Crowbar circuit with TL431 help
Two transistors back-back= 1 SCR
two SCR's back-back= 1 Triac
Since hFE &amp; power rating of transistors have a wide range Vgt threshold is affected by Vbe bulk resistance and Vce(sat) so the gate threshold voltage is not very accurate for low voltage OVP. 


Vgt's can range from 650mV to 5V in datasheets but you can roll your own if you want.  The device must not FUSE before the "fuse" according to SOA and fuse times.


There are better ways than using Vgt , instead, use a precision comparator to drive it.
FYI on TL431 is like using Non-Inverting Op Amp gain on 2.5V reference.