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Breaker vs Fuse
#1
I have decided to install a shunt trip breaker managed by my BMS. 

Currently I have a fuse in line to protect / completely disconnect storage. When I install the breaker, it seems I should no longer need the fuse. Don't they serve the same purpose, except the breaker may be tripped via the BMS as well as an over current condition. 

So the question is Breaker, or Fuse, or Both?

Different question. Lets assume the circuit had a 400A T fuse. At the bottom of the ABB S5 400A breaker pictured below are two adjustments. What settings match a 400A fuse's rating? What should they be set at?

Thanks for any help to these questions!!

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#2
A breaker is NOT a replacement for a fuse.  Where a breaker could fail, a fuse would not.

If that does not answer your question... BOTH.  Use a fuse with the appropriate ratings (voltage and interrupt) for a battery pack, like an HRC fuse.  It is your last line of defense.
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#3
I've never had fuses - so this question didn't come up Smile
I use an ABB SASE for my shunt-trip (triggered by Batrium BMS) and Midnite Solar and Outback CBs for the 1) battery -> inverter,  2) PV -> combiner, 3) combined PV -> charge controller, and 4) charge controller -> battery.    

I believe high quality  - e.g. SASE, Midnite Solar, Outback, and others at that level are perfectly adequate.  I don't feel comfortable with the cheaper circuit breakers such as this - https://www.amazon.com/gp/product/B072JT...UTF8&psc=1 and would leave fuses in place.
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#4
If you look at appliances y ou buy most or all have fuses inside and also breakers.. The fuse inside is last resort failsafe if shit hit the fan....

So The fuse should be 1.25x the breaker if you ask me, I would like the breaker to trigger first in all scenarios so i easily can reset if needed.
This is not a scienetic statement but my personal based on how systems have been installed around me by licensed electricians.

As an example no panel in Sweden today is installed with fuses and only breakers. Larger current applications on other hand have fuses instead.
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