DC breaker polarity orientation

Started by Riley, February 03, 2024, 01:41:56 PM

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Riley

Hi - this is more of a breaker question but since no breaker specific area, this seemed the closest.
I am using the MNEPV breakers in the combiner for each string (on the roof top) and a 60A MNEPV for a PV disconnect in a big baby box as a PV disconnect in my mechanical room.

The polarity orientation is well discussed in the manual for a PV combiner: "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."

My question is: When these breakers are used as a PV disconnect at the charge controller input (combined PV output), where should the positive (+) end of the breaker go? If I was to follow in the same direction of the breakers in the combiner box, the positive would be at the charge controller end. But when I think about it in a logical sense, the higher potential would generally be at the PV input side of the disconnect breaker and not the charge controller side of the breaker.

Advice please..
Off-grid summer home: XW+6048 / 48V FLA battery bank (428 A/H (Rolls S-550 batteries)) / Conext MPPT 60 150 charge controller / SCP / Insight gateway / 12 - 260W solar panels / Kohler 12KW 12-RES propane genset

ClassicCrazy

#1
If it is a polarized breaker - the positive of the source would go to the + on the breaker.
So if it was from PV going to the controller that would be the source.
If the breaker is between the charge controller and the battery then the charge controller would be the source and go to the + marking on the breaker .
But if it was a voltage coming from the battery to a load then that would be the source.
So it depends where in the system the breaker is located.
Does that answer your question ?
Larry
system 1
Classic 150 , 5s3p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system 2
 5s 135w Kyocero , 3s3p 270w Kyocera  to Classic 150 ,   8s Kyocera 225w to Hawkes Bay Jakiper 48v 15kwh LiFePO4 , Outback VFX 3648 inverter
system 3
KID / Brat portable

boB

It's all about what happens when the breaker tries to open and extinguish the DC arc inside.

The PV combiner CBI breakers were supposed to be non-polarized but I'm not sure what's up there right now.  We may be shipping those nose ?  Not sure.

For the battery to charge controller connection, the PLUS should go towards the battery positive.

This is because, if the Classic should short and relay sticks on, the battery positive can have such a high current capability and so the battery positive will be relatively plus while the breaker opens.

For the PV side, Larry has it right in that the PV side is more positive than the Classic.  But even then, there isn't much of an issue when they are backwards only because the PV array is so limited in the current it can supply.  I am pretty sure we have tested all this a long time ago.  You might find some old documentation somewhere on the MidNite web site ?

I haven't looked in quite a while.

boB

K7IQ 🌛  He/She/Me

Riley

The documentation I am reading is for the PV-6 installation manual. The documentation is very clear that for a combiner the POSITIVE goes to the bus bar (effectively the other combined panels) and not to the solar PV string that the breaker is servicing. I assume the logic is that if there's a short on a PV run to a string, the positive is then going to be the combined output of all the other strings (so it's not obvious).

I think there's agreement on the connection between the charge controller and the battery bank (but again that's not obvious).

For the breaker at the charge controller acting as a PV disconnect, the positive needs to go to the PV combiner.

What I've learned is that I shouldn't use polarized breakers for most of these connections because the positive end actually changes under different conditions. I should have bought non-polarized breakers.
Opps...
Off-grid summer home: XW+6048 / 48V FLA battery bank (428 A/H (Rolls S-550 batteries)) / Conext MPPT 60 150 charge controller / SCP / Insight gateway / 12 - 260W solar panels / Kohler 12KW 12-RES propane genset

ClassicCrazy

I have always used the Midnite Mini DC disconnect boxes since they keep all the wiring short and neat. There is a pictorial drawing showing the way to wire the breakers on page four of the manual. ( you can ignore the dotted blue line which is for an optional ground fault device.
https://www.midnitesolar.com/pdfs/mndc125_250_manual.pdf
Larry
system 1
Classic 150 , 5s3p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system 2
 5s 135w Kyocero , 3s3p 270w Kyocera  to Classic 150 ,   8s Kyocera 225w to Hawkes Bay Jakiper 48v 15kwh LiFePO4 , Outback VFX 3648 inverter
system 3
KID / Brat portable