12V 12A resettable crowbar circuit
Try this as an example of using an SCR and a zener: -



Picture source.

If the voltage exceeds Vz + Vgt then the SCR fires and shorts out the supply. Only when the power supply is removed or dropped in voltage to a low level does the short circuit reset.



Added section

If you need a tighter tolerance for crowbar activation than that offered by zener-sensing then use a controller: -



Vz in the circuit can be a precision device that is better than 1% in tolerance quite easily.