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.
watts247.com
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.
To get the paper, click on the orange'' button at the top of the screen. This resource is intended to help the user understand when they need to add Over Current Protection Devices (OCPDs - Fuses or Breakers) to the solar panel array and what...
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