PV disconnect breaker polarity - Using Midnite MNEPV polarity sensitive breaker

Riley · February 3

Hi -

I am using 15A 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 Midnite PV-6 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.

Thoughts?
I have also asked on the Midnite forum but thought I would ask here as well.

BB. · February 4

Another "simple question" with a long answer that ends up "don't use polarity sensitive DC breakers in circuits where current flow reverses"--Such as charging/discharging a battery bank, using these breakers on on/off switches, etc..

The reason for polarity sensitivity for DC breakers (as I understand)--Some (few, some, all?) polarity based breakers use a permanent magnet to "blow" the DC arc of the switch into the quenching channel. If you "reverse the current" flow, the arc can be pulled out the quench channel into the breaker guts instead--And damage the breaker or worse.

https://www.youtube.com/watch?v=Oaq2cvoPBRk (switched under load--Fire example)

https://www.youtube.com/watch?v=S0diAeysxVo (guts of several types of breakers, including a DC polarity breaker example)

https://www.youtube.com/watch?v=Zez2r1RPpWY (example of AC vs DC switching, and how DC arcs "sustain" very nicely)

Note that even 10 Amps of DC current (presumably >>12 VDC) can set a breaker on fire if the breaker is turned off while "reverse current" flows.

When used on the battery bus--The battery is assumed to be the source of very high current--And short circuits/overloads on branch circuits are loads that will trip the breaker(s)... So + of breaker to + of battery or battery bus (circuit short pulls current from battery bank).

The issue with DC polarity rated breakers--They can only "safely" designed to interrupt current flow in the "correct" direction.

For example, if you have 3 or more parallel strings and a combiner box, the current flows from the array to the charge controller when all is working correctly. But current flow is "reversed' in a shorted/faulted solar panel string (i.e., current from "good panels" back feeds the shorted string).

You use the breakers to turn on/off a working string if the + breaker terminal is connected to the solar panel. However, if there is a shorted panel/string, then the current flows "backwards" to the shorted panel, and the breaker + should be facing the combined panel outputs (to the charge controller panel input).

You can have a similar problem with battery banks--Current flows "into the battery" when charging, and flows "out of the battery" when discharging. If the breaker trips, or you manually cycle the breaker you don't know/have control if cycling happens and you run the risk of catching the breakers on fire.

You should not really use polarity sensitive breakers in DC circuits with "bi-directional" current flow.

Some authorities have allowed/used a "do not switch under load" warning level--Which could be used in (for example) a combiner panel--But this is not a "good" solution. The better solution is to use non-polarity sensitive DC rated breakers or fuses.

And many solar combiner installations had the breaker wiring installed incorrectly (Australia was the example I saw in Youtube) and they had roof top fires from the breakers. Plus the installers had to revisit their installations and check the wiring.

-Bill

Riley · February 4

Thanks Bill, excellent answer and I had a feeling this was the case. Good to know.

In the case of the PV input on a charge controller, the current really only flows in one direction ( from panels into the controller). I don't believe the controller can source current out that port.
As a safety precaution, I think I will place the controller into standby mode ( stops current flow) before opening the PV disconnect breaker.

What is more of an issue is the breakers in the combiner box. Like you say, their recommendation works ok for a shorted panel but can't be used to disconnect a functioning string. If I ever wanted to do that, I would need to shut down the PV system as well.

Makes me wonder how they did their disconnecting combiner boxes.

Now that I understand the underlying principles I can work around this.

The greatest issue is the breaker between the controller and battery bank but I'm not replacing that and suspect is a Schneider breaker and came with the PDP breaker box.

BB. · February 4

Riley, you got it. The combiner breakers are installed "backwards" so they can trip if there is a shorted string--But cannot be used to switch under load.

The on/off breaker at the PV input to the charge controller need to be large enough that they will never trip (from over current) and can be used to switch off for service.

What surprised me was that the polarized breakers fail (catch fire) at "at normal operational current" if cycled off with reverse current flow.

-Bill

PV disconnect breaker polarity - Using Midnite MNEPV polarity sensitive breaker

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