MNEPV**-300 Breakders have two connections?

[keithwhare]

From the pictures, it looks like the MNEPV**-300 breakers for the MNPV6-250 combiner box have a positive & negative connections as well as the positive connection for the bus bar.

What's the negative connection for?

Keith

[Halfcrazy]

The 300 volt breakers have a copper jumper in the bottom they are 2 breakers ganged than wired in series. so the two top connections are the input and output. + and - are the same as line and load the + or line side gets the connection with the highest voltage potential here is a little write up on that.


IMPORTANT! Polarity of the din rail DC breakers.  The DC breakers supplied by MidNite Solar are custom manufactured in Lesotho Africa by CBI. These breakers are polarity sensitive. This means that they need to be installed correctly in order to insure that they will be able to trip if called upon to do so. In a PV combiner the + sign marked on the breaker connects to the PV positive output. The same breaker when hooked up to the battery circuit (not in a PV combiner) hooks up a little different. The + sign hooks up to the battery plus. This hook up is not obvious. The + sign designates the highest potential should be connected there. This is an easy one to determine in a PV combiner. Follow the current path through the combiner, into the PV input of a charge controller and out of the controller to an output breaker and then into the battery plus. You would think that the end of the output breaker connected to the controller would be at a higher potential than the battery plus. In normal operation this is true. The main job of this output breaker is to trip when and if there is a catastrophic failure. (Any manufacturer of power electronics will tell you that power electronics can fail). If the output breaker fails to trip, you are at risk of fire from the output wires burning up. When a charge controller fails, they always short from positive output to negative output. Since these two terminals inside the charge controller are normally connected up to a very large battery bank, you have a direct short across the battery bank if the controller fails. During this condition, the controller is acting like a piece of wire. The battery positive terminal is the highest potential! Make sure that the plus (line) of the breaker is connected to the battery plus terminal. If the breaker is connected backwards, it can fuse in the closed position as it attempts to open. That could ruin your entire day!

[keithwhare]

So in the MNPV6-250 combiner, one needs to get the positive wires from the PV strings into the breakers right next to the bus bar?

[Halfcrazy]

That is correct.

[Robin]

Take a look at the picture of the MNPV6-250 on our website. This link goes to our price list, but you can look it up through the PV combiners section of the product category too in case the link doesn't work.
http://www.midnitesolar.com/productPhoto.php?product_ID=264&productCatName=PV COMBINERS&productCat_ID=9
You will notice large cut outs on the big positive busbar. This is to allow the PV wires to enter right next to the output busbar. Do not mess with the bottom busbar on the 300 volt breakers. It is factory installed. This 300 volt breaker is really two of our 150V breakers in series. They are internally ganged and the magnets reversed in one of the breakers. You cannot accomplish the same thing by handle tying two 150V breakers together. The breakers would not trip at the same time. The internal ganging insures that they trip at the same time. I tried putting two 150V breakers together and they breaker just fuses itself in the on position as it attempts to open. This high voltage stuff is quite interesting and also unforgiving. Our 300V breakers are ETL listed for this application, where as two of the 150's are not.

[RegGuheert]

In a PV combiner the + sign marked on the breaker connects to the PV positive output.

This is how I thought I should connect it up, but I was not successful doing it that way.  Using the MNPV12-250, I bolted the two plates together, installed the lug, put them in place and found I could only get the plate to go into the positive terminals, not the negatives as Robin shows here:

Take a look at the picture of the MNPV6-250 on our website. This link goes to our price list, but you can look it up through the PV combiners section of the product category too in case the link doesn't work.
http://www.midnitesolar.com/productPhoto.php?product_ID=264&productCatName=PV COMBINERS&productCat_ID=9

I looked at the bolted-together plates and it looked symmetrical to me, so I did not go through the step of removing it from the unit, moving the lug from one side to the other and then reinserting it to try to get it into the negative terminals as shown.  Thinking about it, I guess it makes sense that would work, since it picked up the positive terminals on BOTH sides the way I assembled it.  In any case, I wasn't overly motivated since I wasn't sure it would make a difference.  Now I know better!

I guess I need to go back and fix that.  Thanks for the detailed explanation on this!

[RegGuheert]

Take a look at the picture of the MNPV6-250 on our website. This link goes to our price list, but you can look it up through the PV combiners section of the product category too in case the link doesn't work.
http://www.midnitesolar.com/productPhoto.php?product_ID=264&productCatName=PV COMBINERS&productCat_ID=9

I've been thinking about this further and I am going to challenge you regarding how you have shown this plate inserted into the '-' terminals of the breakers.  My conclusion is that the breakers will *never* trip in this configuration of a PV combiner.  Here's why:

Note:  All of the below discussion assumes that this is a negative-ground PV system.

