do agree that Imp as a sizing method would need to take into consideration those times when irradience was higher than STC. Using Isc as a stand in or "rule of thumb" has a valid basis.

 

[robbob2112]

Did some back of napkin calculations and if you size for Imp and forget the 1.56 for the NEC things get super hot quick with a short... if you add in 1.25 it helps. Now where it gets dicey is when temps get down close to freezing so current goes up or when it exceeds 40c and you have to derate the wire faster than the current output drops from hot panels.

Lots of ways to slice it and extra factors to throw in.. I didnt even look at wires in coduit and heating in a confined space.

Which is why following the NEC guide for the pv wires and using Isc is the safest way as it results in the largest wires for the least losses and heating.

I think it comes down to following best practices and if you decide to cut a corner know what you are risking, kinda like using a 300amp ANL that cost $3 including holder verse using a class T in a ignition safe mount.

 

[Hedges]

Yeah that was a point about measuring Isc I tried to make but received a hostile response to.

You didn't say to identify bad panels. But you did mention testing diodes.

If you size your circuit protection for Imp than it will react before you can reach Isc.

If you size fuses or breakers for Imp, they will nuisance trip. They should never trip, except in case of a fault.
OCP on output of a PV array will never trip. The only thing that could/should be able to trip it, is for multiple parallel strings if one shorts and 2 or more likely at least 3 backfeed into it. Then it is for protection of the wire or (failed shorted) diodes.

You also aren't likely to have enough failed diodes to serve as a short for other strings to dump current into, unless you sort out (or receive) a bunch of panels with failed diodes and wire them in series. Because diodes fail due to other good panels in same string pushing current through them.

While using Isc for setting the SCC input capability is often mentioned in real use the Imp is what the SCC is tasked to handle. Amperage limits and time are important to heat dissipation capability of the SCC. I can not see a circumstance where you would have Isc at your SCC for any length of time and especially not if your PV circuit is fused for Imp.

If fused for Imp, you won't have Isc for very long but you also may not have Imp for very long either.

Recommended minimum fuse is Isc x 1.56
Recommended maximum, for a string, is what panel label says. Often it is about 2x Isc.

So in my thinking you should plan your PV and SCC requirements using Voc, Vmp and Imp. Isc is simply a curiosity.

SCC requirements, yes. Which can include over-paneling to 2x the Imp it will process.

One place where Isc is meaningful is reverse-polarity protection diode in inverter/SCC.
I suggest connecting only one PV string first. Confirm it works before connecting more strings that exceed short circuit current rating.

 

[KnobCountry]

Some of you make using the ignore button a super easy choice.

Mentioning that grid tie installers have a whole other set of requirements, skills and challenges is not an insult.

The thread was clearly started to generate some casual back and forth, generate ideas and gain new perspectives.

Ideas can be presented without hostility. This forum really suffers from certain personalities and many just simply leave due to the environment.

 

[robbob2112]

I tend to think of the folks that responded to this thread as drinking buddies sitting around a table and there is always someone to start talking smack. Seldom do I see bad behavior in regular threads where these same folks are helping new users.

Part of the problem is the written word doesn't generally convey tone or expression so can be mis-interpreted, especially with different cultures all around the world.