Open a circuit when voltage exceeds a certain value?
I've designed one recently. Here is my memory test: -







simulate this circuit &ndash; Schematic created using CircuitLab

I can't remember what the MOSFETs actually were so sorry for this but it isn't rocket science to simulate this circuit in LTSpice (e.g.) and try a few different MOSFETs. Mine turned off M1 when the input voltage got beyond about 30V - it was protecting a 42V (AMR) switcher on the output side for excessive voltages coming in. M1 is normally on when there are a few volts applied to Vin and as Vin rises past 30V (ish) M2 starts to conduct and shuts off M1.

Both transistors were rated in excess of 100 volts because the generator could lose control easily and shove out well over 50V (this would fry my switcher).

Reading your question, mine as a pretty similar application to yours!

EDIT - I've changed the values in the circuit to reflect what I used in my prototype.