Scenario #1:  There is a short to ground on the positive wire coming from the output of the combiner
Supposedly, this is the fault condition which your arrangement would clear.  However, your PV modules each look like current sources limited to Isc, so they will *never* produce more than about 1.25 times the Isc value at STC, so there is never enough current available for any of the breakers to trip.  The *only* solution here is to ensure that the wire from the PV Combiner is sized to handle 1.25 times the sum of all the Isc values at STC for all the strings being combined.

Scenario #2:  There is a short to ground on the positive wire leading from a string in the PV array to the combiner
In this scenario, the wire from the PV is now at a lower potential than the potential on the combiner plate.  But if you have it connected as shown in the photograph, then it is wired into the '+' terminal and the fault current is flowing the opposite direction from what will trip the breaker.

Now, if you connect the plate into the '+' terminal of the breakers, then the PV output will be connected to the '-' terminal.  When the positive wire from a string shorts to ground, the fault current will flow from '+' to '-' in the breaker leading to that string and if the combined current from the other strings is higher than the breaker rating, the breaker will trip and the fault will clear.

So now I need to go back and reverse the connections on my DC breaker between the charge controller and the batteries so that the '+' terminal is closest to the battery, since I think I got that one wrong.  But I believe I wired the DC combiner box correctly...

Someone please correct me if my understanding is off here.

Reg

[Halfcrazy]

Take a look at the picture of the MNPV6-250 on our website. This link goes to our price list, but you can look it up through the PV combiners section of the product category too in case the link doesn't work.
http://www.midnitesolar.com/productPhoto.php?product_ID=264&productCatName=PV COMBINERS&productCat_ID=9

I've been thinking about this further and I am going to challenge you regarding how you have shown this plate inserted into the '-' terminals of the breakers.  My conclusion is that the breakers will *never* trip in this configuration of a PV combiner.  Here's why:

Note:  All of the below discussion assumes that this is a negative-ground PV system.

Scenario #1:  There is a short to ground on the positive wire coming from the output of the combiner
Supposedly, this is the fault condition which your arrangement would clear.  However, your PV modules each look like current sources limited to Isc, so they will *never* produce more than about 1.25 times the Isc value at STC, so there is never enough current available for any of the breakers to trip.  The *only* solution here is to ensure that the wire from the PV Combiner is sized to handle 1.25 times the sum of all the Isc values at STC for all the strings being combined.

Scenario #2:  There is a short to ground on the positive wire leading from a string in the PV array to the combiner
In this scenario, the wire from the PV is now at a lower potential than the potential on the combiner plate.  But if you have it connected as shown in the photograph, then it is wired into the '+' terminal and the fault current is flowing the opposite direction from what will trip the breaker.

Now, if you connect the plate into the '+' terminal of the breakers, then the PV output will be connected to the '-' terminal.  When the positive wire from a string shorts to ground, the fault current will flow from '+' to '-' in the breaker leading to that string and if the combined current from the other strings is higher than the breaker rating, the breaker will trip and the fault will clear.

So now I need to go back and reverse the connections on my DC breaker between the charge controller and the batteries so that the '+' terminal is closest to the battery, since I think I got that one wrong.  But I believe I wired the DC combiner box correctly...

Someone please correct me if my understanding is off here.

Reg

The + and - markings have no bearing on when the breaker will trip. If it is a 15 amp breaker it will trip with 16 amps (or there about) of current in either direction. The + and - mark have to do with extinguishing an arc inside the breaker. I will roundup Robins explanation on this I think it is here on the forum and make it a FAQ if it is not already.

[RegGuheert]

The + and - markings have no bearing on when the breaker will trip. If it is a 15 amp breaker it will trip with 16 amps (or there about) of current in either direction. The + and - mark have to do with extinguishing an arc inside the breaker. I will roundup Robins explanation on this I think it is here on the forum and make it a FAQ if it is not already.

So you are saying these breakers will trip with overcurrent in either direction, but only one direction is safe in terms of extinguishing the arc.  Still, I believe your picture of the plate installation is incorrect as the arc you try to extinguish from a ground-shorted positive PV wire will have higher potential at the combiner plate and lower potential at the PV wire.  As such, I would think the plate should go in '+' and the wire should go in '-'.  Please correct me if I misunderstand.

[Halfcrazy]

Here is Robins write up on this I will try to find out a definitive answer on the combiner part.

