Question about breaker polarity wiring - Greatest power potential

[hairfarm]

Greetings,

I bought a Midnite Big Baby box. Its purpose is to hold two breakers. The "input" breaker (63A) will be installed between my PV array and CC. The second "output" breaker (80A) will be installed between my CC and my battery bank. Like I said, both will be housed in the Big Baby box.

Midnite states that the "Positive (+) side of their DC breakers must go to the greatest power potential."

So can anyone advise if this is the correct way interpret this:

From my solar array the positive wire will go from the combiner box into the + of the 63 amp breaker then to the CC. Then from the CC, the positive wire will go into the negative side of 80A breaker and out the positive side on the way to the battery bank. That way each + is toward the greatest power potential.

Positive wire path: Array > into positive side of 63A breaker > out negative side of 63A breaker to CC > from CC  into negative side of 80A breaker > out positive side of 80A breaker to battery bank.

Is this correct?

thanks!

Hairfarm

[vtmaps]

Welcome to the forum,

Yes, I think you've got it figured out correctly.  Are these panel mount breakers?  If so, the polarity does not matter.  Polarity does matter for the din rail breakers.

--vtMaps

[hairfarm]

Yes, they are DIN mount.

Thanks for the input!

[TomW]

The "most Positive" is the way to remember this.  The Most Positive connection goes to + on the breaker

The most positive on the PV in is the PV + which goes to the breaker +.

The most positive on the battery circuit is the Controller +. Which goes to the + on the breaker.

If that makes sense. That is how I did mine.

Tom

[Vic]

The "most Positive" is the way to remember this.  The Most Positive connection goes to + on the breaker

The most positive on the PV in is the PV + which goes to the breaker +.

The most positive on the battery circuit is the Controller +. Which goes to the + on the breaker.

If that makes sense. That is how I did mine.

Tom

MN Engineering/Tech has counseled in the past that the Classic positive output must be connected to the negative of the output breaker,  and the positive of that breaker connected to the positive of the battery.

This is because if the CC becomes damaged,  huge fault currents flow from the battery into the output terminal of the CC (in most cases) -- these currents can be thousands of amps.

If this breaker trips due to current  in excess of the breaker rating (in the reverse direction)  this is not harmful to the breaker.

My recollection ...

Just timed out in the first attempt to post this hope that I typed fast enough this time ...    Vic

[Vic]

BOY,  timed out again in a couple of minutes ...  VB

[Resthome]

Tom, Please clarify "The most positive on the battery circuit is the Controller +. Which goes to the + on the breaker."


I believe the Output Breaker for the Classic should have it's + wired to the Battery + and the - of the Output Breaker connected to the Classic +. At least that is the way I read the instructions provided with Combiner boxes.

I believe (IIRC) that the panel mount breakers have a terminal marked LOAD.

Hairfarm has it correct.


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!

[vtmaps]

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

Just to be clearer... in the combiner, the + side of the breaker goes to the PV + from the string of panels.  The negative side of the breaker goes to the PV positive bus bar (where the strings are combined).

This means that in the event of a shorted string, the fault current from the parallel strings will flow backwards through the breaker of the faulted string.   

Midnite has chosen to install the combiner breakers as string switches, rather than as overcurrent protection devices for a shorted string.

--vtMaps

[hairfarm]

Thanks Guys. I just needed verification. It seems like this topic on DC breaker polarity has sparked some debate in the past.

BOY,  timed out again in a couple of minutes ...  VB

VB - before posting on any forum I'll usually type my text into a word document then when I'm done, I'll paste into the forum and hit "post". That way there's no time for the forum to timeout. Assuming that's what you meant.

[Halfcrazy]

Only the CBI din rail breakers are polarity sensitive. They have a + emblem on then and that is the side with the greatest potential.

This is easy on the PV side. The PV is the potential so the + emblem goes towards the PV panel.

