Dimensioning SCR crowbar circuit
It's a bit confusing that you write about a 15V PSU and a 12V circuit. I assume that you meant a 12V power supply and want to have an overvoltage protection that triggers at 15V.

I'm not very familiar whith thyristors but I think your circuit should work, eventually it will trigger already at 13.3V - 14.3V.

However here some questions:


You write that your PSU delivers 9.6A and that your control board draws 12A - that doesn't fit. How many amps does your board draw in your application?
According to your description your PSU already has overvoltage and -current protection built in. Why do you trust in the overcurrent protection but not the overvoltage protection?
Does the PSU switch of the output permanently or does it try to restart?


The working of this circuit is dependent of the behaviour of your power supply. If the PSU tries to do a restart or the internal protection circuits ara broken than you could get an oscillator. Provided your power supply has enough reserves, I would consider an additional fuse between the PSU and your circuit.