IMPORTANT! Polarity of the din rail DC breakers.  The DC breakers supplied by MidNite Solar are custom manufactured in Lesotho Africa by CBI. These breakers are polarity sensitive. This means that they need to be installed correctly in order to insure that they will be able to trip if called upon to do so. In a PV combiner the + sign marked on the breaker connects to the PV positive output. The same breaker when hooked up to the battery circuit (not in a PV combiner) hooks up a little different. The + sign hooks up to the battery plus. This hook up is not obvious. The + sign designates the highest potential should be connected there. This is an easy one to determine in a PV combiner. Follow the current path through the combiner, into the PV input of a charge controller and out of the controller to an output breaker and then into the battery plus. You would think that the end of the output breaker connected to the controller would be at a higher potential than the battery plus. In normal operation this is true. The main job of this output breaker is to trip when and if there is a catastrophic failure. (Any manufacturer of power electronics will tell you that power electronics can fail). If the output breaker fails to trip, you are at risk of fire from the output wires burning up. When a charge controller fails, they always short from positive output to negative output. Since these two terminals inside the charge controller are normally connected up to a very large battery bank, you have a direct short across the battery bank if the controller fails. During this condition, the controller is acting like a piece of wire. The battery positive terminal is the highest potential! Make sure that the plus (line) of the breaker is connected to the battery plus terminal. If the breaker is connected backwards, it can fuse in the closed position as it attempts to open. That could ruin your entire day!

[RegGuheert]

Yes, this text was already posted in this thread.  Here are the parts to which I take exception:

In a PV combiner the + sign marked on the breaker connects to the PV positive output.

This is an easy one to determine in a PV combiner.

But you are go on to describe NORMAL operation of the PV, not a fault on the PV side which reverses the polarity like you described for the battery.  Again, I think the '-' sign needs to connect to the PV positive wire and the '+' wire needs to connect to the metal plate you supply.  To make it clearer, think of the metal plate as your "battery," since this is the "source" of the current which will be flowing into the fault by virtue of fact that it collects the current provided by the other PV strings.

[Robin]

The polarity issue of a combiner breaker can be very confusing to say the least. IT is possible that current WILL be flowing in the wrong direction regardless of how they are connected. The breakers must be connected  in the manner we show to meet UL and NEC requirements. When a combiner breaker is actuated in a normal mode, all the polarities are correct. This is how the breaker is designed to be installed, with the highest potential connected to the + terminal. I guess they don't want the breaker welding itself open just because you turn off one string of panels. It has been mentioned many times here when the PV+ shorts to GROUND......   That is what a DC-GFP is designed to clear. They are also required by the NEC. When a module shorts internally, the adjacent strings will back feed the defective string and attempt to clear the fault. The breaker on the faulted string will be getting back fed with reverse polarity because it has been drug down in voltage. The CBI breakers we use have been tested to clear this type of fault. Does it appear to be backwards during this type of fault, yes. Does it work, yes. I would not suggest hooking up breakers opposite the polarity marking. You will never pass inspection and you run the risk of fusing the breaker in the on position every time you turn the breaker on or off. If there was such a thing as a non polarity DC breaker, we would be using them, but they do not exist. Having sold many millions of breakers for combiners, I can attest to the fact that this system works quite well even though after looking at it, the polarity seems backwards on the faulted breaker.

[RegGuheert]

Hi Robin,

I appreciate the detailed explanation, but I have to say that the way these breakers are recommended for use for PV disconnects is completely nonsensical.  What you are saying is that I am risking fusing my PV disconnect breaker each time I use it to switch off 8A of current from the PV string.  If that is so, what do I risk each time I switch off my Classic charge controller which is putting, say 96A of current into the (-) terminal of the breaker and then out of the (+) terminal of the breaker on its way to the battery?

My only conclusion based on what you write is that this can work for the PV combiner only because the fault current is limited to about 11 times the current of a single PV string.  (12 strings).  Nothing else really makes sense.  To recommend one polarity for the breaker between the CC and the battery and the opposite polarity for the PV sources truly makes no sense, regardless of what testing was done by for NEC approval.

Thanks again,

Reg

[Halfcrazy]

Reg we have sold Millions of these breakers with no issues at all but if this setup really worries you the other option would be to go with fuses and high voltage disconnects. Remember Fuses can not be opened under load.

[RegGuheert]

Reg we have sold Millions of these breakers with no issues at all but if this setup really worries you the other option would be to go with fuses and high voltage disconnects. Remember Fuses can not be opened under load.

I understand.  I'll let it go.  I just suspect the PV disconnect application is much more forgiving than trying to disconnect a fault from a battery due to the limited current available from the array.  Likely both orientations work just fine for PV combining (and I suspect customers are using both orientations) but I also suspect that only the correct orientation works properly for clearing a high-current fault from a battery.  I do understand that only one arrangement has been UL and NEC certified.