Rate my setup please!

MisterSandals

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I do not quite understand what you mean by 'no way to safely use a polarized breaker for the string over-protection device', sorry English is my second language.
I'll take a stab at this unless FilterGuy gets back first:
A polarized breaker would be connected so that the normal current flows from panel to controller (or combiner). This is the easy part and works for single strings (not parallel strings).

As we all know, when you have 3 or more parallel strings you need to fuse each string for potential back feeding (to one string with all the amps of the other parallel strings combined). In such a back feed, the polarized breaker has current flowing in the wrong direction which can produce the results seen in the video in post #15.

I'm sure FilterGuy will correct me if i got this wrong. I'm also certain he'll have more to add, he's a smart guy.
 

MisterSandals

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I cannot see how the backfeed from PV parallel connection if one string shorted out will be fed in the wrong direction to the circuit breaker to the SCC.
That looks like a double combiner box (equivalent of 2 combiner boxes, presumably feeding a 2x MPPT SCC).

This combiner box handles the back feeding potential problem (with diode on +) as well as fusing both + and - array wires.
 

FilterGuy

What, me worry?
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I also see combiner box sold by WATT247 that has fuse holders for + and - and then feeding the ganged polarized circuit breaker, I wonder if that is OK.
Yes. Fuses on the strings and a polarized breaker on the output can be built safely.

I do not quite understand what you mean by 'no way to safely use a polarized breaker for the string over-protection device'
Keep in mind that we are talking about 2 distinctly separate things:

1) PV Disconnect. People often use DC breakers for this even though it will *never* pop if the system is set up correctly. Polarized DC breakers can be used for this if wired correctly.

2) PV String Over-current protection device. This is where a polarized breaker should never be used.

When properly installed, a single string of panels does not need an over-current protection device (OCPD). Even a dead short on the string will not cause a fire because panels are current limited. Even two strings in parallel can be safe without OCPD. However, if there are more than 2 strings in parallel, if one of the strings develops a short, the other strings can dump enough current into the shorted string to cause a fire. Therefor, if there are 3 or more strings in parallel, each string needs an OCPD. Notice however, that the protection is from current coming from the *other* strings. Therefore, to operate correctly, a polarized breaker would have to be installed 'backwards' so it will pop when the current is flowing from the other strings into the shorted strings. The problem is that this means when everything is operating properly, the current will flow backwards through the breaker. Unfortunately, polarized dc breakers can not properly handle a manual turn-off while the current is flowing backwards. The internal arc-snuffing does not work for reverse current so manually turning off the breaker can create a sustained arc that can cause a fire. (See the video I posed earlier)

I wrote a resource to explain the 'when' and the 'why' on putting fuses on PV strings. You may want to review it.

 

FilterGuy

What, me worry?
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This combiner box handles the back feeding potential problem (with diode on +) as well as fusing both + and - array wires.
Hmmm.... that brings up an interesting point. I don't think the NEC allows for blocking diodes as a replacement for string OCPD, but they would prevent the problem. I am guessing the NEC does not consider a diode reliable enough.
 
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