A PV Combiner box has fuses installed. Required by law!
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If it is connected to batteries, it needs fuses installed, even for a single string. Because, as has been said earlier, because a fault condition in a non-isolated MPPT can put the full current of the batteries onto the PV cable.
For a single string on an AC-coupled system, they are not required.
So this scenario would require a fuse to protect a PV array and a MPPT charging batteries?
A non-isolated buck MPPT should also have a disconnect to prevent backfeeding at night. This is usually another FET. But in the event of a failure it is possible to have the battery directly connected to the PV side, where the wires won’t be adequately protected by a fuse that protects the battery wires. So to make sure everything is safe, a fuse should be added to both sides.
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And that is precisely why you do it. Because FETs have this way of welding themselves into a closed circuit when they fail, and then they take the house with them 
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Please provide an update on this…
Installed 20 amp fuses, never had an issue again, but I have16 amp fuses on my own system which is exactly, the same for the last two years ( knock on wood ) and no issues