Protecting BJTs from overvoltage
Here's a half decent-looking site for an explanation of an over-voltage crowbar protector. It has various circuits but ends up talking about the MC3423 crowbar protection IC: -

And, although I've never used this part, it appears to tick all the boxes. Alternatively use a TL431 device (commonly available) like this circuit shown in the link at the top: -

I think this might have components chosen for 5 volt over-protection so some value adjustments will need to be made for a voltage above + 40 volts. Simulation is going to be your best buddy here.
That site then moves on to describe a version that has improved di/dt protection for the triac/SCR/thyristor used. The site also gives some other links to useful pages.
Bottom line is that you will need to have a fuse in series with the 50 volt supply and the crowbar circuit works by detecting an over-voltage and shorting out the supply in microseconds hence, it relies on the fuse blowing.