Over current protection in parallel strings

[cdsolar]

Question about breaker use for 3+ parallel strings:

I understand the following:

1. When combining two series strings, no OCPD is needed, as in the case of a short circuit in the first string, the second string can only push the max of the Isc amp rating of a string into the first, which is less than the max series fuse rating.

2. When dealing with more than two parallel strings (let's say three for simplicity), if there is a short in the first string, then the other two can each push a max of ISc (or just max current at max power) amps into the shorted one, and 2xIsc (or even just 2x current at max power) is greater than the maximum series fuse rating, this requiring an OCPD device on each string.

3. What I don't understand is, why vendors such as MIdnite provide combiners that accept breakers, such as the MNEPVxx series, which are polarized (only break in a single direction, and appear to show the + side toward the PV) hence in the case of a short, the high current would come from the *wrong* direction, which causes the breaker to not put out the arc, or worse, a fire.

I know there are fuse solutions that work in both directions (as opposed to breakers), but why the option for polarized breakers in this case in systems such as the MNPV3 or 6 combiners, and probably other combiners? I also know that MIdnite is advertising some non-polarized breakers that are not yet generally available.

Reference for this saying not to use polarized breakers in this situation:

https://diysolarforum.com/resources/fusing-wire-sizing-guidelines-for-solar-panels.143/download

I suspect this has already been discussed, and if so, I apologize, and ask to be pointed to said posts. Thanks.

 

[timselectric]

3. What I don't understand is, why vendors such as MIdnite provide combiners that accept breakers, such as the MNEPVxx series, which are polarized (only break in a single direction, and appear to show the + side toward the PV) hence in the case of a short, the high current would come from the *wrong* direction, which causes the breaker to not put out the arc, or worse, a fire.

I hadn't really paid much attention to them. I don't use them. But if that's the case. You are right about it being backwards.

 

[timselectric]

I suspect this has already been discussed, and if so, I apologize, and ask to be pointed to said posts. Thanks.

This is the first time I have seen anyone pointing it out.

 

[rhino]

3. What I don't understand is, why vendors such as MIdnite provide combiners that accept breakers, such as the MNEPVxx series, which are polarized (only break in a single direction, and appear to show the + side toward the PV) hence in the case of a short, the high current would come from the *wrong* direction, which causes the breaker to not put out the arc, or worse, a fire.

These are custom made breakers and manual states: "The + sign designates the highest potential should be connected there.". So it is correct to to have the PV positive output connected to the + on breaker.

 

[timselectric]

These are custom made breakers and manual states: "The + sign designates the highest potential should be connected there.". So it is correct to to have the PV positive output connected to the + on breaker.

But the highest potential is on the combiner side of the breaker, and not the PV side. So, I believe that the question is valid.

 

[timselectric]

Polarized breakers should never be used in a combiner box, for the individual strings.
However, it would be ok for the main output.

 

[cdsolar]

So, my question still stands. Why are these polarized breakers an option in Midnite's combiners?

 

[timselectric]

So, my question still stands. Why are these polarized breakers an option in Midnite's combiners?

I don't have a good answer.
Other than, they shouldn't be.

 

[Dsolar]

:

 

[MisterSandals]

So, my question still stands. Why are these polarized breakers an option in Midnite's combiners?

This explains why so many combiner boxes have fuses for the strings. I could never figure that out but now it makes sense.
Everybody likes breakers more than fuses but it doesn't always make sense to have a breaker.

 

[cdsolar]

Just found this (thanks to Google, not the search feature here):

Potential danger using polarize DC breakers in combiner boxes

Anyone else realize this potential problem? Polarized DC breakers such as the ones used in Midnight Solar MNPV combiner box have polarity. The + side must be connected to the highest potential, which would be the PV string. This is required for the arc suppression to function. However...

diysolarforum.com

 

[Shimmy]

2. When dealing with more than two parallel strings (let's say three for simplicity), if there is a short in the first string, then the other two can each push a max of ISc (or just max current at max power) amps into the shorted one, and 2xIsc (or even just 2x current at max power) is greater than the maximum series fuse rating, this requiring an OCPD device on each string.

This is the only one I can give much of an answer on. The breaker on the battery is to protect the battery and ensure it doesn't continue to fault or contribute to a fault. When you combine multiple batteries you need a secondary breaker to protect the cable from the common point to the inverter. The NEC allows you to have "short lengths" of unprotected cables as long as the cable is protected at one end.

So, what is not protected in an installation with three server rack batteries is a terminal fault. Specifically, a high-impendance fault that might only pull 3x a single battery's breaker rating. That is a risk, but not one that code really addresses. 15kW to the case of the battery would likely shut things down in a few minutes though. (After frying everything though.)

 

[Tomthumb62]

Just found this (thanks to Google, not the search feature here):

LoL. I’ve found the search feature on this forum better than most. Some self hosted search engines are a joke.

