In this case it is not clear what is between the 50KA breaker and the substation - I suspect there may be a 'death or glory' fuse that will limit the damage to some extent.

However, even assuming there is,  it is not a sensible design to have a system where when the SPD operates for the last time and becomes a glowing lump of low impedance semiconductor, for the whole site to be disconnected and put out of service until the wiring to the SPD can be unbolted and then the main fuse changed, it is far more sense for the SPD to have its own local fuse, and possibly an alarm that operates if it blows, so when the SPD succumbs to the inevitable, power can be restored as soon as reasonably practical, and the SPD repair scheduled when convenient.

It is worth noting that the really big surge protectors that you get on the 11kV OH lines are exactly like this, and have something akin to an expulsion fuse in concept to blow the wiring off so that the line power can be restored as soon as possible. To be fair compared to the 230V devices,  each SPD has a string of several chunks of rock all of which have to fail, and are sized to put the sort of thing that goes in a CU to shame, so they last many years.

The additional advantage of the explosive disconnector is that a pair of binoculars can reveal the 'tripped' SPD by the obvious dangling link wire, from the safety of the landrover at the roadside, and changing it can be left to a time when the power can be routed around or outage scheduled. I'm not really suggesting that for indoors.