On the battery side it is not so easy. Common sense tells us the charge controller is the source so it is the potential. But out research shows the real issue is when a charge controller fails it shorts thus the breaker is working off the battery at that point. So we want the + emblem to point towards the battery not the charge controller.

[bartman]

This seemed straight forward enough when I read it.  Then I found this cute little wiring diagram that appears to violate the rule - this is what I was I had drawn for my own diagram until I read about the polarity requirements for DIN breakers.

http://www.midnitesolar.com/pdfs/Nigeria%20system.pdf

What gives?

[vtmaps]

Then I found this cute little wiring diagram that appears to violate the rule - this is what I was I had drawn for my own diagram until I read about the polarity requirements for DIN breakers.

http://www.midnitesolar.com/pdfs/Nigeria%20system.pdf

I don't know what gives.  The MNPV6 is a combiner... that means the negative (load) sides of the breakers are combined.  That is not suitable for a power distribution panel.

Of course, as I mentioned earlier in this thread:  I don't really understand how the MNPV6 (which is a combiner) can be used as a combiner: 

The breakers in the combiner have their negative (load) side combined.  That is exactly correct if you wish to use the breakers as switches to disconnect individual PV strings.... the positive output from the string goes to the positive side of the breaker.

The problem is: the PURPOSE of those breakers is to prevent overcurrent through a faulted string.  In the event of a fault, the current from the five other strings will flow retrograde through the faulted string.  During this retrograde overcurrent condition, the highest potential on the faulted string's breaker is on the breaker's negative side.

Each string breaker is sized larger than the PV output of its string... therefore the string breakers should never see an overcurrent condition caused by overcurrent from from the string they are protecting.   The only way for them to ever experience an overcurrent condition is when the current is flowing through them from the negative side to the positive side.

--vtMaps

[Robin]

The confusion on PV combiners is very real. It is impossible to have the breakers in the correct polarity under all circumstances since they are poalarized. Our testing shows that with available currents from a PV array, they work perfectly in both directions. That is more of a problem with batteries although we have not seen contact welding until you use battery voltages over 100 volts. You do want to get the battery plus connection connected to the + of the din rail breaker.
CBI is working on making their breakers non polarized. Even though it isn't a real problem, the NEC is now requiring non polarized breakers. Our panel mount breakers are non polarizxed, but the din rail breakers are not yet non polarized. After having shipped 2 million of these din rail breakers, we have yet to find one that has ever failed to open, so it doesn't sound like it is much of a problem.

[kauaisolarman]

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

Just to be clearer... in the combiner, the + side of the breaker goes to the PV + from the string of panels.  The negative side of the breaker goes to the PV positive bus bar (where the strings are combined).

This means that in the event of a shorted string, the fault current from the parallel strings will flow backwards through the breaker of the faulted string.   

Midnite has chosen to install the combiner breakers as string switches, rather than as overcurrent protection devices for a shorted string.

--vtMaps

any updates or news on this subject?

this is interesting as i was sizing my combiner breakers to be a reverse current protection in case of a faulty string and the current from the other paralleled strings to flow to the faulty set the breaker connected would trip before hitting the panels max reverse current protecting the panels.   

most panels are 10-15A max reverse current

am i getting this correct or is this not possible with these types of DC circuit breakers?

[Vic]

   ...   Our testing shows that with available currents from a PV array, they work perfectly in both directions   ...    After having shipped 2 million of these din rail breakers, we have yet to find one that has ever failed to open, so it doesn't sound like it is much of a problem.

Hi again Kauai..,

Robin is a Founder of MidNite Solar,  he has been heavily involved with the breakers that MN sells,  as well as most other products that they develop and ship  (as you probably know).

Standard wisdom,  is to size each Combiner breaker at the Maximum fuse (or breaker) size specified by the PV manufacturer,  or size UP to the next common breaker size,  if the max size speced would be an odd-ball,  unavailable rating.

Vic