This is a basic schematic of a PV connection to a DC bus. There is a 125A MCCB at the bus to which a 35mm2 2-core cable connects from a four channel optimiser. The cables on the other side of the optimiser are 6mm2. They in turn connect to combiner boxes and to the PV strings of approximately 9Kw per string. There are two strings connected to an optimiser channel. You can see that this occurs 13 times so it is a system getting on for 1Mw. The battery is also 1Mw. 

I have no idea what protection exists, if any, in the optimisers. I am just thinking about a short somewhere between the strings and the optimisers where the conductors are 6mm2. Perhaps it is not possible for current from the bus to pass the optimiser and feed such a short?