DC breaker wiring

gnubie

Photon Sorcerer
The topic of DC breaker wiring polarity (ie, which side of the breaker is the source and which is the sink) has been raised a few times on the forum. I'm not sure if this clip has been posted before, but if not this is a must for everyone to watch.

 

Hedges

Photon Sorcerer
And I say the recommended way to wire polarized DC breakers is bass-ackwards, will not provide the required protection.

Because I know what I'm talking about, and the "experts" who gave the recommendation and designed the certification tests do not know what they are talking about.

First of all, his demo showed that interrupting full voltage from PV panels (feeding a short) while wired backwards caused a failure. True.
If feeding a charge controller, voltage difference between Voc and Vmp is about 10% of Voc, would have disconnected successfully. (But you still don't want it to become an incendiary device if feeding into a short.)

Problem is, we use fuses or breakers to prevent backfeeding into a string due to a fault, when paralleled with other strings.
Consider 9 strings of PV panels each on a separate breaker, paralleled, and connected to another breaker which was supposed to have a 10th equal PV string but instead has a short.

The 9 breakers all do not trip because each string only puts out about 7A, and breaker is 15A.
The one breaker carries 9 x 7 = 63A, backfeeding into a short so it trips. It is now carrying current in the wrong direction, so it burns up.

This fault was the reason code required OCP, but individual breakers failed to provide it. The "experts" who designed certification test and said, "Look Ma (FCC), it works!" did not know what they were doing.

With polarized DC breakers, the only way to use them safely is to have them ganged, so when the backfed one trips, it shuts off the others. The breakers carrying current in forward direction interrupt current and extinguish their arc. The backfed breaker couldn't interrupt its current, only served as an actuator for the others.

Alternative mentioned in the video, non-polarized DC breakers, can be used individually.

So in my professional opinion, the individual polarized DC breakers marketed for string combiners to not provide the protection required by NEC and are not safe when such a fault occurs.
 

gnubie

Photon Sorcerer
The demonstration was to show what happens if the current flow through the device is not in the correct direction for a polarised DC breaker.

Sometimes I wonder why I even bother with this forum any more.
 

Hedges

Photon Sorcerer
Uh, gnubie,

I agree the demonstration was good. Thank you for providing it.

What I'm saying is that the polarized breakers, which are sold for PV disconnects, can't provide OCP for paralleled strings. Neither direction is correct. (when they are separate breakers, not ganged.) The UL listing is a false sense of security.

The video showed the failure which will happen. The way PV strings are being paralleled today, using listed polarized DC breakers, does not provide protection.
 

BiduleOhm

Electronics Sorcerer
Easy and cheap solution: fuses.

I know they can't be reset and must be replaced when blown, and they can't serve as a disconnect, but they are safe in both directions, always.
 

Hedges

Photon Sorcerer
Yes, a fused combiner (which Midnight sells) and a single polarized DC breaker of correct orientation for disconnect would work. Most of their pre-assembled combiners are fused like this one:

http://www.midnitesolar.com/product... Pre-Wired&productCat_ID=35&sortOrder=4&act=p

But this combiner with individual breakers - unless they are ganged so when one trips they all shut off, I don't believe it protects against a shorted string.


I don't know the exact Voc or Isc where failure would occur, but if those breakers can be individually switched I think a retrofit which gangs them should be installed.

Combiners configured by an installer with individual breakers on a DIN rail would have the same problem. Here's a polarized product with two, 150V breakers ganged and wired in series for 300V:


Four for 600V:


I think those are fine as a disconnect for a single high-voltage string. Multiple of those combining strings in parallel is where my concern arises.
 

Sverige

A Brit in Sweden
The topic of DC breaker wiring polarity (ie, which side of the breaker is the source and which is the sink) has been raised a few times on the forum. I'm not sure if this clip has been posted before, but if not this is a must for everyone to watch.

Scary stuff. Thanks for posting.
 

SherylinRM

I read a book that says "DON'T PANIC" on the cover
The topic of DC breaker wiring polarity (ie, which side of the breaker is the source and which is the sink) has been raised a few times on the forum. I'm not sure if this clip has been posted before, but if not this is a must for everyone to watch.

SSSSHHHEEEEEETTTT
Ok that was stupid scary.
I was thinking of getting another circuit breaker.
At least now I know what to get AND how to wire it up properly.
THANK YOU Gnubie for this video.
Wow, that was fast.
Scary as sheeeet too.
Heck wow.

Thanks again Gnubie. This is an eye opener. :)
 
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