It’s not a mistake or by chance that Google is today a behemoth of a tech company. Not sure how old you are, but the early days of search engines were just awful. Seemed like such a simple task, but the way humans asked the search engines, the engines didn’t understand. So if you were a computer engineer, then you understood the required syntax to find what you were looking for, but the average person isn’t a computer engineer. Then Google came along and changed the world, because it better understood what the human was trying to look for.

Yes, Google does allow some poorly developed website (ie this forum) integration that uses their search engine to search a specific website, but it sucks. No one uses it and for good reason, it’s just fuhhgly.

 

[cdsolar]

LoL. I’ve found the search feature on this forum better than most. Some self hosted search engines are a joke.

It’s not a mistake or by chance that Google is today a behemoth of a tech company. Not sure how old you are, but the early days of search engines were just awful. Seemed like such a simple task, but the way humans asked the search engines, the engines didn’t understand. So if you were a computer engineer, then you understood the required syntax to find what you were looking for, but the average person isn’t a computer engineer. Then Google came along and changed the world, because it better understood what the human was trying to look for.

Yes, Google does allow some poorly developed website (ie this forum) integration that uses their search engine to search a specific website, but it sucks. No one uses it and for good reason, it’s just fuhhgly.

My question can be asked in multiple ways. OCPD, over current, combiner, which may seem unrelated, but are. Not very specific and hard for the forum search to handle. I've been thinking and searching on this for several hours.

I was not intending to search this forum specifically with Google. It just happened to return a result from here.

As for my age, my first modem was a 300 baud acoustic coupler. At one time, I owned EVERY video game on the market. (Pong console from Atari, connected to my TV).

 

[Tomthumb62]

I was not intending to search this forum specifically with Google. It just happened to return a result from here.

In this forum and others, I’ve seen the suggestion several times that it’s best to use Google to search instead of the native search bar. I know it seems like it should be an easy task, but if sorting through data was so simple, then Google would’ve been ousted as the search king long ago. They have still the best search engine out there but have also played your business game very well.

So why doesn’t this forum and others just tell people to not use the search bar (or outright disable it) and use Google instead? Well one reason is if people leave a site to go to a place like Google, they could get sucked down the internet rabbit holes and forget what they were doing. The forum/site loses traffic and that’s not good for them. This forum doesn’t even sell anything, but it’s great PR for Will Prowse (I respect him a lot for his contributions). But I think a more human reason for not sending people to Google is it’s embarrassing. It’s embarrassing just how lame poor crappy most ant search engine is that isn’t a big name like Google, Yahoo or DuckDuckGo (not sure how big the Duck is). I suppose Google could license their search engine to replace any built in one, but who knows how much that would cost and I understand this forum costs Will a lot of his own personal money to keep running (he refuses to allow advertising of any kind and is pretty stubborn to pay the costs himself). But I’m not sure Google wants to do that because it means fewer people using their platform which will mean they make less money since they won’t be seeing all those ads they’d see on a Google.com search.

Anyways, I hear you on frustrations not finding what you want with using this forum’s built in search bar. It’s just not this forum that sucks in that regard. Reddit is even worse! Google will find me Reddit posts all the time, but Reddit can’t find those same posts!

 

[cdsolar]

I asked MIdnite this question. They surmise that this is an edge case and that they are primarily concerned with arcs stemming from additional current from events on the + side. (Lightning, dropping rebar on the module, etc.).,[all my words here,]

In my own case, I will have only two strings, so none of this is really an issue, but will need to wait for the non-polarized MNEPV breakers before I (may) expand.

Note: MIdnite offers the combiner boxes with fuses, which eliminates this concern, but if you want to disconnect at the combiner, you'll need either their combined disconnect/combiner, or a separate disconnect switch. However, I like being able to disconnect an individual string, which a separate switch would not allow. Not sure if pulling a live fuse is a good idea. ?

 

[Hedges]

So, my question still stands. Why are these polarized breakers an option in Midnite's combiners?

I think they didn't think of it.
And once they started selling it, never had a problem. Likely rare that the fault develops and trips a breaker.

I asked MIdnite this question. They surmise that this is an edge case and that they are primarily concerned with arcs stemming from additional current from events on the + side. (Lightning, dropping rebar on the module, etc.).,[all my words here,]

I asked Midnight the same, in an email, and they brushed it off.
I suggested that ganging the poles so when reverse current into shorted string trips it would turn off the ones carrying forward current.

I don't consider it an edge case; it is the case for which OCP is required.

I considered setting up the failure scenario and making a video, but eventually code required polarized so let it go.

As for my age, my first modem was a 300 baud acoustic coupler. At one time, I owned EVERY video game on the market. (Pong console from Atari, connected to my TV).

Youngsters!

We used 110 baud acoustic coupler and ASR-33 teletype.